Abstract
Controlled environment (CE) food crop production has existed in the United States for many years, but recent improvements in technology and increasing production warranted a closer examination of the industry. Therefore, our objectives were to characterize historical trends in CE production, understand the current state of the U.S. hydroponics industry, and use historical and current trends to inform future perspectives. In the 1800s, CE food production emerged and increased in popularity until 1929. After 1929, when adjusted for inflation (AFI), CE food production stagnated and decreased until 1988. From 1988 to 2014, the wholesale value of CE food production increased from $64.2 million to $796.7 million AFI. With the recent increase in demand for locally grown food spurring an increase in CE production, both growers and researchers have been interested in using hydroponic CE technologies to improve production and quality. Therefore, we surveyed U.S. hydroponic food crop producers to identify current hydroponic production technology adoption and potential areas for research needs. Producers cited a wide range of technology utilization; more than half employed solely hydroponic production techniques, 56% monitored light intensity, and more than 80% monitored air temperature and nutrient solution pH and electrical conductivity. Additionally, the growing environments varied from greenhouses (64%), indoors in multilayer (31%) or single-layer (7%) facilities, to hoop houses or high tunnels (29%). Overall, producers reported managing the growing environment to improve crop flavor and the development of production strategies as the most beneficial research areas, with 90% stating their customers would pay more for crops with increased flavor. Lastly, taking historical data and current practices into account, perspectives on future hydroponic CE production are discussed. These include the importance of research on multiple environmental parameters instead of single parameters in isolation and the emphasis on not only increasing productivity but improving crop quality including flavor, sensory attributes, and postharvest longevity.
CE food crop production has fluctuated in productivity and evolved in technology over history. For example, early U.S. CE food production generally took place in glasshouses heated by “hot beds” of manure (Dalrymple, 1973), whereas currently food is produced not only in glasshouses but plastic-glazed houses and indoor facilities heated through many methods, most of which are not manure-based. By examining both historical trends and current practices, we can deduce possible future CE hydroponic production trends. Therefore, our goals were to 1) characterize the historical trends in CE production, 2) conduct a national survey with the objectives of identifying current CE hydroponic production technology adoption and the research needs and priorities of the U.S. hydroponics industry, and 3) use historical trends and current practices to inform future perspectives.
Historical Perspectives of U.S. CE and Hydroponic Production
Although CE vegetable production has been reported to have originated during the Roman Empire, it was not until the early 1800s that commercial CE food production emerged in the United States. By 1900, an estimated 1000 facilities were growing 40 ha of winter vegetables in CE with ≈67% of crops grown in greenhouses and ≈33% grown in hotbeds and coldframes (Dalrymple, 1973; Galloway, 1900; Jensen and Collins, 1985). At this time, wholesale and retail annual CE vegetable sales were estimated to be $2.3 and $4.5 million, respectively (Dalrymple, 1973; Galloway, 1900). By 1929, the first year the U.S. Department of Commerce [USDC; these data are now collected by the U.S. Department of Agriculture (USDA)] Census of Horticultural Specialties reported vegetables grown under protected culture, 520 ha of vegetables were produced with 43%, 33%, 18%, and 6% of the crop value consisting of tomato (Solanum lycopersicum), cucumber (Cucumis sativus), lettuce (Lactuca sativa), and other crops, respectively, and sales totaling $10.0 million [$136.9 million adjusted for inflation to 2014 (AFI); USDC – Bureau of the Census, 1930; Table 1]. Although food was grown under protected culture in glass-glazed greenhouses, hydroponic production was limited (Jensen and Collins, 1985).
The number of operations, area under protection, kilograms produced, total sales, and sales adjusted for inflation of food crops grown under protection in the United States between 1929 and 2014 as reported by the U.S. Department of Commerce or the U.S. Department of Agriculture. Values do not include mushroom production.
Hydroponic production can generally be defined as growing plants without mineral soil, using “an inert medium such as gravel, sand, peat, vermiculite, pumice, perlite, coco coir, sawdust, rice hulls, or other substrates” instead and adding the nutrients necessary for growth (Resh, 2013). Although this definition includes soilless substrate growing systems typical to potted floriculture production, by convention, hydroponic production excludes this growing method (Gomez et al., 2019). A more common description used by the USDA Census of Horticultural Specialties and this study, defines hydroponic production as food crops grown “in nutrient solutions without soil” (USDA – National Agriculture Statistics Service, 2015).
Interest in hydroponic production arose from issues with fertilization and soil as a substrate (Dalrymple, 1973). This led to the development of sand culture (Shive and Robbins, 1937), water culture (Gericke, 1933), and then subirrigation (Withrow and Biebel, 1937) hydroponic production systems in the late 1920s and 1930s. Commercial production-scale hydroponics in the United States began during World War II when the U.S. military used hydroponics to produce fresh food on Pacific islands during the war (Jensen and Collins, 1985), and by 1972, commercial hydroponic production greenhouses emerged across the United States (Dalrymple, 1973).
From 1929, protected vegetable cultivation stagnated or decreased until 1988, after which CE food production again increased in popularity. For example, in 1929, food crops worth $136.9 million AFI were produced under 5.2 million m2 (USDC – Bureau of the Census, 1930). By 1988, only $64.2 million AFI worth of vegetables grown under 1.2 million m2 remained, a 53% AFI decrease in sales and a 77% reduction in production area (USDC – Economics and Statistics Administration, 1991; Table 1). The stagnation and decrease in CE food production was due to many factors, possibly including increased trade with Mexico; refrigeration and interstate infrastructure, making long-distance perishable food shipment feasible; and also the burgeoning floriculture industry, the value of which increased from $82 million in 1929 ($555 million AFI to 1988) to just under $2 billion in 1988 (USDC – Bureau of the Census, 1930; USDC – Economics and Statistics Administration, 1991). However, CE vegetable production has been increasing since 1988, and by 2014, $796.7 million worth of vegetables was produced in 8.7 million m2 of protected environments (USDA – National Agriculture Statistics Service, 2000, 2015).
A seemingly large increase in greenhouse food production took place between 1988 and 1998, during which the number of operations nearly doubled from 581 to 1015 producers, and sales grew from $64.2 million AFI to $322.2 million AFI (402% AFI increase in total value; USDC – Economics and Statistics Administration, 1991; USDA – National Agriculture Statistics Service, 2000). However, an even greater increase in the number of greenhouses producing food occurred between 1998 and 2014, when the number of operations increased by 148% (1506 additional operations) and sales increased from $322.2 million AFI to $797.7 million (148% AFI increase). Over those 16 years, large increases in cucumber ($60.0 million AFI increase, 339%), fresh cut herbs ($26.1 million AFI increase, 158%), lettuce ($42.0 million AFI increase, 311%), strawberry (Fragaria ×ananassa; $847,000 AFI increase, 1193%), and tomato ($230.5 million AFI increase, 135%) sales occurred, while pepper (Capsicum annum) remained relatively steady ($13.7 million AFI decrease, 19%; USDA – National Agriculture Statistics Service, 2000, 2015; Table 1).
When comparing 1929 to 2014, both Census of Horticultural Specialties reports cited cucumber, lettuce, and tomato as the most commonly produced CE crops. Additionally, the 2014 report also cited fresh cut culinary herbs, pepper, and strawberry as top CE crops (USDC – Bureau of the Census, 1930; USDA – National Agriculture Statistics Service, 2015; Table 1).
The portion of protected culture crops grown hydroponically was first reported in 2009, with 73% of CE vegetable crops produced in protected culture grown hydroponically. Tomato was the highest value food crop grown in CE, with sales totaling $355 million AFI, and 89% of those fruits were produced hydroponically (USDA – National Agriculture Statistics Service, 2010; Table 1). Similarly, 92% of the cucumber crop produced in CE was grown hydroponically, while only 4% and 3% of pepper and strawberry crops, respectively, grown in CE were produced hydroponically in 2009 (USDA – National Agriculture Statistics Service, 2010). In 2014, the top hydroponically produced CE crops were as follows: cucumber (30.0 million kg), fresh cut herbs (3.4 million kg), lettuce (7.0 million kg), and tomato (75.1 million kg) with 91%, 21%, 70%, and 86% of cucumber, fresh cut herbs, lettuce, and tomato crops, respectively, grown in protected culture produced hydroponically (USDA – National Agriculture Statistics Service, 2015).
Although peer-reviewed aquaponics industry surveys (Love et al., 2014, 2015; Villarroel et al., 2016) and vegetable industry surveys, including a “State of the Vegetable Industry Survey” administered by American Vegetable Grower magazine (Gordon, 2016), have been conducted, these reports have focused primarily on aquaponics or outdoor production. A “State of Indoor Farming” survey was conducted by Agrilyst, a climate control system company, in 2017 reporting data on facility type, crops produced, yield, profitability, technology, and growers’ future plans (Agrilyst, 2017). However, this survey did not identify specific production practices and technology adoption, nor did it identify the U.S. CE hydroponic industry’s research needs. This information is valuable for firms entering CE production or considering it for a near-term opportunity, in addition to educational programming efforts to inform future entrepreneurs. Therefore, a more comprehensive survey focusing on specific production practices and research priorities was needed to focus CE research and extension efforts on topics most beneficial to current and future producers.
Current U.S. Hydroponic Crop Production Practices: A Survey
Approach.
To achieve our objectives, we developed a 23-question online survey for the hydroponics industry (https://osf.io/rvtem/?view_only=ff2affcc7eb442118c12cc4cf08bd78e). The questions were designed to gather information pertaining to 1) demographics and business operations, 2) young plant propagation; 3) hydroponic food production (transplant to harvest), and 4) research priorities. The survey consisted of four open-ended, 16 closed-ended multiple choice, and two constant sum questions. Additionally, we asked respondents to rate several topics relating to hydroponic production on a 4-point Likert scale in terms of their perceived benefit to their operation. The scale ranged from 1 (not at all beneficial) to 4 (very beneficial). Means and standard deviations were calculated from the responses.
With the survey content and methods approved by the Michigan State University committee on research involving human subjects (IRB x17-241e), the survey was created on the SurveyMonkey (San Mateo, CA) online platform to improve ease of access to survey participants. Requests to advertise the survey and recruit participants were sent to several widely read industry trade publications focused on greenhouse and CE crop production, including Growertalks, Greenhouse Product News, Greenhouse Grower, HortAmericas, HortiDaily, Inside Grower, Produce Grower, and Urban Ag News. Each publicized the survey by providing a link to it through their websites, e-newsletters, and blogs (Kuack, 2017). Research topic benefit was analyzed using Tukey’s honestly significant difference test with JMP (version 12.0.1; SAS Institute Inc., Cary, NC). Descriptive statistics were used to represent and compare all other data.
Results and Discussion
Producers and production area.
From the first advertisement (4 Apr. 2017) to the closing date (21 June 2017), we obtained 42 useful responses from 19 states. Fifty-three percent of the respondents produced hydroponic food crops solely, and an additional 30% produced food in-ground and hydroponically. Whereas 10% reported producing hydroponic food crops as well as floriculture crops, 5% were switching from floriculture to hydroponic food production. One firm reported growing hydroponically for breeding and research. Although more than half of respondents only used hydroponic production techniques, 45% grew hydroponically in addition to floriculture or in-ground food production. Increasing diversification potentially increases economic benefits including social capital formation, greater profitability, improved labor management, and improved economic resiliency (Boody et al., 2005; Mishra et al., 2004). Many respondents appear to be food-centric in production, which helps extension programming target specific, focused groups of producers in meetings and the trade press.
The distribution of area dedicated to hydroponic production for the responding firms was wide (Fig. 1). Only one firm reported a hydroponic production area of less than 46 m2, and 19% of the firms reported hydroponic area in production of 9290 m2 or more. Hydroponic area in production for the most frequently reported category was 93 to 464 m2, and the mean and median area were 2629 m2 and 697 m2, respectively. Firms produced crops in greenhouses (64%), indoors in multilayer (31%) or single-layer (7%) facilities, and hoop houses or high tunnels (29%; data not shown).
The area of production dedicated to hydroponics per producer as reported by respondents to a 2017 hydroponic grower survey in the United States (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The area of production dedicated to hydroponics per producer as reported by respondents to a 2017 hydroponic grower survey in the United States (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The area of production dedicated to hydroponics per producer as reported by respondents to a 2017 hydroponic grower survey in the United States (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
Indoor production, used by 38% of the growers surveyed, creates opportunity for increasing environmental control compared with traditional greenhouse production (used by 64% of respondents; data not shown). For example, greenhouses primarily rely on variable solar radiation for the majority or all of the photosynthetically active radiation used by plants, while light (photoperiod, spectrum, and quantity) provided to plants indoors is more precisely controlled. Additionally, temperature in a greenhouse is affected by solar radiation, nighttime long-wave radiation loss, and reduced insulation compared with indoor production, resulting in greater external environmental influences on temperature compared with indoor production. Finally, humidity (or vapor pressure deficit) can be readily increased in both indoor production and greenhouses. Reducing humidity in the two environments requires different strategies depending on outdoor humidity and temperature, energy costs, supplemental carbon dioxide (CO2) use, and the willingness to introduce outside pests to indoor facilities (Gomez et al., 2019). However, the greater ability to control the growing environment in indoor compared with greenhouse production results in higher capital investment and operating costs.
Crops and production systems.
Leaf lettuce (e.g., green leaf, red leaf) was produced by 58% of the respondents (Fig. 2). Head lettuce (e.g., boston/bibb/buttercrunch, and cos/romaine) was produced by nearly half of the firms, and fresh cut culinary herbs [e.g., basil (Ocimum spp.), cilantro (Coriandrum sativum), parsley (Petroselinum crispum), dill (Anethum graveolens), mint (Mentha spp.), rosemary (Rosmarinus officinalis), sage (Salvia officinalis), and thyme (Thymus vulgaris), etc.] were produced by 43% of respondents. Fourteen percent of firms produced microgreens, whereas other leafy greens [e.g., kale (Brassica oleracea), swiss chard (Beta vulgaris subsp. vulgaris), and spinach (Spinacia oleracea)] were grown by a third of the respondents. Five firms reported exclusive production of one of those categories of leafy greens (culinary herbs, head lettuce, leaf lettuce, or microgreens). Combined, leafy greens production accounted for 69% of the production based on the number of producers growing the crop weighted by the percentage of production (Fig. 2).
The proportion of crops grown based on number of producers growing the crop weighted by the percent value of that crop compared with total (left), and the percentage of producers growing each crop (right), based on respondents of a 2017 U.S. hydroponic grower survey (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The proportion of crops grown based on number of producers growing the crop weighted by the percent value of that crop compared with total (left), and the percentage of producers growing each crop (right), based on respondents of a 2017 U.S. hydroponic grower survey (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The proportion of crops grown based on number of producers growing the crop weighted by the percent value of that crop compared with total (left), and the percentage of producers growing each crop (right), based on respondents of a 2017 U.S. hydroponic grower survey (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
In 2014, the USDA Census of Horticultural Specialties only differentiated between the total weight of produce grown under protection and the weight of produce grown hydroponically. Data regarding the number of operations, area in production, and sales of hydroponically produced crops was not available. However, comparisons in trends can be made. Mirroring the 2014 USDA Census of Horticultural Specialties report stating more producers grow lettuce (763 producers) than fresh cut culinary herbs (524 producers) in CE, leaf and head lettuce were the most common hydroponically produced crops by survey respondents, followed by culinary herbs. However, the value of fresh cut culinary herbs produced in CEs in the United States ($70.9 million) was greater than lettuce ($55.5 million; USDA – National Agriculture Statistics Service, 2015; Table 1). The USDA reported tomato as the most commonly produced CE crop based on number of producers, weight, and value; however, only one-third of survey respondents reported producing tomatoes. Similarly, only 19% of the respondents produced cucumbers, whereas the USDA reported a similar number of operations producing cucumbers as lettuce (USDA, 2015; Table 1, Fig. 2). Fewer operations grew eggplant (Solanum melongena; 7%), fruit [e.g., strawberries, blueberries (Vaccinium spp.), and melon (Cucumis and Citrullus spp.), etc.; 17%], pepper (14%), and root crops [e.g., beets (Beta vulgaris), radishes (Raphanus sativus), and carrots (Daucus carota subsp. sativus), etc.; 10%]. Understanding crop diversity is critical for the improvement of extension materials and prioritization of research. Knowing that many firms do not have monocultures underscores the need to identify species-specific environment responses and cultural conditions to classify and group crops, thereby simplifying production and improving production efficiencies.
Nutrient-film technique (NFT) was the most frequently used hydroponic system (48% of respondents, 36% of production area), followed by dutch or bato bucket (33% of respondents, 18% of production area), then raft or deep-flow technique (DFT; 25% of respondents, 14% of production area; Fig. 3). The hydroponic production system used largely depends on the crop being produced. For example, dutch or bato bucket and slab or hanging gutter systems are more commonly used to produce strawberries and high-wire crops such as cucumbers, eggplant, peppers, and tomatoes, whereas NFT, raft or DFT, aeroponics, and ebb-and-flow systems are more commonly used for leafy greens including lettuce, culinary herbs, and microgreens (Jensen, 1997; Peet and Wells, 2005; Fig. 3).
The proportion of hydroponic production systems used by respondents of a 2017 U.S. hydroponic grower survey, based on space dedicated to each system (left), and the percentage of respondents using each type of production system based on frequency (right) (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The proportion of hydroponic production systems used by respondents of a 2017 U.S. hydroponic grower survey, based on space dedicated to each system (left), and the percentage of respondents using each type of production system based on frequency (right) (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The proportion of hydroponic production systems used by respondents of a 2017 U.S. hydroponic grower survey, based on space dedicated to each system (left), and the percentage of respondents using each type of production system based on frequency (right) (n = 42).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
Nutrient and water management.
Municipal water was used by 62% of the firms, whereas well water was used by 29% of respondents. Fewer firms used reverse-osmosis or deionized (14%), reclaimed (19%), or other water sources (19%), including plasma-activated, spring, fish hatchery, gray, rain, and surface or pond water (data not shown). For young plant production, water was applied as overhead irrigation (50%), daily single-event subirrigation (39%), constant subirrigation (29%), or other (18%) including drip, fog, hand (type not specified), and multiple-event-per-day subirrigation (data not shown). In recirculating systems, only 43% reported filtering the nutrient solution, using in-line filters (10%), bio filter, mesh filter, soil sock, ultraviolet, (5% each), or other methods (13%; Table 2).
The types of nutrient solution filters used by respondents of a 2017 U.S. hydroponic grower survey.
Water source is a key component of hydroponic culture. Because water used for plant production can vary widely in pH, alkalinity, electrical conductivity (EC), concentrations and ratios of specific nutrients, other chemical species, and the potential for pathogen contamination, understanding differences in water quality between sources is essential. For example, Argo et al. (1997) analyzed 4306 water samples from greenhouse producers across the United States and found the pH ranged from 2.7 to 11.3, EC varied from <0.01 to 9.8 dS·m‒1, and alkalinity from CaCO3 ranged from 0 to 1120 mg·L‒1. When comparing hydroponic producer responses to a 2013 survey of ornamental plant growers, a higher percentage of ornamental producers (55%) used well water, whereas municipal water was used by a lower proportion of producers (27%; Hodges et al., 2015). Even though municipalities charge for water consumption, the majority of producers in our survey used municipal water. This may be due to the quality standards municipal water is held to by the U.S. Environmental Protection Agency (2009), which sets standards for more than 90 contaminants. As a result, water from municipalities is often more consistent in quality than well or surface water because it is tested and is safe for both human consumption and food crops, including hydroponic, production (Shaw et al., 2015). Additionally, the tendency of CEA facilities to be located in or near urban areas where municipal water is readily available could contribute to this trend. Although only used by 14% of producers surveyed, reverse-osmosis or deionized water is one method of ensuring consistent water quality with low nutrient contamination; however, the cost of these systems may be prohibitive. Reclaimed water (used by 19% of respondents) may reduce the amount of municipal or groundwater needed to produce a crop, but like surface water, it may be susceptible to both nutrient and pathogen contamination (Hanning et al., 2009; Runia, 1993). Due to the wide variation in quality and nutrients already present in the water (Argo et al., 1997) and the potential for pathogen contamination (Hanning et al., 2009), understanding water source inputs gives growers, extension personnel, and researchers a helpful starting point for managing nutrients.
Nutrient solution management is integral to successful hydroponic production. While the EC is a measurement of a solution’s conductivity reflecting the total amount of fertilizer ions, pH of the nutrient solution and the relative proportion and concentration of these ions determines the availability and uptake of specific nutrients by the plant. Both EC and pH can be measured with commonly used sensors (as reported by 85% of respondents; Fig. 4) while more difficult to measure parameters, including specific nutrients and dissolved oxygen, were measured by only 54% and 28% of respondents, respectively. Additionally, 60% of respondents reported using sensors and/or automatic pumps to manage or monitor nutrient solution EC and temperature, whereas 57% and 21% used sensors and/or automatic pumps to manage and monitor pH and dissolved oxygen, respectively.
The environmental and cultural parameters monitored by hydroponic growers in the United States during propagation (n = 38) and finished production (n = 39) based on responses to a 2017 survey of hydroponic growers in the United States.
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The environmental and cultural parameters monitored by hydroponic growers in the United States during propagation (n = 38) and finished production (n = 39) based on responses to a 2017 survey of hydroponic growers in the United States.
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The environmental and cultural parameters monitored by hydroponic growers in the United States during propagation (n = 38) and finished production (n = 39) based on responses to a 2017 survey of hydroponic growers in the United States.
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
Researchers have reported different trends in plant growth and development, nutrient uptake, and quality in response to EC. For example, as EC increased from 1.4 to 3.0 dS·m‒1, lettuce fresh weight decreased when pH was adjusted daily, EC was not adjusted, and solutions were replaced every 2 weeks in a DFT system (Samarakoon et al., 2006). In contrast, as EC increased from 0 to 4.8 dS·m‒1, fresh weight of pakchoi (Brassica campestris L. ssp. chinensis) increased; however, plants were only irrigated three times per week, and the hydroponic production system type is unknown (Ding et al., 2018). Additionally, high-wire tomatoes grown with ECs ranging from 2.5 to 5.0 dS·m‒1 irrigated daily and NFT-grown culinary herbs with ECs from 0.5 to 4.0 dS·m‒1 and daily pH and EC adjustment had similar fresh weight among EC treatments (Currey et al., 2019; Walters and Currey, 2018; Wu and Kubota, 2008). The authors hypothesize that EC may not limit growth when adjusted continuously but may become a limiting factor when nutrient solutions are adjusted periodically in recirculating systems. A more consistent trend is observed between crops regarding nutrient uptake. For example, as EC increased, N concentration increased in pakchoi (Ding et al., 2018) and culinary herbs (Currey et al., 2019; Walters and Currey, 2018) and lycopene, fructose, glucose, and total soluble solids increased in tomato (Wu and Kubota, 2008).
Respondents who used recirculating systems varied in their practices for adjusting nutrient solution EC: 43% added specific nutrients; 38% added a complete, balanced prepackaged mix; 8% did not adjust EC; and 12% took other management steps including mixing their own fertilizer and adding water (data not shown). Growers most commonly replaced nutrient solutions completely every 1 to 3 months (28%), whereas 20% never completely replaced it; the remainder replaced the solution at differing intervals varying from every day to every 3 to 6 months (Fig. 5). The wide variation in methods to account for nutrient depletion and solution replacement aligns with the producers reporting research on nutrient solution formulations (mean = 3.1 ± 1.0) and nutrient solution pH (2.8 ± 1.0) being slightly to very beneficial (Fig. 6). Additionally, with the majority of producers surveyed monitoring and automatically adjusting nutrient concentrations, specific recommendations to aid growers in using automation to manage nutrients is needed. Research on crop-specific recommendations for specific mineral nutrient concentrations and ratios to maintain a balanced nutrient solution could improve crop quality, reduce excessive nutrient accumulation, reduce nutrient deficiencies, and conserve water and labor.
The frequency of nutrient solution replacement as reported by respondents of a 2017 U.S. hydroponic grower survey (n = 40).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The frequency of nutrient solution replacement as reported by respondents of a 2017 U.S. hydroponic grower survey (n = 40).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The frequency of nutrient solution replacement as reported by respondents of a 2017 U.S. hydroponic grower survey (n = 40).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The degree of benefit of research topics as reported by respondents to a 2017 survey of U.S. hydroponic growers. Means ± sd were calculated by assigning values to the responses, 1 = not at all beneficial, 2 = slightly beneficial, 3 = moderately beneficial, 4 = very beneficial. Research topics that share letters do not differ by Tukey’s honestly significant difference test at P ≤ 0.05 (n = 36).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The degree of benefit of research topics as reported by respondents to a 2017 survey of U.S. hydroponic growers. Means ± sd were calculated by assigning values to the responses, 1 = not at all beneficial, 2 = slightly beneficial, 3 = moderately beneficial, 4 = very beneficial. Research topics that share letters do not differ by Tukey’s honestly significant difference test at P ≤ 0.05 (n = 36).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The degree of benefit of research topics as reported by respondents to a 2017 survey of U.S. hydroponic growers. Means ± sd were calculated by assigning values to the responses, 1 = not at all beneficial, 2 = slightly beneficial, 3 = moderately beneficial, 4 = very beneficial. Research topics that share letters do not differ by Tukey’s honestly significant difference test at P ≤ 0.05 (n = 36).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
Growing environment: Monitoring and control.
The number of environmental parameters monitored during propagation was less than those monitored during finished production. For example, 32% and 45% of producers surveyed monitored light intensity and daily light integral (DLI) during propagation, respectively, while 56% and 49%, respectively, monitored these parameters during finished production (Fig. 4). The use of supplemental lighting was more common during propagation (54%) than during finished production (45%), while the use of sole-source lighting was similar between propagation and finished production (21% and 20%, respectively; Fig. 7).
The use of supplemental or sole-source lighting based on responses to a 2017 survey of hydroponic growers in the United States (n = 40).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The use of supplemental or sole-source lighting based on responses to a 2017 survey of hydroponic growers in the United States (n = 40).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
The use of supplemental or sole-source lighting based on responses to a 2017 survey of hydroponic growers in the United States (n = 40).
Citation: HortScience horts 55, 6; 10.21273/HORTSCI14901-20
Even though supplemental lighting was more common during propagation than during finished production (Fig. 7), light intensity and DLI were monitored by more producers during finished production (Fig. 4). Using supplemental lighting to augment low sunlight intensities during propagation can increase yields. For example, McCall (1992) stated that increasing the supplemental photosynthetic photon flux density (PPFD) provided to tomatoes from 30 to 90 µmol·m‒2·s‒1 during transplant production not only increased plant height, leaf number, leaf area, and dry mass, but also yields for the first 16 weeks of production. Walters and Lopez (2018) reported increasing the sole-source light intensity provided to basil seedlings during the first two weeks of growth from 100 to 600 µmol·m‒2·s‒1 PPFD resulted in an 80% increase in fresh cut yield after transplanting and finishing plants in a common greenhouse environment. During finished production, light intensity is a key driver of yield. For example, as DLI increased from 2 to 20 mol·m−2·d−1, basil, cilantro, dill, oregano (Origanum vulgare), thyme, parsley, mint, and sage fresh weight increased by 8.1 g (thyme) to 175.1 g (dill) (Litvin, 2019). Crop quality including postharvest life, appearance, and flavor is also influenced by light intensity. For example, cucumber postharvest shelf life was prolonged, and the fruit color was a more desirable deep green when light intensities during production were higher (Lin and Jolliffe, 1996), and basil favor compounds increased as the DLI increased from 5 to 25 mol·m−2·d−1 (Chang et al., 2008).
By monitoring light intensity and DLI, producers can decide whether ambient light intensities are adequate or if supplemental lighting is necessary to improve production. Producers reported research on light quality (mean = 3.2 ± 1.0) and DLI (mean = 3.1 ± 1.1) as being moderately to very beneficial (Fig. 6). Light quality is particularly important to the 20% of respondents who used sole-source lighting, as 100% of the light is provided by electrical lighting compared with supplemental lighting, where a larger proportion of light is provided by the sun (Poel and Runkle, 2017). Light quality can influence crop quality; in tomato, red light increases while far-red light decreases carotenoid accumulation (Alba et al., 2000). Additionally, increasing blue light increases the plant quality of a variety of crops, including increasing lycopene and β-carotene in tomato fruits (Gautier et al., 2004) and increasing the concentration of flavor compounds basil, dill, and parsley (Ichimura et al., 2009; Litvin et al., 2020).
Given choices of air, substrate, and nutrient solution temperature, respondents indicated that air temperature was the most commonly monitored environmental parameter; 74% of producers measured air temperature during propagation and 82% measured it during finished production (Fig. 4). The rate of plant development is primarily driven by temperature (Heins et al., 1998). Because temperature can be a readily controlled environmental factor with a large impact on growth, development, and plant quality, it is commonly manipulated by growers. Target temperatures for crop production depend on many factors including the cardinal temperatures for different crop species, interactions with other environmental parameters, target finishing or harvest dates, desired size and quality, crop production stage, cost of cooling and heating, the ability to control the environment, time of year, and growing location. Models quantifying air temperature effects on cucumber (Slack and Hand, 1983), culinary herbs (Chang et al., 2005; Currey et al., 2016; Walters and Currey, 2019), lettuce (Scaife, 1973; Seginer et al., 1991), pepper (Nilwik, 1981), and tomato (Adams et al., 2001) have been published. In general, researchers have found that as temperature increases above the base temperature of a crop, the growth rate increases linearly until a species-dependent optimum temperature, above which the growth rate decreases. Some researchers have also investigated the interaction of temperature, light intensity, and plant age determining the optimal temperature may also be dependent on those factors (Nilwik, 1981; Pearce et al., 1993). However, more robust models including more data points across a broader temperature range and the interaction of temperature and other environmental parameters including light intensity, photoperiod, and CO2 concentration, as well as cultural parameters including nutrient concentration and proportions, are needed. Additionally, temperature extremes can reduce fruit-set of flowering food crops and should be taken into consideration (Erickson and Markhart, 2002). Given commercial producers’ ability to monitor temperature and the availability of research models on which to base growing decisions, the producers surveyed identified research on temperature as the fourth-lowest research priority, being moderately beneficial (mean = 3.0 ± 0.8; Fig. 6). However, production recipes, including the interaction of temperature and other environmental parameters, are deemed the second-highest research priority (mean = 3.3 ± 1.1; Fig. 6).
Thirty-six percent of producers surveyed monitored CO2 concentration during finished production, whereas 32% monitored CO2 during propagation. One-third of respondents injected CO2 to reach target concentrations ranging from 350 to more than 1500 ppm (Table 3). During propagation and finished production, the same number of producers reported monitoring relative humidity or vapor pressure deficit (46% to 47%; Fig. 4).
The target carbon dioxide (CO2) concentration as reported by respondents to a 2017 survey of hydroponic growers in the United States.
Increasing CO2 concentration to species-specific saturation points increases yield of cucumber (Wittwer and Robb, 1964), lettuce (Knecht and O’Leary, 1983; Wittwer and Robb, 1964), pepper (Fierro et al., 1994), strawberry (Wang and Bunce, 2004), and tomato (Morgan, 1971; Wittwer and Robb, 1964). However, only 36% of growers monitored CO2 during finished production. CO2 management was deemed the lowest research benefit as growers ranked it as slightly to moderately beneficial (mean = 2.7 ± 1.1; Figs. 4 and 6).
Crop quality.
When asked if their customers would pay more for crops with increased flavor, 90% of respondents responded affirmatively, whereas 10% reported only their wholesale customers would pay more if the crop had improved nutrition or color (data not shown). Managing the growing environment to improve crop flavor was cited as the most beneficial research area (mean = 3.4 ± 0.7; Fig. 6). Many environmental and cultural factors can influence the flavor of crops including light intensity (Chang et al., 2008), light quality (Alba et al., 2000; Gautier et al., 2004; Litvin et al., 2020; Weisshaar and Jenkins, 1998), air temperature (Chang et al., 2005), CO2 concentration (Wang and Bunce, 2004), and nutrition (Benard et al., 2009). However, increases in secondary metabolites do not necessarily result in improved flavor. For example, in a sensory panel, consumers deemed the flavor of basil grown at 23 °C under 400 or 600 µmol·m−2·s–1 PPFD too intense, whereas plants grown under 100 µmol·m−2·s–1 PPFD were not flavorful enough; however, plants grown under 200 µmol·m−2·s–1 PPFD had preferred characteristics (Walters et al., 2019). Similarly, increasing secondary metabolite production in brassicas can lead to increased health-promoting glucosinolates, but with the side effect of a bitter taste (Bell et al., 2018). With producers indicating their consumers would pay more for increased flavor, continued research to determine which type and intensity of flavor is preferred and by which consumer segments is essential to improving crop quality to increase prices.
Research priorities.
Given the range of production systems, cultural practices, and environmental conditions affecting hydroponic food crop production, coupled with the lack of science-based production recommendations, additional research is warranted. However, research priorities should reflect the needs of the commercial industry. The most important research priorities, on a scale of 1 to 4, as reported by growers were manipulating the growing environment to improve crop flavor (mean = 3.4 ± 0.7), production recipes (e.g., lighting, CO2, temperature, nutrients) (mean = 3.3 ± 1.1), light quality [e.g., supplying different wavelengths of light using light-emitting diodes (LEDs)] (mean = 3.2 ± 1.0), food safety guidelines (mean = 3.2 ± 0.9), postharvest recommendations (3.1 ± 0.9), and energy-use and resource-use management (mean = 3.1 ± 1.0). The topic with the lowest mean score was CO2 management (mean = 2.3 ± 1.1; Fig. 6). These priorities, based on their perceived benefit to growers, should be taken into consideration when determining research priorities for hydroponic food crop production.
Future Perspectives and Conclusions
Although CE hydroponic production has been used to grow food crops for many years, the recent increase in CE food production is creating need for additional research to increase production efficiencies and profitability, improve yields and produce quality, and address production challenges. Through this producer survey, we are establishing baseline data on the variability in production type, technology adoption, and research needs of hydroponic food crop producers. Educators, extension specialists, and researchers can use these data to better understand the current state of the U.S. hydroponics industry, identify gaps in both knowledge and technology adoption, provide extension resources or research to aid in filling those gaps, and educate future industry members on current practices.
On the basis of the survey results and the authors’ professional opinion, research should not only emphasize increasing productivity but should also examine crop quality, especially how to improve crop flavor, sensory attributes, and postharvest longevity. This will increase potential for CE producers to grow and market a premium crop, creating opportunities for increasing profitability (Bi et al., 2012). Additionally, much recent research has focused on lighting technology, including LEDs and the effects of light spectrum (Mitchell et al., 2015). However, growers rated creation of production recipes as equally important. Although more difficult to research than light spectra in isolation, research on how environmental parameters and cultural practices interact will likely lead to more effective production strategies, taking both yield and plant quality into account. However, as interactions become more complex, data interpretation, analysis, and implementation will as well (Boote et al., 1996). Some hydroponic production models exist for CE produced lettuce such as the NiCoLet model predicting nitrate concentrations and growth (Seginer et al., 1998), a modified SUCROS87 model (Spitters et al., 1989) to simulate the effects of DLI, ADT, and plant density on lettuce (Both, 1995), and an evapotranspiration prediction model based on CO2 and DLI (Ciolkosz et al., 1998). Additionally, many researchers have focused on modeling to determine biomass production without considering crop quality (Marcelis et al., 1998), an aspect of production that is increasingly important (Sadílek, 2019). Finally, although hydroponic nutrient solution research has been conducted for years, it has become clear that, as Hoagland and Arnon (1950) stated: “There is no one composition of nutrient solution which is always superior to every other composition.” Increased efficiencies can be realized in nutrient management by investigating the interaction of specific nutrient concentrations and the proportions relative to each other to determine what is necessary for both optimal growth and quality of the range of species grown commercially (Ahn, 2019).
Taken together, multiparameter research working toward optimizing environmental (light quality, quantity, temperature, etc.) and cultural (nutrient solution, cropping duration, etc.) parameters is integral to improving CE hydroponic production. The “optimization” of these parameters is dependent on production goals, which have traditionally focused on improving yield; however, as apparent from this research, many producers are now also focusing on improving crop quality. Therefore, the “optimization” of growing parameters should take both productivity and quality into account.
Literature Cited
Adams, S.R., Cockshull, K.E. & Cave, C.R.J. 2001 Effect of temperature on the growth and development of tomato fruits Ann. Bot. 88 869 877
Agrilyst 2017 State of indoor farming. 3 Mar. 2019. <https://www.agrilyst.com/stateofindoorfarming2017/>
Ahn, T. 2019 Establishment of theoretical models for the interpretation of nutrient variations in electrical conductivity-based closed-loop soilless culture systems. Soul Natl. Univ., Soul, South Korea, PhD Diss
Alba, R., Cordonnier-Pratt, N.M. & Pratt, L.H. 2000 Fruit-localized phytochromes regulate lycopene accumulation independently of ethylene production in tomato Plant Physiol. 123 363 370
Argo, W.R., Biernbaum, J.A. & Warncke, D.D. 1997 Geographical characterization of greenhouse irrigation water HortTechnology 7 49 55
Bell, L., Oloyede, O.O., Lignou, S., Wagstaff, C. & Methven, L. 2018 Taste and flavor perceptions of glucosinolates, isothiocyanates, and related compounds Mol. Nutr. Food Res. 62 18 1700990
Benard, C., Gautier, H., Bourgaud, F., Grasselly, D., Navez, B., Caris-Veyrat, C., Weiss, M. & Genard, M. 2009 Effects of low nitrogen supply on tomato (Solanum lycopersicum) fruit yield and quality with special emphasis on sugars, acids, ascorbate, carotenoids, and phenolic compounds J. Agr. Food Chem. 57 4112 4123
Bi, X., House, L., Gao, Z. & Gmitter, F. 2012 Sensory evaluation and experimental auctions: Measuring willingness to pay for specific sensory attributes Amer. J. Agr. Econ. 94 562 568
Boody, G., Vondracek, B., Andow, D.A., Krinke, M., Westra, J., Zimmerman, J. & Welle, P. 2005 Multifunctional agriculture in the United States Bioscience 55 27 38
Boote, K.J., Jones, J.W. & Pickering, N.B. 1996 Potential uses and limitations of crop models Agron. J. 88 704 716
Both, A.J. 1995 Dynamic simulation of supplemental lighting for greenhouse hydroponic lettuce production Cornell Univ., Ithica, PhD Diss.
Chang, X., Alderson, P. & Wright, C. 2005 Effect of temperature integration on the growth and volatile oil content of basil (Ocimum basilicum L.) J. Hort. Sci. Biotechnol. 80 593 598
Chang, X., Alderson, P.G. & Wright, C.J. 2008 Solar irradiance level alters the growth of basil (Ocimum basilicum L.) and its content of volatile oils Environ. Expt. Bot. 63 216 223
Ciolkosz, D.E., Albright, L.D. & Both, A.J. 1998 Characterizing evapotranspiration in a greenhouse lettuce crop Acta Hort. 456 255 262
Currey, C.J., Smith, J.R. & Walters, K.J. 2016 Air temperature affects the growth and development of cilantro, dill, and parsley HortScience 51 S219 (abstr.)
Currey, C.J., Walters, K.J. & Flax, N.J. 2019 Nutrient solution strength does not interact with the daily light integral to affect hydroponic cilantro, dill, and parsley growth and tissue mineral nutrient concentrations Agronomy 9 389 403
Dalrymple, D.G. 1973 Controlled environment agriculture: A global review of greenhouse food production. U.S. Dept. Agr. Econ. Res. Serv. No. 145622. 13 Mar. 2019. <https://ageconsearch.umn.edu/record/145622/files/faer89.pdf>
Ding, X., Jiang, Y., Zhao, H., Guo, D., He, L., Liu, F., Zhou, Q., Nandwani, D., Hui, D. & Yu, J. 2018 Electrical conductivity of nutrient solution influenced photosynthesis, quality, and antioxidant enzyme activity of pakchoi (Brassica campestris L. ssp. Chinensis) in a hydroponic system PLoS One 13 8 e0202090
Erickson, A.N. & Markhart, A.H. 2002 Flower developmental stage and organ sensitivity of bell pepper (Capsicum annuum L.) to elevated temperature Plant Cell Environ. 25 123 130
Environmental Protection Agency 2009 National primary drinking water regulations. 1 July 2019. <https://www.epa.gov/sites/production/files/2016-06/documents/npwdr_complete_table.pdf>
Fierro, A., Gosselin, A. & Tremblay, N. 1994 Supplemental carbon dioxide and light improved tomato and pepper seedling growth and yield HortScience 29 152 154
Galloway, B.T. 1900 Progress of commercial growing of plants under glass. USDA. 13 Mar. 2019. <https://naldc.nal.usda.gov/download/IND43620781/PDF>
Gautier, H., Rocci, A., Buret, M., Grasselly, D., Dumas, Y. & Causse, M. 2004 Effect of photoselective filters on the physical and chemical traits of vine-ripened tomato fruits Can. J. Plant Sci. 85 439 446
Gericke, W.F. 1933 Fertilizing unit for growing plants in water. U.S. Patent 1,915,884
Gomez, C., Currey, C.J., Dickson, R.W., Kim, H., Hernandez, R., Sabeh, N.C., Raudales, R.E., Brumfield, R.G., Laury-Shaw, A., Wilke, A.K., Lopez, R.G. & Burnett, S.E. 2019 Controlled environment food production for urban agriculture HortScience 54 1448 1458
Gordon, R. 2016 State of the vegetable industry survey shows growers optimistic about 2016. Amer. Veg. Grower. 13 Mar. 2019. <https://www.growingproduce.com/vegetables/state-of-the-vegetable-industry-survey-shows-growers-optimistic-about-2016/>
Hanning, I.B., Nutt, J.D. & Ricke, S.C. 2009 Salmonellosis outbreaks in the United States due to fresh produce: Sources and potential intervention measures Foodborne Pathog. Dis. 6 635 648
Heins, R.D., Liu, B. & Runkle, E.S. 1998 Regulation of crop growth and development based on environmental factors Acta Hort. 513 17 28
Hoagland, D.R. & Arnon, D.I. 1950 The water-culture method for growing plants without soil. Circular. 2nd ed. California Agr. Expt. Sta. 347
Hodges, A.W., Khachatryan, H., Palma, M.A. & Hall, C.R. 2015 Production and marketing practices and trade flows in the United States green industry in 2013 J. Environ. Hort. 33 125 136
Ichimura, M., Watanabe, H., Amaki, W. & Yamazaki, N. 2009 Effects of light quality on the growth and essential oil content in sweet basil Acta Hort. 907 91 94
Jensen, M.H. 1997 Hydroponics worldwide Acta Hort. 481 719 730
Jensen, M.H. & Collins, W.L. 1985 Hydroponic vegetable production Hort. Rev. 7 483 558
Knecht, G.N. & O’Leary, J.W. 1983 The influence of carbon dioxide on the growth, pigment, protein, carbohydrate, and mineral status of lettuce J. Plant Nutr. 6 301 312
Kuack, D. 2017 Industry survey seeks information on hydroponic food production. HortAmericas blog. 13 Mar. 2019. <https://hortamericas.com/blog/news/industry-survey-seeks-information-on-hydroponic-food-production/>
Lin, W.C. & Jolliffe, P.A. 1996 Light intensity and spectral quality affect fruit growth and shelf life of greenhouse-grown long English cucumber J. Amer. Soc. Hort. Sci. 121 1168 1173
Litvin, A.G. 2019 Quantifying the effects of light quantity and quality on culinary herb physiology. Iowa State Univ. Ames, PhD Diss
Litvin, A.G., Currey, C.J. & Wilson, L.A. 2020 Effects of supplemental light source on basil, dill, and parsley growth, morphology, aroma, and flavor J. Amer. Soc. Hort. Sci. 145 18 29
Love, D.C., Fry, J.P., Genello, L., Hill, E.S., Frederick, J.A., Li, X. & Semmens, K. 2014 An international survey of aquaponics practitioners PLoS One 9 7 e102662
Love, D.C., Fry, J.P., Li, X., Hill, E.S., Genello, L., Semmens, K. & Thompson, R.E. 2015 Commercial aquaponics production and profitability: Findings from an international survey Aquaculture 435 67 74
Marcelis, L.F.M., Heuvelink, E. & Goudriaan, J. 1998 Modelling biomass production and yield of horticultural crops: A review Scientia Hort. 74 83 111
McCall, D. 1992 Effect of supplementary light on tomato transplant growth, and the after-effects on yield Scientia Hort. 51 65 70
Mishra, A.K., El-Osta, H.S. & Sandretto, C.L. 2004 Factors affecting farm enterprise diversification Agr. Financ. Rev. 64 151 166
Mitchell, C.A., Dzakovich, M.P., Gomez, C., Lopez, R., Burr, J.F., Hernández, R., Kubota, C., Currey, C.J., Meng, Q., Runkle, E.S. & Bourget, C.M. 2015 Light-emitting diodes in horticulture Hort. Rev. 43 1 87
Morgan, J.V. 1971 The influence of supplementary illumination and CO2 enrichment on the growth, flowering and fruiting of the tomato Acta Hort. 22 187 199
Nilwik, H.J.M. 1981 Growth analysis of sweet pepper (Capsicum annuum L.) 2. Interacting effects of irradiance, temperature and plant age in controlled conditions Ann. Bot. 48 137 146
Pearce, B.D., Grange, R.I. & Hardwick, K. 1993 The growth of young tomato fruit. I. Effects of temperature and irradiance on fruit grown in controlled environments J. Hort. Sci. 68 1 11
Peet, M.M. & Welles, G.W.H. 2005 Greenhouse tomato production, p. 277–278. In: E. Heuvelink (ed.). Tomatoes. CABI Publishing, Wallingford, UK
Poel, B.R. & Runkle, E.S. 2017 Spectral effects of supplemental greenhouse radiation on growth and flowering of annual bedding plants and vegetable transplants HortScience 52 1221 1228
Resh, H.M. 2013 Hydroponic food production: A definitive guidebook for the advanced home gardener and the commercial hydroponic grower. 7th ed. CRC Press Taylor and Francis Group, Boca Raton, FL
Runia, W.T. 1993 A review of possibilities for disinfection of recirculation water from soilless cultures Acta Hort. 382 221 229
Sadílek, T. 2019 Perception of food quality by consumers: Literature review Eur. Res. Stud. 22 1 758 767
Samarakoon, U.C., Weerasinghe, P.A. & Weerakkody, W.A.P. 2006 Effect of electrical conductivity (EC) of the nutrient solution on nutrient uptake, growth and yield of leaf lettuce (Lactuca sativa L.) in stationary culture Trop. Agr. Res. 18 13 21
Scaife, M.A. 1973 The early relative growth rates of six lettuce cultivars as affected by temperature Ann. Appl. Biol. 74 119 128
Seginer, I., Shina, G., Albright, L.D. & Marsh, L.S. 1991 Optimal temperature setpoints for greenhouse lettuce J. Agr. Eng. Res. 49 209 226
Seginer, I., Straten, G.V. & Buwalda, F. 1998 Nitrate concentration in greenhouse lettuce: A modeling study Acta Hort. 456 189 198
Shaw, A., Currey, C.J. & Evans, M.R. 2015 Focus on soilless and hydroponic systems Greenhouse Grower 33 5 758 767
Shive, J.W. & Robbins, W.R. 1937 Methods of growing plants in solution and sand cultures. New Jersey Agr. Expt. Sta. Bul. 636
Slack, G. & Hand, D.W. 1983 The effect of day and night temperatures on the growth, development and yield of glasshouse cucumbers J. Hort. Sci. 58 567 573
Spitters, C.J.T., Van Keulen, H. & Van Kraalingen, D.W.G. 1989 A simple and universal crop growth simulator: SUCROS87, p. 147–181. In: R. Rabbinge, S.A. Ward, and H.H. van Laar (eds.). Simulation and systems management in crop protection. Ctr. Agricultural Publishing Documentation, Wageningen, Netherlands
Villarroel, M., Junge, R., Komives, T., König, B., Plaza, I., Bittsánszky, A. & Joly, A. 2016 Survey of aquaponics in Europe Water 8 468
Walters, K.J. & Currey, C.J. 2018 Effects of nutrient solution concentration and daily light integral on growth and nutrient concentration of several basil species in hydroponic production HortScience 53 1319 1325
Walters, K.J. & Lopez, R.G. 2018 Quantifying the influence of light intensity and CO2 concentration during sweet basil seedling production on subsequent growth, development, and volatile content HortScience 53 S89 (abstr.)
Walters, K.J. & Currey, C.J. 2019 Growth and development of basil species in response to temperature HortScience 54 1915 1920
Walters, K.J., Behe, B.K. & Lopez, R.G. 2019 Consumer sensory preferences in response to manipulating fresh basil flavor through controlled environment light, temperature, and carbon dioxide management HortScience 54 S90 (abstr.)
Wang, S.Y. & Bunce, J.A. 2004 Elevated carbon dioxide affects fruit flavor in field-grown strawberries (Fragaria × ananassa Duch) J. Sci. Food Agr. 84 1464 1468
Weisshaar, B. & Jenkins, G.I. 1998 Phenylpropanoid biosynthesis and its regulation Curr. Opin. Plant Biol. 1 251 257
Withrow, R.B. & Biebel, J.P. 1937 Nutrient solution methods of greenhouse crop production. Purdue Univ. Agr. Expt. Station. Circ. 232
Wittwer, S.H. & Robb, W.M. 1964 Carbon dioxide enrichment of greenhouse atmospheres for food crop production Econ. Bot. 18 34 56
Wu, M. & Kubota, C. 2008 Effects of high electrical conductivity of nutrient solution and its application timing on lycopene, chlorophyll and sugar concentrations of hydroponic tomatoes during ripening Scientia Hort. 116 2 758 767
U.S. Department of Commerce (USDC) – Bureau of the Census. 1930 Fifteenth census of the United States, horticulture, statistics for the United States and for states, 1929 and 1930. U.S. Govt. Printing Office. 13 Mar. 2019. <https://archive.org/stream/fifteenthcensuso1930unse_1#mode/2up>
U.S. Department of Commerce (USDC) – Bureau of the Census. 1952 1949 U.S. Census of Horticultural Specialties. U.S. Govt. Printing Office. 13 Mar. 2019. <http://usda.mannlib.cornell.edu/usda/AgCensusImages/1950/05/01/1815/34101814v5p1ch1.pdf>
U.S. Department of Commerce (USDC) – Bureau of the Census. 1962 1959 U.S. Census of Horticultural Specialties. 13 Mar. 2019. <http://usda.mannlib.cornell.edu/usda/AgCensusImages/1959/05/01/1959-05-01.pdf>
U.S. Department of Commerce (USDC) – Social and Economics Statistics Administration. 1973 1970 U.S. Census of Horticultural Specialties. 13 Mar. 2019. <http://usda.mannlib.cornell.edu/usda/AgCensusImages/1969/05/10/1969-05-10.pdf>
U.S. Department of Commerce (USDC) – Bureau of the Census. 1982 1979 Census of Horticultural Specialties. 13 Mar. 2019. <http://usda.mannlib.cornell.edu/usda/AgCensusImages/1978/05/07/1978-05-07.pdf>
U.S. Department of Commerce (USDC) – Economics and Statistics Administration. 1991 1988 Census of Horticultural Specialties. 13 Mar. 2019. <http://usda.mannlib.cornell.edu/usda/AgCensusImages/1987/03/03/1987-03-03.pdf>
U.S. Department of Agriculture (USDA) – National Agriculture Statistics Service. 2000 1998 Census of Horticultural Specialties. 13 Mar. 2019. <http://usda.mannlib.cornell.edu/usda/AgCensusImages/1997/03/04/1997-03-04.pdf>
U.S. Department of Agriculture (USDA) – National Agriculture Statistics Service. 2010 2009 U.S. Census of Horticultural Specialties. 13 Mar. 2019. <https://www.nass.usda.gov/Publications/AgCensus/2007/Online_Highlights/Census_of_Horticulture_Specialties/HORTIC.pdf>
U.S. Department of Agriculture (USDA) – National Agriculture Statistics Service. 2015 2014 U.S. Census of Horticulture Specialties. 13 Mar. 2019. <https://www.nass.usda.gov/Publications/AgCensus/2012/Online_Resources/Census_of_Horticulture_Specialties/>
U.S. Department of Labor (USDL) – Bureau of Labor Statistics. 2019 CPI Inflation Calculator. 14 Dec. 2019. <https://www.bls.gov/data/inflation_calculator_inside.htm>
