Abstract
Saffron is an extremely high-value spice made from the dried stigmas of the saffron crocus (Crocus sativus). The saffron crocus flowers in the fall, grows vegetatively during the winter and spring, and is dormant during the summer. It is native to the Mediterranean region and most of the global saffron crop is produced in the Mediterranean and western Asia. Summer rainfall is rare in these regions. However, saffron crocus is becoming increasingly popular as a niche crop in the northeastern United States and other areas where summer rainfall is common. This study evaluated four methods of weed control during summer dormancy to determine the relative costs and the effects on saffron yield. Hand cultivation (HC) was the most expensive method and resulted in significantly decreased saffron yields. Covering the saffron crocus field with a light-proof plastic tarp (occultation) was the least expensive and had no effect on saffron yield. Annual planting of saffron crocus also had no effect on saffron yield but required more labor than occultation. Intercropping basil (Ocimum basilicum) resulted in the highest revenue per unit area and did not negatively affect yields of either saffron or basil. Based on this study intercropping is recommended for summer weed management for producers seeking to maximize revenue per unit area, while occultation is recommended for producers seeking to minimize production costs.
Saffron is an extremely high-value spice made from the dried stigmas of the saffron crocus (C. sativus). Saffron has been used as a culinary spice, textile dye, and component of cosmetics and herbal medicines since ancient times. More recently, saffron has shown promise in pharmacognosy research as a treatment for cancer, glaucoma, drug-resistant infections, and other conditions (Gohari et al. 2013). The saffron crocus is a sterile annual herbaceous plant that reproduces vegetatively through corm multiplication; it is usually cultivated as a perennial (Cardone et al. 2020). The saffron crocus flowers in the autumn, grows vegetatively through the winter and spring, and initiates new flower buds just before going dormant in the early summer (Koocheki and Seyyedi 2020). This annual growth cycle is controlled by physiological responses to changes in daylength and soil temperature (Gresta et al. 2009; Molina et al. 2005; Mollafilabi 2004) and is well adapted to the Mediterranean or monsoonal climates of traditional production regions.
However, saffron crocus does not require a climate without summer rainfall (Gheshm and Brown 2021). It grows well in most temperate zone areas as long as the soil drainage is sufficient. Saffron production has been expanding outside traditional production areas, driven by the growing popularity of foods from traditional saffron production areas and by interest in pharmaceutical use of saffron (Kumar et al. 2008). In North America, consumers are willing to pay a premium for domestically produced saffron and saffron crocus has attracted the attention of farmers as an ultra-niche crop (Skinner et al. 2021). Flower harvesting and separation of the stigmas from the flowers is done entirely by hand, so there is limited economy of scale for saffron production. This allows small farms to be competitive, particularly if they are able to sell saffron directly to consumers or use it in value-added products. The fertile soils and abundant rainfall in eastern North America result in excellent saffron yields (Gheshm and Brown 2021); however, growers must adapt traditional saffron crocus production practices to suit a more mesic climate.
Weeds do not directly compete with saffron crocus during the summer, as the corms are fully dormant. However, weeds can provide cover for rodents that eat the corms (Bazoobandi et al. 2020) and the weed biomass needs to be removed before saffron crocus emergence in the fall. Most global saffron production is in climates where there is no rainfall during the summer; in these climates a single cultivation event or herbicide application early in the dry season will control weeds all summer (Abbasian et al. 2013). In the mesic climate of the northeastern United States weed growth continues throughout saffron crocus dormancy. The choice of summer weed control strategy directly influences cost of production and can indirectly influence saffron yields by altering the temperature and moisture levels of dormant corms.
Four methods of summer weed management were selected based on potential fit with small-scale diversified market farming operations in eastern North America: Hand cultivation, annual planting, intercropping, and occultation. Hand cultivation is widely used in saffron crocus production, although fields can be cultivated using tractor-mounted cultivation implements as long as the tractor wheels do not travel over the dormant corms (Bazoobandi et al. 2020). Annual planting is a traditional solution to summer weed control used in the Abruzzo region of Italy. Farmers dig up the saffron corms every year at the beginning of the dormant season and replant in a new location in late summer. This minimizes the risks of rodents, fungal pathogens, or high temperatures damaging the corms and allows saffron to be rotated with annual crops to avoid buildup of perennial weeds, insect pests, and pathogens (Gresta et al. 2016). However, digging and replanting corms is labor intensive. Intercropping is defined as the simultaneous production of two or more crops on the same piece of land (Martin-Guay et al. 2018). It is promoted as a means of reducing problems with pests and weeds while increasing yields and income, particularly for farmers with limited access to land. Researchers in Iran have shown that intercropping chamomile (Matricaria chamomilla), cumin (Cuminum cyminum), or small grains can increase total productivity and help control weeds without reducing saffron yields (Darbaghshahi et al. 2012; Koocheki et al. 2009; Maleki et al. 2020). These studies grew the intercrops during saffron vegetative growth to avoid potential damage to the dormant corms from irrigation. However, Koocheki et al. (2019) found that biweekly irrigation did not harm dormant corms and intercropping irrigated oilseed pumpkin (Cucurbita pepo) and watermelon (Citrullus lunatus) during saffron dormancy increased saffron yields because the intercrop canopy shaded the soil surface and reduced soil temperatures during flower bud initiation. We chose to intercrop basil (Ocimum basilicum) as it is likely to appeal to consumers of saffron and is more widely grown than chamomile or cumin in eastern North America (Matthews et al. 2018). Occultation, or tarping, is the practice of controlling unwanted vegetation by covering fallow land with opaque tarps (Lounsbury et al. 2022). It has become a popular method on small market farms to control weeds without tillage. Occultation is similar to solarization but relies on a combination of light deprivation and increased temperature to kill weeds. The black plastic tarps are also used to protect seedbeds that have been prepared in late fall for early spring planting as they increase soil temperatures and reduce infiltration of rain and snowmelt (Lounsbury et al. 2022). The objective of this study was to evaluate the four methods of summer weed control described above for saffron crocus production in climates with wet summers.
Materials and methods
Study location and design
Field preparation
The study was conducted at the Gardiner Crops Research Center in Kingston, RI, USA (lat. 41.49°N, long. 71.54°W). The climate is mesic and mild. Average annual precipitation is 52.9 inches, evenly distributed throughout the year. The average high temperature is 62 °F and the average low temperature is 40 °F. Soil is Bridgehampton silt loam. Before beginning field preparation for this study, the soil had a pH of 5.5, 3.6% organic matter, 16.5 lb/acre phosphorus and 272 lb/acre potassium. The experiment was conducted from Jun 2020 to Nov 2021. The year before study establishment, the field was planted to a mustard (Brassica juncea) cover crop to smother perennial weeds. The covercrop was winter-killed and the field was covered with 6-mil black/white silage sheet (Berry Plastics, Evansville, IN, USA) from 12 Apr to 26 May 2020 to further reduce weed pressure. On 26 May ground limestone and fertilizer were broadcast and incorporated with a disk harrow. This increased soil pH to 6.6 and provided nitrogen at 150 lb/acre and potassium at 70 lb/acre. No phosphorus was applied as soil tests indicated that levels were sufficient. The study was set up as a randomized block design with four saffron crocus weed control treatments: Hand cultivation (HC), annual planting (AP), intercropping (IC), and occultation (OC). Plot size was 2 m by 1 m; a 0.6-m unplanted buffer separated adjacent plots. All treatments were replicated four times, and the experimental unit was the plot.
Saffron crocus planting
Saffron corms for this study were produced on the research farm and dug in Jun 2020. Plots for treatments HC, IC, and OC were planted from 2 Jul to 4 Jul 2020 using corms weighing 12 to 15 g. Corms were planted at 20-cm depth into augured holes in rows 11 cm apart with 10 cm between corms in the row for a density of 90 corms/m2. Because annually cropped saffron crocus is always planted in late summer, plots for treatment AP were planted on 8 Sep 2021 using corms that had been held indoors over the summer at 65 °F and 50% RH. Planting depth and spacing were the same as for the other treatments. Treatment AP corms were dug up in Jun 2021 and stored indoors at 65 °F and 50% RH until replanting at 90 corms/m2 on 9 Sep 2021.
Basil planting
Seeds of O. basilicum ‘Rutgers Obsession DMR’ (Van Drunen Farms, Momence, IL, USA) were planted into 102-cell plug trays (Landmark Plastics, Akron, OH, USA) with a cell volume of 22 cm³ filled with Miracle-Gro performance organics container mix (Scotts Miracle-Gro, Marysville, OH, USA) on 29 May and 28 May in 2020 and 2021, respectively. Seedlings were grown in a passively ventilated greenhouse set to 75 °F for 20 d, then moved to a shade tunnel (30% shade) until transplanting.
Management during summer dormancy
In both years, saffron crocus in Kingston, RI, USA, was in full summer dormancy by 21 Jun. Plots for treatment OC were covered with 6-mil black/white silage sheet (Berry Plastics) with the black side up on 4 Jul 2020 and 6 Jul 2021; tarps were removed on 22 Sep in both years. Plots for treatment HC were hoed weekly during July, August, and September to control weeds. Plots for treatments IC and AP were used to produce basil during saffron crocus dormancy. The difference between the two treatments is that saffron crocus corms remained in the soil of the IC plots during basil production while the AP plots had not been planted to saffron crocus. Microembossed black polyethylene plastic mulch (1.0-mil thickness; Berry Plastics) was laid by hand to cover the entire plot. On 7 Jul 2020 and 8 Jul 2021, basil seedlings were transplanted into the plots. Each 2-m2 plot had three rows of basil 40 cm apart, with plants 33 cm apart within the row.
Maintenance
Saffron crocus does not require irrigation in the mesic climate of Rhode Island. Rainfall and evaporation amounts are shown in Fig. 1. Plots containing basil were irrigated by hand at transplanting in both years and five additional times in 2020; frequent rainfall in 2021 meant no further irrigation was needed.
Daily precipitation and pan evaporation amounts for Kingston, RI, USA, from 1 May 2020 through 30 Nov 2021. Evaporation data are only collected from 1 May to 31 Oct each year.
Citation: HortTechnology 34, 6; 10.21273/HORTTECH05488-24
All nutrients for 2020 were applied at trial establishment. In 2021, plots were side-dressed in early March with granular fertilizer (12N–0.3P–10K) to provide 40 lb/acre nitrogen and with urea (46N–0P–0K) in early April and early May to provide 40 lb/acre nitrogen at each application. Plots for treatments IC and AP received fertilizer to provide 100 lb/acre nitrogen and 41.5 lb/acre potassium before laying plastic mulch and transplanting basil. On 25 Sep 2021, 30N–0P–16.6K was applied to all plots to provide 62.5 lb/acre nitrogen.
Summer weed management for plots is described previously. Weekly HC was sufficient to keep the HC plots weed free, while the tarp used in treatment OC and the plastic mulch used in basil production prevented weed growth in the other plots. All spaces between plots were hand cultivated weekly to control weeds. All treatments were hand cultivated in October of each year before the beginning of saffron crocus flowering. After flower harvest in 2020 the entire trial area was mulched with a 5-cm layer of locally sourced shredded deciduous tree leaves to suppress annual winter weed growth. Weeds emerging through the saffron crocus canopy were hand-pulled in late March and late May.
No disease or insect issues required control in saffron crocus. In 2020, basil plants remained free of disease, but high temperatures and humidity in Aug 2021 led to an outbreak of basil downy mildew (Peronospora belbaharii). Basil plants were sprayed with a tank mix of fluopicolide (Presidio 4SC; Valent USA, San Ramon, CA, USA) at 4 oz per acre and phosphites (Phiticide; Drexel Chemical Company, Memphis, TN, USA) at 15 mL per gallon on 25 Aug and 7 Sep to control downy mildew.
Soil temperature and precipitation monitoring
Saffron crocus corms are sensitive to soil temperatures during dormancy, with temperatures above 27 °C resulting in flower bud abortion (Molina et al. 2005). To assess the effect of the weed management strategies on soil temperature data, loggers (RC-51; Elitech Technology, Milpitas, CA, USA) were placed between plants in the center of each plot at the same depth as the saffron crocus corms and used to record soil temperatures at 4-h intervals. In both years, data loggers were installed on 11 Jul and removed on 21 Sep. Precipitation and open pan evaporation data were collected at the University of Rhode Island Weather Station, which is located at the Gardiner Crops Research Center.
Basil harvest
In 2020, basil was harvested five times. The first harvest was pruning to shape basil plants on 2 Aug. Four later harvests were done on 9 Aug, 26 Aug, 9 Sep, and 21 Sep. In 2021 we harvested basil plots on 18 Aug, 3 Sep, and 20 Sep. Data were collected on marketable fresh weight and dry weight from all plots in both years. Dry weight was calculated by drying samples of known fresh weight in an oven at 43 °C until there was no change further change in weight to determine percent dry matter.
Saffron harvest
Saffron crocus flowering began on 13 Oct in both years and ended on 14 Nov in 2020 and on 11 Nov in 2021. Blossoms were picked by hand every morning. After harvest, flowers were kept refrigerated at 4 °C until processing. Stigmas were separated from anthers and tepals and dried in a commercial dehydrator (78450; Proctor Silex, Glen Allen, VA, USA) at 35 °C for 12 h and weighed on an enclosed balance.
Cost-benefit analysis
The cost of all inputs that differed between treatments were tracked. Labor time was tracked for tasks that differed between treatments and labor was valued at $20 per hour. Boston Terminal Market prices published by US Department of Agriculture–Agricultural Marketing Service were used to determine the value of the basil crop. Value of the saffron crop was based on the average price reported in the annual survey of North American saffron producers conducted by the North American Center for Saffron Research and Development (Skinner M, personal communication).
Data Analysis
Basil data were collected on marketable fresh weight and dry weight per m2. Saffron crocus data were collected on the number of flowers per m2, the weight of dried pistils per m2, and the average single pistil dry weight. Years were analyzed separately for saffron yield, as is standard practice in saffron crocus production studies (Khademi et al. 2013). Yield data were analyzed using mixed measures analysis of variance (ANOVA) with the pairwise comparison of least mean squares (SAS software version 9.4; SAS Institute, Cary, NC, USA). Soil temperature data were analyzed using repeated measures ANOVA to compare means.
Results and discussion
Flower number and saffron yield
Saffron yields are determined by corm density, number of flowers per corm, and stigma weight. Because the experimental plots were established in the summer of 2020 using fully dormant corms that had already initiated floral buds, differences in 2020 yields were due only to differences in the number of floral buds that produced flowers and in stigma weight. In 2020, treatment OC produced significantly more flowers than any other treatment, averaging 178 flowers/m2 (Table 1). There were no significant differences in flower number among the other three treatments. Treatment AP produced the heaviest stigmas, averaging 6.4 mg each (Table 1). There were no significant differences in stigma weight among the other three treatments. Treatment OC had the highest yield in 2020, averaging 1.04 g/m2; treatment AP was similar, averaging 1.01 g/m2 (Table 1). Treatment OC yields were significantly greater than yields from treatments IC and HC, whereas treatment AP yields were significantly greater than yields from treatment HC but similar to treatment IC.
Effects of weed control method during summer dormancy on flower number, stigma size, and saffron yield for saffron crocus grown in Rhode Island in 2020.
Corm density and saffron yields in 2021 were influenced by saffron crocus vegetative growth during Winter 2020–21, daughter corm formation and flower bud initiation in Spring 2021, and flower bud survival over Summer 2021. Treatment AP directly affected corm density in Fall 2021 as the daughter corms produced in Spring 2021 were dug up, and the plots were replanted in Sep 2021 at 90 corms/m2 using the largest and healthiest corms from each plot. Corm density and health were not assessed for the other treatments. Treatment IC produced the most flowers in 2021, averaging 215 flowers/m2 (Table 2). However, variability between plots was higher than in 2020 and the only significant difference was between treatment IC and treatment HC, which produced the fewest flowers. There were no significant differences between treatments for stigma weight in 2021, although treatment AP again had the heaviest stigmas. Treatment IC was the most productive, averaging 1.2 g/m2 of dried saffron. However, the only significant difference was with treatment HC, which had the lowest yield.
Effects of weed control method during summer dormancy on flower number, stigma size, and saffron yield for saffron crocus grown in Rhode Island in 2021.
HC had the lowest flower number and saffron yield in both years, and it was the only treatment in which flower number decreased from 2020 to 2021. Normally flower number in perennial saffron crocus plantings increases from year 1 to year 2 (Koocheki and Seyyedi 2020). Wet soils are the most likely cause of the yield decrease in HC, given the high rainfall in 2021. Rahimi et al. (2008) report that corm rot and mite infestation frequently reduce saffron yields in areas with summer rainfall. Under the low rainfall conditions in 2020 treatment, OC produced significantly more flowers than the other treatments and had significantly higher yields than intercropping. These results suggest that the tarp may have reduced soil moisture and improved flower bud survival. Treatment OC did not differ from treatment IC or treatment AP in 2021 but the plots used in this study were small enough that lateral movement of soil moisture from outside the plot could have counteracted any reduction in infiltration due to the tarp.
Transpiration is a major source of subsurface drying of soils during the growing season so intercropping has potential to reduce corm rot. Treatment IC produced saffron yields similar to HC in 2020 despite the additional moisture from irrigating the basil. In 2021 when frequent rainfall allowed the basil to be grown without irrigation treatment IC had the highest yields of any treatment. Koocheki et al. (2019) reported that biweekly summer irrigation to support an intercrop of pumpkin did not harm saffron corms in Iran. Our results suggest that in climates with summer rainfall, intercropping may help protect corms by decreasing soil moisture, particularly in wet years.
Basil yields
Basil fresh yield averaged 3.96 kg/m2 in 2020 and 2.87 kg/m2 in 2021. Dry weight yields were 0.482 kg/m2 in 2020 and 0.377 kg/m2 in 2021. The significantly (P < 0.05) lower yield in 2021 was primarily because of infection with basil downy mildew, caused by the oomycete Peronospora belbahrii.
Saffron crocus corms have been shown to release allelopathic compounds (Feizi et al. 2018; Hosseini and Rizvi 2007) that could complicate intercropping. In 2020, the basil plants intercropped with the dormant saffron crocus corms (treatment IC) yielded significantly less fresh weight than the basil plants grown in soil without saffron crocus corms (Table 3). However, in 2021, the presence of saffron crocus corms had no effect on fresh basil yields. Most importantly, in this study the presence of dormant saffron corms did not significantly affect dry basil yields in either year (Table 3). The lack of effect on dry yields suggests that the difference in fresh yields in 2020 was due to differences in plant water content at harvest.
Yields of sweet basil grown in monoculture and intercropped with dormant saffron crocus during the summer in Rhode Island.
Soil temperature
Soil temperature was significantly affected by both year and weed management method (Figs. 2 and 3). Mean soil temperature was 25.0 °C in 2020 and 24.4 °C in 2021 (P < 0.05); this was likely because of greater rainfall in 2021. Covering the soil with the black silage sheet (treatment OC) resulted in a mean soil temperature of 26.4 °C with a range from 16.6 to 31.1 °C. The mean soil temperature under the basil intercrop (treatment IC) averaged 23.9 °C with a range from 16.0 to 28.7 °C. The mean soil temperature in the bare ground plots (treatment HC) also averaged 23.9 °C with a range from 16.4 to 28.2 °C. Treatment OC was significantly warmer than the other treatments (P < 0.01) but the soil did not become warm enough to cause flower bud abortion. Rahimi (2016) increased the soil surface temperature to 65 °C for 6 weeks through solarization to control weeds and mites in saffron fields in Iran without harming saffron yields.
Daily mean soil temperatures at corm depth under black silage sheet (OC), a basil intercrop with black polyethylene mulch (IC), and bare ground (HC). Temperature data were recorded during saffron crocus dormancy in 2020.
Citation: HortTechnology 34, 6; 10.21273/HORTTECH05488-24
Daily mean soil temperatures at corm depth under black silage sheet (OC), a basil intercrop with black polyethylene mulch (IC), and bare ground (HC). Temperature data were recorded during saffron crocus dormancy in 2021.
Citation: HortTechnology 34, 6; 10.21273/HORTTECH05488-24
Cost-benefit analysis
Cost data were collected during the summer of 2021 for inputs of labor and materials which differed between summer weed management methods (Table 4). Costs and revenue are reported per square meter rather than per acre because saffron crocus production fields in North America rarely exceed 10,000 square feet in area. Annual planting (AP) required 6 min per m2 to dig, sort, and pack the corms in June and 4 min per m2 to prepare the field and replant the corms in September, for a total of 10 min per m2 and a cost of $3.33. Mesh bags for storing the corms cost $0.31 each with one bag being sufficient for 1 m2. We assumed that indoor storage space was available free of charge. Intercropping with basil (IC) required $5.50/m2 for basil transplants and $0.20 for field supplies, including black plastic mulch, drip tape, and fertilizer. Labor costs for intercropping were $0.44/m2 for laying plastic mulch and drip tape and transplanting basil, $6.00/m2 for basil harvest, and $0.30/m2 for clearing away mulch and drip tape in September. The silage cover used for occultation (OC) cost $1.25/m2 but can be used for 5 years, leading to an annual cost of $0.25/m2. Laying the cover in June and removing it in September requires only 1.5 min/m2. HC required 3.2 min/m2 each week from 21 Jun to 21 Sep for a total of 48 min/m2 and a cost of $16.
Cost-benefit estimates for four summer weed management strategies used in saffron crocus production in Rhode Island. Calculations include only the inputs associated with summer weed management. All calculations are based on 1 square meter as saffron plantings in the northeastern United States rarely exceed 0.1 ha.
Revenue data for basil use the average wholesale price at the Boston Terminal Market for Sep 2020 ($13.20/kg) and Sep 2021 ($11.00/kg). Most small-scale growers sell directly to consumers at significantly higher prices. Revenue data for saffron use $25/g which is typical for direct-market sales of locally grown saffron in the northeastern United States (Skinner M, personal communication).
Under mesic conditions, HC to maintain clean fallow is the least economically sustainable method of summer weed control since it has the highest costs and the lowest revenue. In mesic climates annual cropping of saffron requires less labor than maintaining bare ground during summer dormancy and in this study it resulted in the heaviest stigmas. Heavier stigmas are desirable because increasing stigma weight increases yields without increasing effort required for harvesting and processing the flowers. AP has potential to be economically sustainable for large-scale saffron crocus production in mesic areas if corm digging and planting are mechanized. However, growers in eastern North America currently dig and plant corms by hand.
Intercropping a high-value crop such as basil costs more than annual cropping but less than maintaining bare ground, and it has the highest revenues since the revenues from the intercrop are added to those from the saffron crop. Intercropping may also increase saffron yields in wet years since the intercrop would remove moisture from the soil through transpiration, reducing the potential for corms to rot. Intercropping basil with saffron did not significantly affect dry basil yields in this study, and intercropping could be an excellent strategy for small-scale growers. However, intercropping can increase the production costs for the intercrop because growers must avoid damaging the dormant corms during tillage, bedshaping, and planting. For growers who are not land-limited, AP or occultation may be more economically sustainable approaches to controlling summer weeds in saffron crocus production. Occultation had the lowest cost and saffron yields were similar to AP and intercropping. Silage sheets are available in widths up to 50 ft and lengths up to 1000 ft so occultation could be used on saffron plantings of 1 acre or more in size.
Conclusions
Based on the results of this study, saffron producers in the northeastern United States and other areas with summer rainfall should consider occultation or intercropping for summer weed management. Producers who are land-limited can use intercropping to increase their revenue per unit area. However, options for bed preparation are limited by the need to avoid damaging the saffron corms with the weight of equipment or during tillage. For example, raised beds are recommended for basil production in New England but the plastic mulch was laid by hand over flat beds for this study because tillage to produce raised beds risked damaging the saffron crocus corms. Producers who have more land may prefer to use occultation due to the extremely low cost, particularly if intercropping increases the cost of producing the intercrop. Annual cropping of saffron has higher production costs than occultation and does not increase yields. Producers should avoid trying to maintain bare fallow through frequent cultivation as it significantly reduced saffron yields in this study.
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