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Amanda Bayer, John Ruter and Marc W. van Iersel

Excessive irrigation and leaching are of increasing concern in container plant production. It can also necessitate multiple fertilizer applications, which is costly for growers. Our objective was to determine whether fertilizer and irrigation water can be applied more efficiently to reduce leachate volume and nutrient content without negatively impacting aboveground growth of Gardenia jasminoides ‘MAGDA I’. Plants were fertilized with one of three rates of a controlled-release fertilizer (subplots) (Florikan 18–6–8, 9–10 month release; 18.0N–2.6P–6.6K) [100 (40 g/plant), 50 (20 g/plant), and 25% of bag rate (10 g/plant)] and grown in 5.4-L containers outside for 137 days. Soil moisture sensor-controlled, automated irrigation was used to provide plants with one of four irrigation volumes (whole plots) (66, 100, 132, or 165 mL) at each irrigation event. All plants were irrigated when the control treatment (66 mL irrigation volume, 100% fertilizer treatment) reached a volumetric water content (VWC) of 0.35 m3·m−3. Plants in the different irrigation treatments were irrigated for 2, 3, 4, or 5 minutes, thus applying 66, 100, 132, or 165 mL/plant in the different irrigation treatments. Fertilizer rate had a greater effect on aboveground growth than irrigation volume with the 25% fertilizer rate resulting in significantly lower shoot dry weight (18.7 g/plant) than the 50% and 100% rates (25.3 and 27.3 g/plant respectively). Growth index was also lowest in the 25% fertilizer rate. Leachate volume varied greatly during the growing season due to rainfall and irrigation volume effects on leachate were most evident during the third, eighth, and ninth biweekly leachate collections, during which there was minimal or no rainfall. For these collections the control treatment of 66 mL resulted in minimal leachate (less than 130 mL over the 2-week leachate collection period), whereas leachate volume increased with increasing irrigation volumes. Pore water electrical conductivity (EC), leachate EC, NO3-N content, and PO4-P content were all highest with the 100% fertilizer rate, with the 66 mL irrigation treatment having the highest leachate EC for all fertilizer treatments. Cumulative leachate volumes for the 66 and 100 mL irrigation treatments were unaffected by fertilizer rate, whereas the 132 and 165 mL had greater leaching at the 25% fertilizer rate. Lower irrigation volumes resulted in reduced water and nutrient leaching and higher leachate EC. The higher leachate EC was the result of higher concentration of nutrients in less volume of leachate. The results of this study suggest that a combination of reduced fertilizer rates (up to 50%) and more efficient irrigation can be used to produce salable plants with reduced leaching and thus less environmental impact.

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Peter Alem, Paul A. Thomas and Marc W. van Iersel

Production of poinsettias (Euphorbia pulcherrima) often involves intensive use of plant growth retardants (PGRs) to regulate height. Height control is necessary for visual appeal and postharvest handling. Since PGRs do not always provide consistent height control and can have unwanted side effects, there is interest in alternative methods of height control. Since turgor potential drives cell expansion, and thus stem elongation, drought stress has potential for regulating plant height. Through soil moisture sensor-controlled irrigation, the severity of drought stress can be both monitored and controlled. The objective of our study was to compare poinsettia ‘Classic Red’ height control using PGRs (spray, mixture of daminozide and chlormequat at 1000 mg·L−1 each and drench, 0.25 mg·L−1 paclobutrazol) with the use of controlled water deficit (WD). Graphical tracking of plant height, using a final target height of 43.5 cm, was used to determine when to apply PGR or controlled WD. In the WD treatment, substrate volumetric water content (θ) was reduced from 0.40 to 0.20 m3·m−3 when actual height exceeded the expected height. PGR applications (spray or drench) reduced poinsettia height to 39 cm, below the final target level of 43.5 cm. WD resulted in a height of 44.5 cm, closest to the target height, while control plants were taller (49.4 cm). There was no effect of PGR drenches or WD on bract size, while spraying PGR reduced bract size by ≈ 40%. Bract chroma was unaffected by WD or PGR treatments. There was no difference in shoot dry weight between PGR- and WD-treated plants. Lateral growth was reduced by the PGR treatments, but not by WD. These results indicate that controlled WD can be used to regulate poinsettia height.

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Peter Alem, Paul A. Thomas and Marc W. van Iersel

Rising concerns over environmental impacts of excessive water and fertilizer use in the horticultural industry necessitate more efficient use of water and nutrients. Both substrate volumetric water content (θ) and fertilizer affect plant growth, but their interactive effect is poorly understood. The objective of this study was to determine the optimal fertilizer rates for petunia (Petunia ×hybrida) ‘Dreams White’ grown at different θ levels. Petunia seedlings were grown at four levels of θ (0.10, 0.20, 0.30, and 0.40 m3·m−3) with eight different rates of controlled-release fertilizer (CRF) (Osmocote 14-14-14; 14N–6.1P–11.6K; rates of 0 to 2.5 g/plant, equivalent to 0 to 6.25 kg·m−3 substrate). Shoot dry weight increased as the CRF rate increased from 0 to 1.67 g/plant but decreased again at even higher CRF rates. The effect of CRF rate on growth was more pronounced at higher θ. Leaf size doubled as the θ thresholds increased from 0.10 to 0.40 m3·m−3. Flowering was reduced by a combination of high CRF rates (greater than 0.63 g/plant) and high θ (0.30 and 0.40 m3·m−3), indicating that optimal conditions for vegetative growth are different from those for maximal flowering. These results suggest that without leaching, high-quality petunias can be grown with lower CRF rates than commercially recommended rates.

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Jongyun Kim, Marc W. van Iersel and Stephanie E. Burnett

Many ornamental plant growers water excessively to reduce the risk of drought stress. Scheduling irrigation in greenhouses is challenging because there is little quantitative information about ornamental plant water requirements and how water use changes when plants are grown in varying greenhouse environmental conditions. Models to estimate the daily water use (DWU) of greenhouse crops may provide a useful tool to conserve irrigation water. Our objective was to develop a model to predict DWU based on plant age and easily acquirable environmental data. Two petunia (Petunia ×hybrida) cultivars, Single Dreams Pink and Prostrate Easy Wave Pink, were grown in different sized containers (diameter = 10, 12.5, and 15 cm) to quantify their DWU for 6 weeks. The substrate water content (θ, v/v) was maintained at 0.40 m3·m−3 using an automated irrigation system with capacitance soil moisture sensors. Every irrigation event was recorded by a data logger, and this information was used to calculate the DWU of the plants. On overcast days early in the experiment, plants used only 4.8 to 13.8 mL·d−1. The maximum DWU of ‘Single Dreams Pink’ was 63, 96, and 109 mL·d−1 in 10-, 12.5-, and 15-cm containers, respectively. Late in the experiment, ‘Prostrate Easy Wave Pink’ petunia used more water than ‘Single Dreams Pink’ because of their more vigorous growth habit. DWU was modeled as a function of days after planting (DAP), daily light integral (DLI), vapor pressure deficit (VPD), temperature, container size, and interactions between these factors and DAP (R 2 = 0.93 and 0.91 for ‘Single Dreams Pink’ and ‘Prostrate Easy Wave Pink’, respectively). Days after planting and container size were the most important factors affecting DWU and are indicative of plant size. Daily light integral was the most important environmental factor affecting DWU. These models, describing the DWU as a function of the DAP and environmental conditions, may be used as guidelines for accurately watering petunias in greenhouses and may improve irrigation scheduling.

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Lucas O’Meara, Matthew R. Chappell and Marc W. van Iersel

As a result of the lack of quantitative data regarding specific water requirements of ornamental species, precision irrigation can be a difficult task for nursery growers. One challenge for growers is that it is not clear how much of the water in soilless substrates is actually available for plant uptake. Substrate moisture release curves (MRC) have been used to predict the amount of plant-available water in soilless substrates, yet there is little information about whether there are differences among species in their ability to extract water from substrates. The objectives of this study were to determine 1) the hydraulic properties of a composted pine bark substrate; and 2) how water uptake in Hydrangea macrophylla and Gardenia jasminoides was affected by decreasing substrate volumetric water content (VWC). As the substrate VWC decreased from 0.38 to 0.17 m3·m−3, substrate matric potential decreased from –4.0 to –69 kPa, whereas hydraulic conductivity decreased from 0.115 to 0.000069 cm·d−1. To measure plant water uptake in a drying substrate, growth chambers were used to provide stable environmental conditions that included continuous lighting to prevent diurnal fluctuations in water use. Water use by H. macrophylla ‘Fasan’ started to decrease at a higher VWC (0.28 m3·m−3) than G. jasminoides ‘Radicans’ (0.20 m3·m−3). Plant water uptake stopped at a VWC of 0.16 m3·m−3 in H. macrophylla and 0.12 m3·m−3 in G. jasminoides. The results show that H. macrophylla is less adept at extracting water from a drying substrate than G. jasminoides. Traditionally, plant-available water in soilless substrates has been studied using substrate MRCs. Our data suggest that substrate hydraulic conductivity may be an important factor controlling water availability to the plants. In addition, there are important differences among species that cannot be detected by only looking at substrate hydraulic properties.

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Matthew Chappell, Sue K. Dove, Marc W. van Iersel, Paul A. Thomas and John Ruter

Water quality and quantity are increasingly important concerns for agricultural producers and have been recognized by governmental and nongovernmental agencies as focus areas for future regulatory efforts. In horticultural systems, and especially container production of ornamentals, irrigation management is challenging. This is primarily due to the limited volume of water available to container-grown plants after an irrigation event and the resultant need to frequently irrigate to maintain adequate soil moisture levels without causing excessive leaching. To prevent moisture stress, irrigation of container plants is often excessive, resulting in leaching and runoff of water and nutrients applied to the container substrate. For this reason, improving the application efficiency of irrigation is necessary and critical to the long-term sustainability of the commercial nursery industry. The use of soil moisture sensing technology is one method of increasing irrigation efficiency, with the on-farm studies described in this article focusing on the use of capacitance-based soil moisture sensors to both monitor and control irrigation events. Since on-farm testing of these wireless sensor networks (WSNs) to monitor and control irrigation scheduling began in 2010, WSNs have been deployed in a diverse assortment of commercial horticulture operations. In deploying these WSNs, a variety of challenges and successes have been observed. Overcoming specific challenges has fostered improved software and hardware development as well as improved grower confidence in WSNs. Additionally, growers are using WSNs in a variety of ways to fit specific needs, resulting in multiple commercial applications. Some growers use WSNs as fully functional irrigation controllers. Other growers use components of WSNs, specifically the web-based graphical user interface (GUI), to monitor grower-controlled irrigation schedules.

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Stephanie E. Burnett, Svoboda V. Pennisi, Paul A. Thomas and Marc W. van Iersel

Polyethylene glycol 8000 (PEG-8000) was applied to a soilless growing medium at the concentrations of 0, 15, 20, 30, 42, or 50 g·L-1 to impose controlled drought. Salvia (Salvia splendens F. Sellow. ex Roem & Shult.) seeds were planted in the growing medium to determine if controlled drought affects morphology and anatomy of salvia. Polyethylene glycol decreased emergence percentage and delayed emergence up to 5 days. Stem elongation of salvia treated with the five lowest concentrations was reduced up to 35% (21 days after seeding), and salvia were a maximum of 53% shorter and the canopy was 20% more narrow compared to nontreated seedlings 70 days after seeding. These morphological changes were attributed to PEG-8000 mediated reduction in leaf water potential (Ψw). The growing medium Ψw ranged from -0.29 to -0.85 MPa in PEG-8000 treated plants, and plant height was positively correlated with Ψw 21 days after seeding. Stem diameter of PEG-treated seedlings was reduced up to 0.4 mm mainly due to reductions in vascular cross-sectional area. Xylem cross-sectional area decreased more than stem and phloem cross-sectional area. Polyethylene glycol 8000 reduced vessel element number, but not diameter.

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Katherine F. Garland, Stephanie E. Burnett, Michael E. Day and Marc W. van Iersel

Two investigations were conducted to determine the morphological and physiological impacts of varying light and substrate water levels on Heuchera americana ‘Dale's Strain’ (american alumroot). Both investigations used a capacitance sensor automated irrigation system to maintain constant substrate volumetric water contents (θ = volume of water/volume of substrate). In the first study, the substrate was maintained at one of eight θ ranging from 0.15 to 0.50 L·L−1. Leaf area of plants grown at the highest θ was more than twice that of plants grown at the lowest θ. Shoot dry weight also responded positively to θ increasing from 0.15 to 0.35 L·L−1, but plants did not have greater dry weights when maintained at θ higher than 0.35 L·L−1. The second experiment assessed american alumroot's performance under four daily light integrals (DLIs) (7.5, 10.8, 14.9, and 21.8 mol·m−2·d−1) with θ maintained at 0.35 L·L−1. Increasing DLI from 7.5 to 21.8 mol·m−2·d−1 caused shoot dry weight, leaf area, maximum width, and leaf count to change quadratically. Dry weight and leaf area reached their maximum at 10.8 mol·m−2·d−1, whereas leaf count was greatest at 14.9 mol·m−2·d−1. Increasing DLI to 21.8 mol·m−2·d−1 negatively impacted leaf area and leaf count but did not lower shoot dry weight. Leaf area ratio and petiole length of the uppermost fully expanded leaf decreased with increasing DLI. Measures of leaf-level net photosynthesis, light response curves, and CO2 response curves indicated no physiological differences among plants grown under different water or light levels. In both studies, long-term, whole crop measures of water use efficiency based on shoot dry weight and water applied (WUEc) did not reflect the same water use trends as instantaneous, leaf-level measures of WUE based on leaf gas exchange (WUEl). WUEc decreased with increasing θ and DLI, whereas WUEl was not influenced by θ and increased with increasing DLI. WUEl is often used to provide insight as to how various abiotic and biotic factors influence how efficiently water is used to produce biomass. However, these findings demonstrate that there are limitations associated with making such extrapolations.

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Rhuanito Soranz Ferrarezi, Geoffrey Matthew Weaver, Marc W. van Iersel and Roberto Testezlaf

Subirrigation is a greenhouse irrigation method that relies on capillary action to provide plants with water and nutrients from below their containers. The first documented subirrigation system was described in 1895, and several variations on the basic design were used for research purposes before the modern ebb-and-flow type systems emerged in 1974. Most subirrigation systems apply the fertilizer solution to a waterproof bench or greenhouse section, allowing the substrate to absorb the water through holes in the bottom of the containers. Because there is little or no leaching, subirrigation typically allows for the use of lower fertilizer solution concentrations. Although excess fertilizer salts typically accumulate in the top layer of the substrate, this does not seem to have a negative impact on plants. Subirrigation can conserve nutrients and water, reduce labor costs, and help growers meet environmental regulations. A challenge with subirrigation is the potential spread of pathogens via the fertilizer solution. When this is a concern, effective disinfection methods such as ultraviolet radiation, chlorine, or ozone should be used. Sensor-based irrigation control has recently been applied to subirrigation to further improve nutrient and water use efficiencies. Better control of irrigation may help reduce the spread of pathogens, while at the same time improving crop quality. The primary economic benefit of subirrigation is the reduction in labor costs, which is the greatest expenditure for many growers.

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Marc W. van Iersel, Geoffrey Weaver, Michael T. Martin, Rhuanito S. Ferrarezi, Erico Mattos and Mark Haidekker

Photosynthetic lighting is one of the main costs of running controlled environment agriculture facilities. To optimize photosynthetic lighting, it is important to understand how plants use the provided light. When photosynthetic pigments absorb photons, the energy from those photons is used to drive the light reactions of photosynthesis, thermally dissipated, or re-emitted by chlorophyll as fluorescence. Chlorophyll fluorescence measurements can be used to determine the quantum yield of photosystem II (ΦPSII) and nonphotochemical quenching (NPQ), which is indicative of the amount of absorbed light energy that is dissipated as heat. Our objective was to develop and test a biofeedback system that allows for the control of photosynthetic photon flux density (PPFD) based on the physiological performance of the plants. To do so, we used a chlorophyll fluorometer to measure ΦPSII, and used these data and PPFD to calculate the electron transport rate (ETR) through PSII. A datalogger then adjusted the duty cycle of the light-emitting diodes (LEDs) based on the ratio of the measured ETR to a predefined target ETR (ETRT). The biofeedback system was able to maintain ETRs of 70 or 100 µmol·m−2·s−1 over 16-hour periods in experiments conducted with lettuce (Lactuca sativa). With an ETRT of 70 µmol·m−2·s−1, ΦPSII was stable throughout the 16 hour and no appreciable changes in PPFD were needed. At an ETRT of 100 µmol·m−2·s−1, ΦPSII gradually decreased from 0.612 to 0.582. To maintain ETR at 100 µmol·m−2·s−1, PPFD had to be increased from 389 to 409 µmol·m−2·s−1, resulting in a gradual decrease of ΦPSII and an increase in NPQ. The ability of the biofeedback system to achieve a range of different ETRs within a single day was tested using lettuce, sweetpotato (Ipomoea batatas), and pothos (Epipremnum aureum). As the ETRT was gradually increased, the PPFD required to achieve that ETR also increased, whereas ΦPSII decreased. Surprisingly, a subsequent decrease in ETRT, and in the PPFD required to achieve that ETR, resulted in only a small increase in ΦPSII. This indicates that ΦPSII was reduced because of photoinhibition. Our results show that the biofeedback system is able to maintain a wide range of ETRs, while it also is capable of distinguishing between NPQ and photoinhibition as causes for decreases in ΦPSII.