Location, dates, and edaphic and weather conditions.

The experiment was conducted as late spring plantings in 2017 and 2019 at the University of Rhode Island’s Gardiner Crops Research Center in Kingston (lat. 41°N, long. 71°W). In 2017, the transplanting date was 5 May, and lettuce was harvested on 6 July. In 2019, seedlings were transplanted to the experimental plots on 29 Apr., and lettuce was harvested on 26 June. The field soil is sandy loam, with a pH of 5.9 and 3.6% organic matter. The air temperatures during the 2017 growing season ranged from a minimum of 6.9 °C in May to a maximum of 26.6 °C in June. In 2019, air temperatures during the study ranged from 3.3 °C in May to 31.1 °C in June. The mean air temperatures for the growing season were 18.5 °C in 2017 and 16.7 °C in 2019. Cumulative solar radiation during the experiment was 1197 MJ·m⁻²·d⁻¹ in 2017 and 1069 MJ·m⁻²·d⁻¹ in 2019. Precipitation during the experiment totaled 7.1 inches in 2017 and 8.23 inches in 2019.

Crop production methods.

On 23 Mar. 2017 and 21 Mar. 2019, seeds of two full-size romaine lettuce cultivars, Ridgeline and Coastal Star, were seeded into 102-cell plug trays with a cell volume of 22 cm³ (Landmark Plastics, Akron, OH) filled with a bark and peatmoss-based greenhouse medium with starter nutrients (Metromix 510; Sun Gro Horticulture, Agawam, MA). ‘Ridgeline’ is best suited for spring and autumn production. ‘Coastal Star’ is a heat-tolerant cultivar developed for organic production systems. Seeds of these cultivars were obtained from Johnny’s Selected Seeds (Albion, ME). In 2017, the planted trays were kept for 10 d in the greenhouse at 25 °C; then, transplants were grown in the high tunnel for 30 d. In 2019, seedlings were grown in the greenhouse at 25 °C for 30 d and hardened outdoors for 7 d before transplanting. On 5 May 2017 and 29 Apr. 2019, plants were transplanted to raised beds. Beds were 115 ft long, 30 inches wide, and 7 inches high, and were a nominal 5 ft apart on center. Before making raised beds, fertilizers were broadcast and incorporated in the soil to provide 96 lb/acre nitrogen (N), 35 lb/acre phosphorus (P), and 58 lb/acre potassium (K). Each bed had two lines of drip tape with 12-inch emitter spacing and a 0.25-gal/h flow rate (Chapin BTF; Jain Irrigation, Watertown, NY) and was spaced 10 inches apart underneath the mulch. In 2017, the field was irrigated four times for 6 h each time. A total of 18.5 m³ of water was applied. In 2019, the field was irrigated three times for 6 h each time, and the total volume of water was 13.9 m³. Soil temperature data loggers were placed between plants in the center of each subplot at 2 inches below the soil surface. Air temperature loggers were installed at 10 inches above the surface in one subplot per replication. In 2017, the data loggers were iButton units (Maxim Integrated, San Jose, CA); in 2019, they were RC-51H units (Elitech Technology, Milpitas, CA). Air and soil temperatures were recorded at 4-h intervals. Plants were irrigated immediately after transplanting and as needed during growth. Experimental plots in 2017 were fertigated on 12 and 23 May. In 2019, fertigation was conducted on 24 May and 5 June. During both years, a water-soluble fertilizer (Aquatrols, Paulsboro, NJ) was used to provide 25.3 lb/acre N, 9.0 lb/acre P, and 18.9 lb/acre K.

Experimental design and mulching materials.

Our experimental design was a split plot with four replications. Mulch treatments were applied to the main plots and cultivars were applied to subplots. The main plots were 8 m long; subplots were 4 m long and 0.75 m wide. Each plot had two rows of lettuce 40 cm apart. In-row spacing between plants was 30 cm (8.3 plants/m²). The main plots were separated by an unplanted buffer 60 cm long, and the entire experiment was surrounded by a buffer planted to romaine lettuce. Main plot treatments were bare ground, compost mulch, black polyethylene mulch, and shredded leaf mulch. The compost was produced from a mixture of feedstocks (animal manure, wood chips, leaves, fish scraps, shellfish waste, seaweed, spent mushroom substrate, food scraps, coffee grounds, and wood ash) in outdoor-turned windrows using an 18-month process (Earth Care Farm, Charlestown, RI). The nutrient content of oven-dried compost was estimated at 1.3% N, 0.11% P, and 1.0% K in 2017; in 2019, the content was estimated at 1.0% N, 0.08% P, and 0.12% K. Locally sourced deciduous tree leaves collected in Autumn 2016 and Autumn 2018 and stored outdoors in uncovered piles were air-dried and shredded to make shredded leaf mulch. Both compost and shredded leaf mulches were applied to a depth of 2 inches. Both low-nutrient composts such as the one used in this study and aged deciduous tree leaves have been shown to have minimal effects on soil nutrient status when used as a short-duration surface mulch (Schonbeck and Evanylo, 1998b; Valenzuela-Solano and Crohn, 2006). Standard 1.0-mil micro-embossed black polyethylene mulch (Berry Plastics, Evansville, IN) was laid using a mulch layer (2550; Rain-Flo Irrigation, East Earl, PA). In both years, weeds in the bare ground plots were controlled by hand. Bare ground plots were weeded on 11 and 24 May 2017, 20 May 2019, and 3 June in 2019. Alleys and buffers were mulched with locally sourced aged wood chips in 2017. In 2019, landscape fabric (UltraWeb 3000; Dewitt, Sikeston, MO) was used to prevent weeds in alleys.

Data collection methods.

Data regarding canopy size, yield, leaf area index, plant height, and plant dry weight (shoot only) were collected. Canopy size was calculated using a variance of the method of Bumgarner at al. (2012). Every 10 d during vegetative growth (7 May to 19 June 2017 and 29 Apr. to 20 June 2019), digital images were obtained using a white quadrat (35 × 35 cm) placed around two randomly selected plants in the middle of each subplot. Images were obtained from a height of 60 cm above the quadrat, perpendicular to the ground. The percentage of the green cover was calculated using Caneopeo (Patrignani and Ochsner, 2015) and converted to an area based on the quadrat area. Lettuce was harvested at the final stage of head development on 6 July 2017 and 26 June 2019. Leaf area was measured for two marketable plants randomly selected at harvest time. The leaves were stripped from the stem and spread in a single layer inside a 1-m² quadrat; leaf area was measured using digital imagery and Caneopeo. The leaf area index (LAI) was calculated from the measured leaf area for each plant. To measure dry weight, leaves were cut into small pieces and dried in a 43 °C oven until there was no change in weight.

Data analysis.

Single measures data were analyzed using PROC GLM analysis of variance (ANOVA) and Duncan’s multiple range tests in SAS (version 9.4; SAS Institute, Cary, NC). Growth rate and soil temperature data were analyzed using PROC MIXED multivariate repeated-measures ANOVA with year, cultivar, and mulch as fixed effects, block as a random effect, and date as the repeated effect. Compound symmetry covariance was used with the repeated effect when analyzing the growth rate to address structured unequal variance across dates. Levene’s test for homogeneity of variance was used to compare variances across mulch treatments for soil temperatures. Graphs were created using the software packages Microsoft Office Excel (version 2016; Microsoft Corp., Redmond, WA) and Veusz 3.1 (Sanders, 2019).

Effects of year.

The year effect was significant for all measurements. This was expected because weather conditions vary widely from year to year in southern New England. The year × mulch effect was significant for soil temperature, canopy size, LAI, and yield; therefore, results are presented by year below. The year × cultivar effect was not significant. Plants were larger in 2019 than in 2017 for all parameters measured (Table 1). Growth occurred significantly faster in 2019 than in 2017, particularly during the first 30 d after transplanting (Fig. 1)

Table 1.Effects of year on soil temperature, leaf area index (LAI), dry weight, plant height, and head diameter across all mulch treatments and two romaine lettuce cultivars.

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Effects of mulch type on soil temperatures.

Mulch treatment effected both the mean soil temperature and diurnal fluctuation in soil temperature. In 2017, mean root zone soil temperatures ranged from 16.9 °C in plots with shredded leaf mulch to 18.0 °C in plots with black polyethylene mulch (Table 2). Variance ranged from 19.5 in the black polyethylene mulch treatment to 13.6 in the shredded leaf mulch treatment. Black polyethylene and compost mulches resulted in similar mean soil temperatures, but plots with compost mulch had significantly (P < 0.001) lower variations near the mean. Shredded leaf mulch had the greatest ability to moderate soil temperature changes, resulting in both the lowest mean soil temperature (16.9 °C) and significantly (P < 0.0001) lower variance. Mean soil temperature in the bare ground plots was similar to that of all other treatments, and variance was similar to that in plots mulched with black polyethylene.

Table 2.The effect of mulch type on soil temperature at a depth of 2 inches (5.1 cm) below the soil surface in 2017 and 2019. The treatment × year interaction was significant (P = 0.032).

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As in 2017, mean root zone soil temperatures in 2019 were significantly (P < 0.01) different between plots, ranging from 17.0 °C in the bare ground plots to 18.9 °C in plots with black polyethylene mulch (Table 2). The black polyethylene and compost treatments were significantly warmer than the bare ground and leaf mulch treatments. Interestingly, in 2019, the black polyethylene and compost mulch treatments also had similar variance, with the variance in the black polyethylene mulched plots being significantly (P = 0.002) lower than that in the bare ground plots. The bare ground treatment had the highest variance in 2019. As in 2017, the shredded leaf mulch resulted in significantly lower variance than any of the other treatments (P < 0.0001).

Both mean root zone temperature and variance were similar across years in the bare ground and treatment. Mean root zone temperature was similar across years in the shredded leaf mulch treatment, but variance was significantly (P < 0.0001) lower in 2019 than in 2017. In contrast, the mean root zone temperatures were significantly (P < 0.05) higher in 2019 than in 2017 in the compost and black polyethylene mulch treatments, and variance was lower. Average air temperatures during the experiment in 2019 were 1.8 °C cooler than in 2017, and conditions were wetter, which would be expected to decrease soil temperatures. In fact, the mean soil temperature in the bare ground treatment was slightly lower in 2019, although the difference was not statistically significant. However, soil temperatures increased in the mulched treatments and when means were compared by years across all treatments (Table 1). This may be due to the use of black landscape fabric between beds rather than light-colored wood chips (Fig. 2). The landscape fabric likely raised temperatures in the alleys on sunny days, whereas the woodchips likely acted as an insulating layer, suppressing soil warming in the alleys.

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Overall, the mean temperatures of the soil in the plastic mulch and compost treatments were warmer than in the shredded leaf and bare ground treatments, suggesting that the dark-colored compost absorbed solar radiation like the plastic mulch (Fig. 3). Kamal and Singh (2011) and Schonbeck and Evanylo (1998a) similarly reported that the highest soil temperatures occurred under black polyethylene mulch in their experiments. Schonbeck and Evanylo (1998a) reported that black paper mulch and dark-colored compost mulch both increased soil temperatures in the morning but decreased them in the afternoon relative to bare ground. The narrower range and lower variance for shredded leaf mulch (Table 2) suggest that the mulch reduced absorption of solar radiation during the day and radiative heat loss from the soil at night. In effect, leaf mulch stabilized soil temperatures rather than altering the mean temperature. These results agree with our results of spring high tunnel romaine production (Gheshm and Brown, 2018). Other researchers have reported that organic mulch can stabilize the root zone temperatures in summer extremes and slow cooling in the fall, but note that the effect of mulching on soil temperature entirely depends on the type of mulch (Kosterna, 2014).

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All types of mulches were effective for suppressing weeds; no weeds emerged in the black polyethylene and compost treatments. Some weeds emerged through the shredded leaf mulch, but there were too few to require control. The bare ground plots required hand weeding twice during the growing season.

Effects of mulch type on lettuce canopy size.

The repeated measures analysis showed that there were significant differences (P < 0.0001) among treatments in both years. Black polyethylene and compost mulches resulted in a significantly larger canopy size than bare ground and shredded leaf mulch in both years. In 2017, there was no difference between black polyethylene mulch and compost. Bare ground and shredded leaf mulch were both significantly different from all other treatments (P < 0.001), with canopy size larger in bare ground plots than in plots mulched with shredded leaves (Fig. 4). Forty days after transplanting in 2017, more than 85% of the soil was covered by the lettuce canopy in both compost and black polyethylene mulch treatments. In bare ground and shredded leaf treatments, just 76% and 62% of ground area were shaded by lettuce canopy (Fig. 4). In 2019, black polyethylene mulch and compost again resulted in similar canopy sizes. However, bare ground plots had a canopy size similar to that of compost plots (P = 0.13) and significantly larger canopies than plots mulched with shredded leaves (P = 0.04) (Fig. 4). Forty days after transplanting, more than 95% of the soil was covered by the canopy in the black polyethylene and compost treatments. On that date, the lettuce canopy covered 88% and 78% of the soil area with the bare ground and shredded leaf mulch treatments, respectively (Fig. 4). In 2017, shredded leaf mulch plots had the coldest root zone temperature; however, in 2019, the bare ground plots had the lowest root zone temperature (Table 2). The lettuce growth rate is highly correlated with both accumulated soil degree days (SDD) and accumulated photosynthetically active radiation (PAR); the growth rate is exponential, especially in the heading stage (Salomez and Hofman, 2007; Wurr et al., 1981). Higher soil temperatures accelerate leaf enlargement and development and assist the crop in reaching the maximum leaf area quickly (Dufault et al., 2009).

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Effects of mulch type on lettuce leaf area, yield, and plant size.

In 2017, shredded leaf mulch significantly (P < 0.01) reduced LAI relative to the other treatments. Black polyethylene mulch significantly increased leaf area over bare ground, whereas compost was similar to both black polyethylene mulch and bare ground (Table 3). In 2019, LAI values were similar for plots mulched with black polyethylene and with compost, and both treatments significantly (P < 0.0001) increased LAI over bare ground and shredded leaf mulch. (Table 3)

Table 3.Effects of mulch type on leaf area (LAI) and yield of romaine lettuce grown in the field during Spring 2017 and Spring 2019.

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In 2017, the compost treatment had the highest yields, averaging 6.68 kg·m⁻² (Table 3). Black polyethylene mulch was similar at 6.38 kg·m⁻². Plots mulched with shredded leaves had significantly (P < 0.01) lower yields than the other treatments, averaging only 4.96 kg·m⁻². Bare ground yields averaged 5.94 kg·m⁻², which was significantly (P < 0.05) lower than the compost treatment yields but similar to that of black polyethylene yields (Table 3). In 2019, the black polyethylene mulch treatment produced the highest yields, but yields in the compost treatment were similar. Both treatments significantly (P < 0.0001) out-yielded the bare ground and leaf mulch treatments, which were similar to each other (Table 3).

Data for dry weight, height, and head diameter were pooled across years because the mulch × year interactions were not significant. Plants in the compost and black polyethylene treatments averaged 0.33 kg·m⁻² of dry matter, which was significantly (P < 0.0001) more than the averages of 0.29 and 0.27 kg·m⁻² for bare ground and leaf mulch plots, respectively (Table 4). The average height for plants grown on black polyethylene mulch was 38.2 cm, which is only 0.4 cm taller than plants grown with compost mulch. Both treatments were significantly (P < 0.0001) taller than plants grown with leaf mulch or on bare ground (Table 4). Compost mulch produced the greatest head diameter, averaging 25.25 cm, which is slightly more than that of black polyethylene mulch. Plants in both treatments were significantly (P < 0.0001) larger than plants in either bare ground or shredded leaf mulch treatments. Bare ground and shredded leaf mulch were similar at 23.9 and 23.2 cm, respectively (Table 4).

Table 4.Effects of mulch type on dry weight, plant height, and head diameter of late spring romaine lettuce grown in the field in Rhode Island.

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Cultivar effects.

Cultivar effects were consistent across years, and data have been pooled. All mulch treatments affected both lettuce cultivars similarly, with no significant interaction effects. ‘Ridgeline’ grew significantly (P = 0.012) faster than ‘Coastal Star’, and plants were larger at harvest (Fig. 5). At the last sampling, ‘Ridgeline’ covered 88.6% of the plot, which is significantly (P = 0.046) more than the 80.1% covered by ‘Coastal Star’ (Fig. 5). Cultivars differed significantly (P < 0.01) regarding dry weight, plant height, and head diameter. Yields differed significantly at α = 0.1, but not at α = 0.05 (P = 0.098) (Table 5). ‘Ridgeline’ was taller, had a larger diameter, and had a greater dry weight than ‘Coastal Star’.

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Table 5.Effects of romaine lettuce cultivar on leaf area index (LAI), yield, dry weight, plant height, and head diameter in the field. Data are pooled across years.

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Vargas et al. (2017) found no interactions between mulch and cultivar in romaine lettuce for characteristics like heart diameter, shank diameter, leaf height, and leaf width. Lee et al. (2015) found a significant interaction between temperature and cultivar effects on yields of lettuce grown in high tunnels over eight seasons. However, their study included cultivars from multiple market classes and examined effects of seasonal changes in air temperature rather than mulch-induced changes in soil temperature. Lee et al. (2015) also reported a range of responses to air and soil temperatures among lettuce cultivars, which might result in differing responses to mulches.