Site and experimental growing system.

Two experiments were conducted at the Horticulture and Crop Science Department Farm of the Ohio Agricultural Research and Development Center (OARDC) in Wooster, OH. The experiments were conducted simultaneously and followed duplicate methodological approaches. The experiments differed only in the setting in which they were completed: one experiment was conducted in a high tunnel, and the second was conducted outdoors in an adjacent open field. Both experiments were repeated in two spring and fall seasons in 2008–10.

Both experiments employed a split-plot design containing four replications per treatment. Treatments represented eight combinations of four microclimates and two cultivars also functioning as main and subplot factors. Eight wood-framed raised beds (2 ft × 8 ft × 6 inches) contained the four main plot microclimates: 1) control (no low tunnel or subsurface heating), 2) subsurface-heated with heating cable, 3) aerial-covered with low tunnel, and 4) subsurface-heated and aerial-covered. Within these main plots were four randomized subplots (2 × 2 ft) containing the two lettuce cultivars. Each of the eight microclimate × cultivar treatments was replicated four times in each experiment for a total of 32 subplots. The Fall 2008, experiments tested only ‘Outredgeous’ lettuce in the four microclimates, while all subsequent experiments tested two lettuce cultivars as subplot treatments. Also, in Fall 2008, a 13 × 40 × 6-ft single-layer 6-mil plastic tunnel was used, while all subsequent high-tunnel experiments were carried out in a 30 × 80 × 13-ft single-bay, gothic style, single-layer 6-mil clear polyethylene-covered high tunnel.

A woven black weed barrier (Hummert International, Earth City, MO) was placed under all raised beds and covered with a 1 to 1.5-inch layer of sand (Woodland Mulch, Kidron, OH). A second layer of weed barrier (3 × 9 ft) was stapled to the raised bed frame to contain the medium. The growing medium consisted of (v/v) 35% peat (Premier Horticulture, Quakerstown, PA), 35% dairy manure compost (OARDC), 15% shredded organic red clover hay (OARDC), and 15% silt loam field soil (OARDC). A mixer (model 12193; Bouldin and Lawson, McMinnville, TN) was used to completely blend the materials. Each main plot was filled with ≈5.2 ft³ of new medium (average 5.8 pH, 1.1% nitrogen, 19.0% carbon) for each experiment. Polystyrene foam dividers (Dow Chemical Co., Midland, MI) separated the subplots within each main plot.

Control plots consisted of unheated raised beds also lacking a low-tunnel cover. Root-zones in treatments 2 and 4 were heated with a 40-ft automatic electric heating cable (Wrap-On Co., Bedford Park, IL) under the rooting medium at about 4-inch depth and triggered to function at medium temperatures below 23 °C. The heating cable was attached to the upper layer of woven weed barrier at 3-inch spacing to provide even heating and prevent cable contact, and the operation of each cable was governed by a thermostat with its sensing unit in continual contact with the rooting medium at 4-inch depth. Treatment 3 main plots were covered by a single layer of 0.8-mil slitted polyethylene (Hummert International) stretched across four wire hoops and attached to the ground with landscape staples to create an 18 × 30-inch low tunnel. Treatment 4 main plots were polyethylene-covered and cable-heated as described earlier.

About 1000 preweighed primed and pelleted seeds of the two red leaf romaine lettuce cultivars, Outredgeous (Johnny's Selected Seeds, Winslow, ME) and Flagship (Shamrock Seeds, Salinas, CA) were sown in seven parallel rows at 3-inch spacing within each subplot, as recommended for baby leaf lettuce production. Seeds were evenly placed on the substrate and covered by hand on 9 Oct. 2008, 21 Mar. 2009, 10 Oct. 2009, and 15 Mar. 2010. All plots were overhead watered by hand as needed by microclimate treatment, generally twice daily with ≈1 L/subplot of clear water. No additional fertilizer was added.

Environmental data.

Air and soil temperatures were recorded continuously at 30-min (Fall 2008, Spring 2009) and 15-min (Fall 2009, Spring 2010) intervals in both experiments using data loggers (Hobo ProV2; Onset Computer Co., Bourne, MA). All main plots were equipped with a separate data logger, and sensors were protected with a radiation shield. Air and soil temperatures were recorded 8 inches above and 1 to 1.5 inches below the medium surface, respectively. Data from the monitoring of each main plot replication are summarized in Table 1. Periodic readings of photosynthetically active radiation (LI250-A; LI-COR Biosciences, Lincoln, NE) were also taken at canopy level in each experiment.

Table 1.

Shoot- and root-zone temperatures in high-tunnel and outdoor raised beds under four methods of microclimate modification in eight experiments in 2008–10 in Wooster, OH.

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Crop data.

Microclimate effects on plant growth and yield were measured at four developmental stages using destructive and nondestructive sampling. Data are presented for two of these stages. Early stand establishment was measured via nondestructive counts of emerged seedlings taken 5 d after sowing on 10 inches of the center of the third row in each subplot. Plant number and biomass were measured through plant samples collected ≈2 and 3 weeks after sowing (WAS). In this process, all plants were removed intact and by hand from a 10-inch section of a randomly chosen row (excluding edge rows 1 and 7) in each subplot, placed in a sealed plastic bag, and cooled (5 °C) until weighing. All plants from each subplot sample were counted and weighed in bulk. Thereafter, 10 representative plants were selected from the bulk subplot sample, and their roots and shoots were separated with a razor blade and weighed. Finally, yield was recorded ≈4 WAS (7 Nov. 2008, 22 Apr. 2009, 9 Nov. 2009, and 14 Apr. 2010) by collecting all above-ground biomass from a standardized 2-ft² section of each subplot. Leaves and stems were placed in plastic bags and stored on ice in coolers until being processed within 4 h of harvest.

Germination data were collected on a thermogradient table (model DB2000; Van Dok and De Boer, Enkhuizen, The Netherlands). Petri dishes (100 × 15 mm; Fisher Scientific, Waltham, MA) were lined with blotting paper (3.25-inch discs; Anchor Paper Co. St. Paul, MN) and filled with ≈40 mL of the above described medium. Twenty-five primed and pelleted seeds were evenly spread on the surface of the medium and moistened. Germination was tested at 10 discrete temperatures in 2 °C increments from 10 to 28 °C. A total of 80 petri dishes were placed on the thermogradient table in rows of eight, with each row containing four replications of each cultivar at each temperature. Emerged and viable seedlings were counted and removed from the petri dishes 3, 5, 7, and 10 d after sowing.

Data analysis.

The high-tunnel and outdoor experiments were not replicated in space; therefore, data from these experiments were analyzed separately. Data collected at 2 and 4 WAS were also analyzed separately within experiment to uncover potential treatment effects on growth through time. A Proc Univariate procedure was carried out to test for normality on all data. Normally distributed data were analyzed using untransformed data in Proc Mixed. Data with a nonnormal distribution were log-transformed before analysis (SAS version 9.2; SAS Institute, Cary, NC), a common step in the handling of data displaying a skewed distribution. Means were then back-transformed for inclusion in tables. Microclimate and cultivar were analyzed as fixed effects and replications within years were analyzed as random effects in Proc Mixed. Treatment means were separated using a pdiff difference statement at a P ≤ 0.05 level of significance when microclimate and cultivar fixed effects were significant at P ≤ 0.05.

High-tunnel experiments.

Yield in the high-tunnel setting was influenced by microclimate treatment in three of four experiments, the lone exception being Fall 2008 (Table 3). Interestingly, plant population recorded 2 WAS was unaffected by microclimate in Fall 2009 and Spring 2010 (Table 2) although treatment effects on biomass production were typically evident at the same and later harvest in 2009 and 2010 (Tables 2 and 3). Yield in the high-tunnel setting was clearly influenced by microclimate and typically lowest in uncovered and unheated plots. The relative impacts of root, shoot, and root- and shoot-zone heating on yield also varied by year, season, and crop stage. Still, the data clearly indicate most trends established by 2 WAS tended to hold at 4 WAS. Overall, these results indicate that a slitted polyethylene cover established at sowing, whether alone or in combination with root-zone heating, often resulted in the greatest biomass production.

Outdoor experiments.

Biomass production in the outdoor setting was influenced by microclimate treatment in all four experiments (Table 3) but, perhaps, in a manner more mitigated by plant population than in the high-tunnel setting. Plant populations recorded 2 WAS were lowest in uncovered, unheated control plots in three of four experiments; only the rank order and significance of treatment effects on plant population varied slightly by year and season (Table 2). Yield recorded 2 and 4 WAS displayed a similar trend (lowest in the control, variable rank order among the root and shoot alone heated treatments). However, as in the high-tunnel setting, root- and shoot-zone heating tended to result in the greatest yield. Interestingly, the relative impacts of root- and shoot-zone heating when practiced alone varied slightly among year, season, and crop stage. Still, the use of a slitted polyethylene covering alone resulted in statistically higher yield values than using root-zone heating alone in two of four experiments.

High-tunnel experiments.

Cultivar main effects on biomass production were significant only in Spring 2010 when ‘Outredgeous’ out-yielded ‘Flagship’ 2 and 4 WAS (Tables 2 and 3). A significant microclimate × cultivar interaction was detected for shoot fresh weight 4 WAS in Spring 2009 and plant population 2 WAS in Fall 2009 (Table 4).

Outdoor experiments.

As in the high-tunnel experiments, cultivar main effects on yield were less common than microclimate effects in the outdoor setting. ‘Outredgeous’ was greater than ‘Flagship’ yield 4 WAS in Spring 2009 and Spring 2010 (Table 3). A significant microclimate × cultivar interaction was detected only at 4 WAS in Spring 2010 (Table 4).

High-tunnel experiments.

Plant populations were affected by microclimate in only one of four high-tunnel experiments [Spring 2009 (Table 3)]. Plant populations were generally similar among microclimates, suggesting that conditions supported stand establishment and that differences in yield followed growth. These outcomes are supported by previous work suggesting that high-tunnel-field soil temperature differentials may exceed air temperature differentials during some seasons (Both et al., 2007; Knewtson et al., 2010). If true, stand establishment of direct-seeded lettuce could be promoted in high-tunnel systems during periods when low external temperatures would suppress it regardless of whether other strategies are applied.

Outdoor experiments.

Microclimate treatment, more than cultivar, clearly influenced plant populations recorded 2 WAS in all four outdoor experiments (Table 2). Treatment rank order differences tended to hold across experiments: covering, heating, or covering and heating raised beds generally resulted in greater populations compared to the control as would be suggested by germination data collected in a controlled environment (Fig. 1) but over the temperature range recorded during these experiments (Table 1). However, microclimate effects on population tended to be less consistent than microclimate effects on yield.

High-tunnel experiments.

High-tunnel systems are noted for their ability to shield crops against abiotic and biotic stress, especially when a second, interior layer of plastic is used near the crop, as in this study (Lamont, 2005; Lamont et al., 2003). Here, significant differences among four interior microclimates were noted in three of four experiments, Fall 2008 the exception. Air and soil temperature averages were highest in Fall 2008 (Table 1), suggesting that the constant heating capacity of the three treatments was unnecessary or detrimental to optimize growth in that experiment. The warmer high-tunnel environment with potentially lower air volume and circulation was also conducive to plant disease, an additional potential drag on biomass production.

Plant health was sometimes a factor in the high-tunnel experiments but less so in the outdoor experiments. Fungal disease, primarily Rhizoctonia, was occasionally noted in aerial and subheated plots within the high tunnel, an outcome reported as likely when air movement is reduced, higher temperatures prevail, and free moisture is consistently abundant in the root-hypocotyl zone (Grosch and Kofoet, 2003). Electric heating cables surrounded by a high organic matter rooting medium covered with a dense canopy required a consistently high level of soil moisture to be maintained, a condition favoring disease. Upon disease identification, ventilation was increased, and in the Fall 2009 and Spring 2010 experiments, etridiazole with thiophanate-methyl (Banrot 40% wettable powder; Scotts, Marysville, OH) was applied to limit disease progression. Variations in root-zone-heating method, more frequent ventilation, a rooting medium lower in organic matter, and/or other tactics may limit this weakness of our experimental system when applied elsewhere.

Outdoor experiments.

Microclimate modification resulted in greater yields, relative to control plots, in all outdoor experiments. Installing plastic covers over horticultural crops is known to increase growth and yield in multiple systems and crops (Nair and Ngouajio, 2010; Rekika et al., 2009; Soltani et al., 1995; Wells and Loy, 1985). However, this approach relies exclusively on passive solar and soil warming. The influence of air- or water-mediated active root-zone heating is most often studied in growth chambers or greenhouses (Economakis, 1997; Elwell et al., 1985; Li et al., 1994; Shedlosky and White, 1987). Fewer recent studies (Bumgarner et al., 2012; Hunter et al., 2010) have capitalized on earlier reports (e.g., Cameron Brown and Gray, 1957; Canham, 1952; Rykbost et al., 1975) and considered the impacts of direct soil heating in differently engineered low- and high-tunnel systems, especially in temperate climates.

The experimental system used here permitted direct comparisons of three microclimates and a control group in outdoor and high-tunnel settings. Outdoors, aerial-covered and subheated plots produced up to 40 times more above-ground biomass than control or other experimental plots. Covering raised beds or heating the root-zone they contain separately also increased yield relative to no action but to lesser extents than combining the two steps. Data reported here suggest that the on-farm value of covering or subheating alone or in combination be examined thoroughly and that such tests account for potential shifts in irrigation and/or disease management that the strategies may involve. Relative to no intervention, covering and subheating appear to be required to maximize fall and spring outdoor, raised bed baby lettuce yield in most years under the experimental conditions experienced here.

Seasonal environmental conditions unquestionably alter the impact of crop management techniques. However, unlike in the high-tunnel experiments, spring and fall seasonal yield averages tended to be similar in outdoor experiments. Interestingly, the average air and soil temperatures for the two lowest yielding outdoor experiments were similar to those recorded for the two higher yielding experiments (Table 3). This observation corroborates earlier assertions that temperature effects on crop growth and tissue composition vary both with the crop stage at which certain temperatures are experienced and the mean temperature of the entire study period (Bumgarner et al., 2012; Gazula et al., 2005; Kleinhenz and Wszelaki, 2003; Radovich et al., 2005). Direct-seeded, quick-cycling crops such as baby lettuce may respond strongly to dynamic environments involving rapid and potentially extreme temperature and light fluctuations over narrow timeframes influencing germination and growth. Growers and scientists accept the challenge of identifying microclimate management techniques that are adaptable and consistently effective and economical. Here, we provide further evidence that root- and shoot-zone microclimate modification can enhance lettuce biomass production, especially when used simultaneously in outdoor growing systems.

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