Abstract
We examined the effect of harvest schedule on the yield of ‘Red Russian’ kale (Brassica napus ssp. napus var. pabularia) grown during the winter in 16 × 32-ft high tunnels in northern New Mexico. We conducted the study for two growing seasons: 2013–14 and 2014–15. All plots were sown on 16 Oct. and harvested four times according to four harvest schedules: A) 8, 16, 20, and 24 weeks after sowing; B) 10, 17, 21, and 25 weeks after sowing; C) 12, 18, 22, and 26 weeks after sowing; and D) 14, 19, 23, and 27 weeks after sowing. The first harvest of each treatment was the greatest, averaging 216 g/ft2, compared with 88, 109, and 104 g/ft2 for harvests 2, 3, and 4, respectively. Season total yield of treatments B, C, and D (harvests beginning at 10, 12, and 14 weeks after sowing) yielded significantly more than treatment A, but only in year 2, when delayed growth resulted in very low yields for treatment A at harvest 1. Considering the entire 240-ft2 cropped area of the high tunnel, staggered harvests of 60 ft2 at a time can yield 2.6 to 17.5 kg per harvest or up to 124 kg over an entire season. Although we examined the yield of mature leaves, harvests could possibly begin earlier than in this study for “baby” kale or salad mixes, and the area harvested could be tailored to plant growth stage and market demand.
High tunnels (i.e., hoop houses) are plastic-covered, passively heated, walk-in, semipermanent structures sited on field soil (Jimenez et al., 2005). High tunnels were developed in the United States (Emmert, 1955), but the adoption of tunnels has been slower there than in other countries (Blomgren and Frisch, 2007). High tunnels can reduce crop damage (Upson, 1998), wide diurnal temperature swings, and other unpredictable growing conditions (Lamont et al., 2003). They can change the growing environment equivalent to moving three Plant Hardiness Zones to the south (Coleman, 2009; Wells and Loy, 1993). In the United States, high tunnels have been used to grow a variety of crops from tomato (Solanum lycopersicum), to cut flowers (Knewtson et al., 2010), to leafy greens (Borrelli et al., 2013). This last group of crops is important because of their tolerance of cold temperatures and freezing (Maynard and Hochmuth, 2006). If such crops can be grown through the winter in a protected environment, they represent an important addition to total year-round productivity to the small scale grower. According to the U.S. Department of Agriculture (USDA) Agricultural Marketing Service, 31% of the nation’s 8797 farmers’ markets are open in winter (USDA, 2020). The New Mexico Department of Agriculture (NMDA) reported 75 statewide farmers’ markets with combined sales of up to $9 million per year; 10 of these are open during the winter (New Mexico Farmers’ Market Association, 2019; NMDA, 2015). To help farmers gain access to winter markets, we explored the use of high tunnels for the production of kale.
Kale is a general term for leafy Brassica crops, mostly included in Brassica oleracea Acephala (nonheading) group but also including Brassica napus ssp. napus var. pabularia. We used ‘Red Russian’ kale, which falls into the latter group. Kale is a valuable fresh-market item with high levels of phytonutrients, such as anthocyanins, glucosinolates, carotenoids, tocopherols, polyphenolics, general antioxidants, terpenes (Björkman, et al., 2011; Manchali, et al., 2012) and mineral elements (Kopsell and Sams, 2013). In addition, kale is a frost hardy plant (Kadam and Shinde, 1998) and has the potential to maximize winter production because leafy material can be harvested several times (“cut and come again”) with a relatively short regrowth period (35–50 d or 5–7 weeks) (Swiader and Ware, 2002). Between 2009 and 2017, the acreage of organic kale in Washington state rose from 15 to 99 acres, pointing to the crop’s growing popularity (Kirby and Granatstein, 2018). Kale can be harvested at early growth stages as “microgreens” or “baby” leaves, although these terms lack legal definitions (Treadwell et al., 2010). Waterland et al. (2017) defined “microgreens” as plants with only cotyledons or two true leaves, “baby” kale as plants with four to six true leaves, and adult kale as plants with more than six leaves. The objective of this study was to examine the effect of the timing of repeated harvests of mature adult leaves on the yield of kale grown in a high tunnel.
Materials and methods
This study was conducted during the winters of 2014–15 (season 1) and 2015–16 (season 2) on certified organic acreage at the New Mexico State University Sustainable Agriculture Science Center at Alcalde, NM, at an elevation of 5680 ft and receiving average annual precipitation of 10.0 inches. Based on 1953–2019 data from the Alcalde weather station, compiled by the New Mexico Climate Center (2019), the average last spring freeze (32 °F) date was 11 May and the average first fall freeze date is 4 Oct., for a 146-d growing season. According to data from the Western Regional Climate Center (2019) for Alcalde, the average maximum daily temperature in January, the coldest month, is 45.7 °F and the average minimum daily temperature in January is 15.2 °F (WRCC, 2019). The study was conducted on Fruitland soil [coarse-loamy, mixed, calcareous, mesic Typic Torriorthents (USDA, 2008)]. The study took place in two high tunnels (16 × 32 ft in size with the longest dimension in east–west alignment). A large south-facing sidewall increases the transmission of solar radiation when the sun is at lower angles (Giacomelli, 2009). The design is based on Jimenez et al. (2005) and modified to have sidewalls and endwalls framed by dimensional lumber (Hecher et al., 2014). The high tunnel was constructed with 3-ft-high sidewalls, a peak height of 8.5 ft, and 3 × 7-ft doors on the west and east ends. The high tunnel was glazed with a heavyweight woven plastic containing an ultraviolet light inhibitor treatment (SOLARIG ROOF 172; J&M Industries, Ponchatoula, LA).
Soil fertility management.
In the spring of each year of this study, ≈300 lb of manure (≈210, 23, and 222 lb/acre nitrogen, phosphorus, and potassium, respectively) were applied and incorporated with a small rototiller to a depth of ≈6 inches. Manure nutrient content analysis was conducted by the Colorado State University Soil, Water and Plant Testing Laboratory (Ft. Collins, CO). In early summer, a cover crop of sudangrass (Sorghum ×drummondii) was broadcast across the entire floor of the high tunnel. The cover crop was mown several times through the summer and incorporated with a small rototiller in late summer.
Experimental design.
The high tunnel was divided into 24 plots (experimental units), 12 on each side of a central aisle, each 5 ft long by 2 ft wide and separated by a 7-inch buffer. The study was a randomized complete block design with four treatments and six blocks.
Crop management.
In each year of the study, seeds were sown 16 Oct. in three rows 8 inches apart. Within each 2 × 5-ft plot, 0.42 g (equivalent to 4 lb/acre) of ‘Red Russian’ kale seed was sprinkled across the three rows, lightly covered in soil, and firmed by foot. The soil surface was kept moist using overhead micro sprinklers to ensure germination, and on hotter days, high tunnel doors were opened for venting. After emergence, plots were covered with spun-bonded fabric rowcover (Agribon AG-15, J&M Industries), which rested directly on top of plants. Rowcover was placed over the crop on 25 Nov. in each year of the study and removed for irrigation and harvesting. We irrigated the kale with micro sprinklers weekly through November; biweekly in December, January, and February; weekly in March; and twice weekly in April. Approximately 0.75 to 1.25 inches was applied in each irrigation event.
In each season of the study, we defined four treatments based on the time between sowing and the first harvest of mature kale leaves, and each treatment was harvested a total of four times. The treatments were as follows: A) 8, 16, 20, and 24 weeks after sowing; B) 10, 17, 21, and 25 weeks after sowing; C) 12, 18, 22, and 26 weeks after sowing; and D) 14, 19, 23, and 27 weeks after sowing. Figure 1 illustrates the sowing and harvesting schedules. By early May in each year of the study, cabbage aphids (Brevicoryne brassicae) had infested a majority of the trial plots, and individual plants had begun to bolt.
Sowing and harvesting schedules for ‘Red Russian’ kale sown 16 Oct. 2014 and 2015 in 16 × 32-ft (4.9 × 9.8 m) high tunnels in Alcalde, NM. Labels in the diagram refer to weeks from sowing to each of four harvests. Wedges represent growth periods and truncations on the right-hand side of each wedge denote harvests.
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
Harvesting.
Before harvest, we took three measurements of plant height (inches) in the middle row of each plot and averaged these. For the first two harvest cycles (dates) plants were in a basal rosette form, and fully expanded leaves with elongated petioles were gathered in hand and clipped with scissors 2 to 4 inches above the ground and allowed to regrow until the following harvest. Care was taken to leave the apical meristem and young leaves emerging therefrom intact, but in some cases, especially during the two latter harvest cycles, the growing point was cut accidentally. In those cases, leaves emerged from lower nodes (Fig. 2). Three linear feet of the central row in each plot were harvested to avoid edge effects, and after sampling, we harvested the remainder of each plot in the same way. Figure 3 shows kale plants in three different stages of regrowth. Following each harvest, we counted the plants clipped in the harvested area and weighed harvested leaf material.
Image taken 24 Mar. 2015 showing ‘Red Russian’ kale plant where the apical portion was removed and new growth issued from lower nodes. This was the harvest 3 (23 weeks after sowing) of treatment 4 (harvests at 14, 19, 23, and 27 weeks after sowing).
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
Image taken 6 Jan. 2015, 12 weeks after sowing, showing three stages of growth/regrowth of ‘Red Russian’ kale sown 16 Oct. 2014. The plot in the left-center was harvested just before image acquisition (treatment C: harvested at 12, 18, 22, and 26 weeks after sowing), the plot in the right-center was harvested 12 Dec. 2014 and shows 2 weeks’ regrowth (treatment B: harvested at 10, 17, 21, and 25 weeks after sowing), and the plots to the far left and right had not been harvested yet (both treatment D: harvested at 14, 19, 23. and 27 weeks after sowing).
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
Temperature monitoring.
Temperatures were recorded at 30-min intervals using temperature data loggers (HOBO U12; Onset Computer Corp., Bourne, MA). Sensors were placed 1 ft above the soil in solar radiation shields and horizontally 3 inches below the soil surface at each of three locations: 1) outside the high tunnel; 2) inside the high tunnel, uncovered; and 3) inside the high tunnel under the rowcover material.
Statistical analysis.
Plant height, plant density, and fresh weight yield were analyzed using a mixed model that included fixed effects for year, treatment, harvest, and all interactions among these three factors. Random effects for block within year and harvest by block within year interaction were included to account for blocking. To account for nonconstant variances and correlations among the repeated factor levels, an unstructured covariance was fitted. Analysis was completed using SAS (version 9.4; SAS Institute, Cary, NC) with significance defined as P ≤ 0.05.
We mainly examined season total yield (STH) but also analyzed repeated harvests to better understand production at different times of the year. When significant interactions were detected, we examined simple main effects for those factors. We used the SLICE command, and significant differences among means were determined with Tukey’s honestly significant difference test with α = 0.05. Yields are reported as grams harvested per square foot.
Results and discussion
Plant height at time of harvest varied widely across year, treatment, and harvest, and there were many complex interactions (Table 1). Plant height ranged from 3.0 to 21.4 inches for individual plots. The overall average plant height at time of harvest was 11.4 inches. Looking at year 1, plant height did not vary significantly at harvests 1 and 4 but varied erratically at harvests 2 and 3. In year 2, plant heights varied significantly across treatments at all harvests (Fig. 4).
Analysis of variance table showing sources of variation in plant height, plant density, and fresh weight yield of ‘Red Russian’ kale sown 16 Oct. 2014 and 2015 in 16 × 32-ft (4.9 × 9.8 m) high tunnels and harvested four times by four schedules: 8, 16, 20, and 24 weeks after sowing; 10, 17, 21, and 25 weeks after sowing; 12, 18, 22, and 26 weeks after sowing; and 14, 19, 23, and 27 weeks after sowing at Alcalde, NM.
Plant height by treatment across year and harvest of ‘Red Russian’ kale yield sown in 16 × 32 ft (4.9 × 9.8 m)–high tunnels on 16 Oct. 2014 (year 1) and 2015 (year 2) and harvested according to four harvest schedules: treatment A (8, 16, 20, and 24 weeks after sowing), treatment B (10, 17, 21, and 25 weeks after sowing), treatment C (12, 18, 22, and 26 weeks after sowing), and treatment D (14, 19, 23, and 27 weeks after sowing). Where significant differences exist, means with the same letter within a grouping are not significantly different. Least square means (LSMEANS) were derived from SAS using a mixed-model analysis of variance (SAS version 9.4; SAS Institute, Cary, NC); 1 cm = 0.3937 inch.
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
Mean overall plant density was 8.3 plants/ft2 and ranged from 2.3 to 15.0 plants/ft2, and there was significant interaction between year, treatment, and harvest. Although plant densities were generally higher in year 2 (8.8 vs. 7.1 plants/ft2), this was not significant by itself. Plant density varied significantly across treatments only at harvest 3 in year 2, and otherwise varied without an apparent pattern. The plant densities appeared to rise and fall across harvests, and this could be due to counting errors, plant death, and/or late germination of seeds, and to the shifting of the 3-ft sample row length toward one end of the plot or the other. To avoid this in future studies, a standard sample area could be marked with small stakes to reduce error.
Examining fresh weight yield, there was not a significant difference in mean yield across years, but there was significant variation among treatments and harvests, and complex year × treatment, treatment × harvest, and year × harvest interaction. Although STH for treatment A was lowest in both years, it was only significantly lower in year 2. Mean STH for treatment A, across years, was 415 g/ft2 compared with 521, 550, and 579 g/ft2 for treatments B, C, and D, respectively. There was more variability across treatments at the first three harvests but no significant difference between treatments at harvest 4 (Table 2). Across all treatments, harvest 1 was the largest harvest, but freeze damage reduced the marketable yield from harvest 1 of treatments C and D [Fig. 5 (yield loss was not quantified, but damaged leaves were excluded from yield measurements)]. This damage seemed to be most prominent on leaves that were directly contacting the rowcovers. Across treatments, the mean second harvest was significantly lower in year 1; in year 2, it was similar to harvests 3 and 4. For treatment A, harvest 3 was consistently the smallest harvest. A relatively larger harvest 1 was correlated with relatively smaller later harvests, but smaller initial harvests were correlated with larger subsequent harvests.
Mean fresh weight yield of ‘Red Russian’ kale by harvest and season sown 16 Oct. 2014 and 2015 in 16 × 32 ft (4.9 × 9.8 m)–high tunnels for four harvests and season total harvest (STH) for 2014–15 (Year 1) and 2015–16 (Year 2) at Alcalde, NM.
Example of freeze damage on ‘Red Russian’ kale leaves noted in both year 1 (2014–15) and year 2 (2015–16) of this study (photo from 6 Jan. 2015). We excluded damaged leaves from marketable yield.
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
The second harvest cycle occurred with the coldest temperatures, the shortest photoperiod, and the lowest light intensity. Lefsrud et al. (2005, 2006) reported a linear increase in kale fresh matter yield as light levels increased from 125 to 620 µmol·m−2·s−1 and likewise as photoperiod increased from 6 to 24 h. It is reasonable that the lowest yields occurred during this time period. As temperatures warmed, photoperiod became longer and light more intense; however, the harvest interval became shorter, but yields were about the same. For example, treatment A averaged 98 g/ft2 for harvest 2, grown over 8 weeks, but produced an average of 104 g/ft2 for harvest 4 after only 4 weeks (Table 2).
Our yields were greater than 136 to 168 g/ft2 reported in Kentucky (Ward and Bomford, 2013), the 168 g/ft2 reported in Tennessee (Sandhu and Nandwani, 2017), or 204 g/ft2 (with heavy rows covers) to 354 g/ft2 without rowcovers in Kentucky (Wyatt, 2018), but New Mexico, and the southwestern United States in general, experiences more winter insolation than those locations. These yield estimates may not apply to other cultivars of kale, but in a Colorado study, Red Russian yielded more than Dinosaur and Winterbor (Yoder, 2014). Compared with ‘Bloomsdale Longstanding’ spinach (Spinacia oleracea) grown in the same soils and in the same structures before this study, fresh weight yields were greater and were distributed over a harvest season that both began earlier and ended later (Heyduck et al., 2019). In another winter study at Alcalde, ‘Red Russian’ kale out-yielded ‘Bloomsdale Longstanding’ spinach 442 to 201 g/ft2 when grown together in the same high tunnel (Cormier et al., 2020).
For all treatments, the first harvest was the largest contribution to significant differences in season total harvest (Table 2). These all had harvests after 8 to 14 weeks of growth, including the warmer and longer days of October. Because yields never rebound to these levels in subsequent harvests, it may make sense to begin harvesting in the fall 6 to 8 weeks after sowing, sacrificing some of the larger initial harvest for an earlier, although smaller, harvest, and to give longer periods of regrowth in midwinter. Figure 6 shows the seasonal distribution of yield under the conditions and schedule that we used. The market farmer is not under the same constraints of harvesting a set area composed of randomized plots that we used in this trial and could tailor harvested area to market needs based on plant development and growth.
Seasonal distribution of ‘Red Russian’ kale yield sown in 16 × 32 ft (4.9 × 9.8 m)–high tunnels on 16 Oct. 2014 (year 1) and 2015 (year 2) and harvested according to four harvest schedules: treatment A (8, 16, 20, and 24 weeks after sowing), treatment B (10, 17, 21, and 25 weeks after sowing), treatment C (12, 18, 22, and 26 weeks after sowing), and treatment D (14, 19, 23, and 27 weeks after sowing). Yield is expressed as fresh weight per 60-ft2 (5.6-m2) harvest area, which is the total area harvested at each harvest interval for each treatment; 1 kg/60 ft2 = 0.1794 kg·m−2 = 0.5879 oz/ft2.
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
Temperatures.
Daily maximum and minimum air temperatures in the high tunnel and under the rowcover (°F) during the trial period (16 Oct. 2014 to 28 Apr. 2015 and 21 Oct. 2015 to 21 Apr. 2016) are displayed in Fig. 7. Average temperature in the high tunnel and under the rowcover in season 1 was 54.2 °F while outside ambient air temperature for the same period was 41.7 °F. In year 2, these averages were 53.1 and 47.9 °F, respectively. Inside the high tunnel and under the rowcover, maximum air temperatures reached 113.6 °F in season 1 and 112.8 °F in year 2. Uncovered interior air temperature maximums were only slightly lower at 112.7 and 105.0 °F in years 1 and 2, respectively. In both years, temperatures over 100 °F were recorded in all months except October. The extreme temperatures may have hastened bolting in the later part of the growth cycle and may have reduced growth and yield at harvest 3 and 4. Lefsrud et al. (2005) reported reduced growth of ‘Winterbor’ kale at temperature between 77 and 86 °F, even as lutein, β-carotene, and chlorophyll concentrations increased through this temperature range.
Minimum (Min) and maximum (Max) daily air temperatures in the growing environment of ‘Red Russian’ kale sown 16 Oct. 2014 and 2015, 1 ft (0.3 m) above soil level under the rowcover in the high tunnel from 16 Oct. 2014 to 28 Apr. 2015 (Y1) and 21 Oct. 2015 to 21 Apr. 2016 (Y2). Temperatures were logged every 30 min, and daily minimum and maximum were acquired by preselected data filter; (°F – 32) ÷ 1.8 = °C.
Citation: HortTechnology hortte 30, 5; 10.21273/HORTTECH04584-20
In year 1, the absolute minimum outside air temperature was 2.2 °F, while in year 2, it was 6.9 °F. On these cold nights, temperatures under the rowcover were 22.4 and 13.7 °F, respectively. Year 1 had 7 nights with outdoor minimums below 10 °F, whereas season 2 had only 5, but undercover temperatures were generally lower in year 2. This, coupled with several days with high temperatures below 60 °F, may have had some correlation with lower yields at harvest 2 that year. In year 1, temperatures under the rowcover averaged 4.9 °F warmer than outside ambient temperature, but in season 2, the temperature under the rowcovers was, on average, 12.2 °F warmer than ambient outdoor temperature.
Conclusions
Our study showed that the harvest schedule for ‘Red Russian’ only significantly affected season-long yield in the second year of the study. Uneven or late germination and delayed growth caused a yield penalty on the first harvest for treatment A, thus reducing season-long yield, although it did not appear to affect later harvests of the same units. Kale grown in high tunnels during the winter in northern New Mexico yielded 415 to 579 g/ft2 over the length of a season, and similar yields may be possible with other varieties of kale. Total seasonal yield from the entire cropped area of the high tunnel (240 ft2) ranged from 122 to 127 kg yielding from 2.6 to 17.5 kg per harvest (60 ft2 per harvest) throughout the harvest season. High tunnels with roll-up sides and larger end doors allow more flexible temperature controls, and more careful attention to venting to avoid extreme temperatures could allow earlier sowing, prolong the harvest season, and improve crop quality.
Units
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