Cover cropping strategies for year-round weed control on mixed vegetable farms in southern New England

Plot establishment. The study was established in the fall of 2009 on roughly one half hectare of Bridgehampton silt loam soil in Kingston, RI.  A randomized complete block design with four treatments and three blocks was used.  The individual treatments within the blocks were 10 X 30 m. Within each treatment, six 1.5 m wide rows were established. The two outside rows were maintained as buffers between plots, while the four interior rows were planted to vegetables. Each year the vegetable crops shifted one row to the east with the fourth row becoming the first to eliminate the use of the same beds for the same crops over multiple years.   Drip tape with 30 cm emitter spacing was used to irrigate each crop.  All vegetables received one line of drip tape except the carrot / lettuce row, in which two lines were used.

Treatments

Conventional (CT). Each fall, winter rye was planted at a rate of 123 kg·ha^-1. Seeding dates for each field season were the 20th, 21st, and 24th days of October (2009, 2010, and 2011), respectively. The following spring, the winter rye stand was incorporated using a moldboard plow. After plowing, the plots were disked twice.  Five planting beds, each 1.5 m in width, were created using the tractor wheelbase as aisles.  After planting, weeds were controlled with a tractor-mounted tiller for the empty rows, and an 8hp walk-behind tiller for the walkways and shoulders of the beds. Various hand-weeding tools were used for additional weeding around the crops.  Each replication was tilled a total of 3 times in 2011 and 4 times in 2012.  The melon/cucumber row was covered with black plastic mulch; all other crops were grown on bare ground.

Rolled crimped / zone builder (RCZB). Winter rye was planted at a rate of 123 kg·ha^-1 on the 20th and 21st days of October (2009 and 2010) respectively. In 2011, the seeding rate was increased to 184 kg·ha^-1 to increase cover crop biomass; planting occurred on October 24. The winter rye was rolled at anthesis using a front mounted roller crimper (I & J Manufacturing). After flattening the rye, a zone builder (Monroe Tufline™ 2S-24-60 subsoiler with Unverferth® zone strip coulters and roller basket) was used to make 30 cm wide planting beds within the treatment.  Each planting bed, except for the cabbage, was made with one tractor pass using a single shank set 30 cm into the ground. For the cabbage, two zones spaced 75 cm apart were made.

Perennial ryegrass and Dutch white clover perennial living mulch (PLM). In the spring of 2010, a mixture of turf-type perennial ryegrass (27.20 kg·ha^-1) and Dutch white clover (2.72 kg·ha^-1) was seeded in the aisles between four raised beds. This cover crop was designed to provide perennial soil coverage between the planting rows throughout the course of the three-year experiment. The 2010 tomato and all melon and cucumber plantings were covered with black plastic mulch to help control weeds, warm the soil, and preserve soil moisture. After harvest mulch was removed and the planting beds were rototilled with a walk-behind tiller and seeded with winter rye at a rate of 123 kg·ha^-1 using a drop seeder. The following spring, the rye was mowed and the beds were rototilled with a walk-behind tiller to prepare them for planting.  Each spring, the PLM treatment was reseeded as needed. In 2010 mulch was laid with a Rainflo II mulch layer; in 2011 and 2012 mulch was laid by hand.

Crimson clover annual living mulch (CC).  Each spring the CC plots were plowed and disked twice before planting the vegetable crops. Following vegetable crop planting, in June, the crimson clover was seeded throughout the plot. The crimson clover seed was mixed with pelletized lime at a 1:2 ratio and seeded at a rate of 25 kg·ha^-1 using a drop seeder. Vegetable and cover crop plant residue was left on the soil surface until the following spring when the plot was plowed and disked before being planted.  

Vegetable Crops

Tomato. All of the 2010 tomato transplants were provided by Confreda’s Greenhouses and Farms (Hope, RI). In 2011 and 2012, the tomatoes were grown in the URI greenhouse. Seeds were started on the 17^th and 21^st of April (2011 and 2012) respectively. The tomatoes were transplanted by hand during the last week of May (2011) and first week of June (2010 and 2012).  In 2011, sparrows in the greenhouse damaged several flats of transplants. To make up for the loss, new seeds were started on the 27^th of April, and additional tomato transplants were obtained from Confreda’s Greenhouses and Farms. The additional plants led to an increase in the number of varieties planted. In 2012, three varieties, ’Celebrity’, ‘Polbig’ and ‘Valley Girl,’ were chosen for production. Kocide® 3000 and Bravo Weather Stik® were used as needed to control fungal outbreaks. In 2012 Dipel® DF was used to control Lepidoptera species.

1. The first year of the study (2010), broccoli was directly seeded in the spring.  Late in the season, cross-striped cabbageworms became a large problem in the crowns of the crop. To prevent this problem from recurring, broccoli was replaced with cabbage for the 2011 and 2012 seasons.  The 2011 cabbage crop was hand seeded on the 3rd of June. A double row of 150 plants (300 plants total) spaced 30 cm apart was established in each treatment.  Heavy weed pressure in 2011 led us to grow transplants for the 2012 season. The 2012 cabbage crop was started in the URI greenhouse on the 15th of May, before being transplanted into two rows of 150 plants on the 18th of June.  The brassicas were sprayed with Dipel® DF as needed to control Lepidoptera species.

Melon / Cucumber. In 2010, several melon varieties including Sarah’s Choice, Honey Orange, Delicious 51 PMR, and Halona were planted.  In the following years, Diplomat (melon) and Marketmore (cucumber) were seeded into 38-cell trays at the URI greenhouse on the 5th and 7th days of May 2011 and 2012, respectively. For the 2011 and 2012 seasons, the row was divided into 6 subplots, alternating melon and cucumber down the row. A total of 54 melons and 42 cucumbers were planted per row in 2011, and 45 melons and 33 cucumbers per row in 2012. Both melons and cucumbers were planted with 60 cm between plants.

Lettuce / Carrot. In 2010, the 50 m row was equally divided into lettuce and carrot plantings. For 2011 and 2012, 15.5 m and 10 m of carrots were seeded, respectively. Carrot seeding was done using an Earthway® (1001 –B) seeder in 2010 and 2011, and a Jang (Jang Automation Co., JP-1) seeder in 2012. In all three years, four rows of carrots were seeded in each treatment. To seed the salad, two passes, each 15.5 m in length, were made using a Johnny’s six-row seeder. In 2011 and 2012, 10 m sections of lettuce were seeded to start the season. The initial seeding was followed by biweekly plantings that were 3 m in length.  Both crops were hand weeded and thinned as necessary.  A hand held weed burner was used to manage the succession planting areas in 2012. Each week, six meters of bed space was burned.  For each 3 m planting, weeds were burned twice prior to seeding.  After seeding, the beds were watered, and covered with floating row cover. Plots were watered daily until the first true leaf was visible.

Cornell Soil Health Test:

Soil samples were collected each April (2010, 2011, 2012, and 2013) prior to the start of the growing season. For the baseline sampling in 2010 a bulk sample was collected for each block prior to treatment establishment. In subsequent years, each treatment within each block was sampled individually.  All samples were sent to Cornell University’s Nutrient Analysis Laboratory (CNAL) for soil health test evaluations. The soil was collected using the “W” method as suggested by CNAL. At each sampling site a hole, 15cm deep, was made using a hand trowel. The loosened soil was stirred to maximize homogeneity and 600 mL were extracted and placed in a one gallon interlocking plastic bag. Samples were mixed and a 1L subsample was submitted for analysis.

Soil microbial activity:

Starting in 2011, biweekly soil samples were collected and analyzed for respiration using the Solvita soil health kit. Forty-eight sampling sites were established across the field, with each treatment having four sampling sites in each replication (12 total across all three replications). Within the treatments, two sampling sites were randomly selected in both the lettuce and tomato rows. Six soil core samples to a depth of 15cm were collected from each sampling site. The samples were combined in a paper bag, weighed and dried for 30 hours at 50 °C. The dried samples were weighed and then crushed using a mortar and pestle. The samples were then hand shaken through a 250 mm mesh sieve for twenty seconds, and a 40 g subsample was collected.  

A small cellulose filter was placed in the bottom of a 50 mL plastic beaker that had three holes drilled in the bottom. 25 mL of de-ionized water was added to a screw top, 250 mL glass jar. The 40 g of soil were then poured into the plastic beaker and the plastic beakers were placed inside the glass jars. Solvita low soil CO₂ paddles were placed in the soil and the jars were sealed. The jars were placed out of direct light in a temperature-controlled room at 21 °C. After twenty-four hours, the Solvita paddles were removed and read with the Solvita digital color reader.

Nitrate:

An additional 20 g of sieved soil was set aside for nitrate analysis. To extract the nitrate, 50 mL of a 0.04 M ammonium sulfate extractant was added to the 20 g of soil. It was stirred for 15 minutes and then filtered (Fisher Scientific P4) twice. Spectrophotometric nitrate analysis was performed using a 1:1 mixture of nitrate extract and vanadium (III) acid solution.  The samples were allowed to sit at room temperature for five hours before analysis at 540 nm using KC Junior Nitrate analytical software.

Statistical Analysis

The harvested crops were graded as marketable at farmers market or non-marketable, weighed, and counted. Yield results were evaluated for significant difference from the conventional (CT) treatment. Due to the high variability between replications within treatments, all yield data was transformed using log (x+1) to normalize the distribution. Yield, soil microbial activity, and nitrate data were analyzed using repeated measures ANOVA. Treatment means were separated (α = 0.05) using Fisher’s LSD. Differences between treatments with the Cornell Soil Health Test results were evaluated by percent change over the three-year study.