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

Final Report for LNE10-293

Project Type: Research and Education
Funds awarded in 2010: $117,360.30
Projected End Date: 12/31/2014
Grant Recipient: University of Rhode Island
Region: Northeast
State: Rhode Island
Project Leader:
Dr. Rebecca Brown
University of Rhode Island
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Project Information


This project examined alternative strategies for protecting soil health in intensive vegetable production systems. The strategies were 1) zone tillage into a killed rye mulch; 2) permanent beds with a perennial living mulch; and 3) intercropped crimson clover. They were compared to the standard practice of intensive tillage and a winter rye cover crop in a three year study with three replications that assessed effects on yield, soil health, and economics. The vegetable production system included tomatoes, lettuce, carrots, melons/cucumbers, and Brassicas (broccoli and cabbage).

None of the strategies significantly increased yield relative to the standard practice for any of the crops. The killed rye mulch insufficiently suppressed weeds, and the zone tillage treatment gave the lowest yields for all crops in all years. Zone tillage yields ranged from 0 to 91% of standard practice yields, and never achieved satisfactory levels. The perennial living mulch treatments produced yields similar to the standard practice for crops grown on plastic mulch (tomatoes in 2010, cucumber in 2011 and 2012, and melons in all three years) but plastic had to be laid by hand to avoid damaging the living mulch, cancelling out any economic benefits of this practice. Crops grown without plastic mulch showed mixed results, with carrot yields ranging from 70% to 96% of the standard, lettuce yields from 21% to 113% of the standard, Brassica yields from 4% to 107% of standard, and tomato yields from 31% to 79% of the standard. The crimson clover intercrop severely suppressed yields of direct-seeded cabbage in 2011, but otherwise gave yields comparable to the standard practice across all crops and years. Competition from weeds was the primary cause of reduced yields.

Soil health was quantified using the Cornell Soil Health test on an annual basis from 2010 – 2013, and by measuring soil microbial activity and nitrate levels on a biweekly basis from May through September of 2011 and 2012. The standard practice resulted in a 15% decrease in soil organic matter over the three years and a decrease in aggregate stability, but soil water holding capacity increased and overall soil health was not changed. Soil organic matter levels remained constant in the zone tillage plots, and overall soil health increased by 12%. Results were similar for the perennial living mulch plots, with overall soil health increasing by 11%. Intercropping with crimson clover gave results similar to the standard practice with the exception that the loss of soil organic matter was decreased to only 10% over three years. Soil microbial activity was significantly higher in the zone tillage and living mulch plots than in the standard, and was slightly higher in the crimson clover plots. Nitrate levels peaked in June and July of each year, and were significantly higher in the crimson clover treatment than in the other treatments. The zone tillage and living mulch plots had the lowest nitrate levels; in 2012 they were significantly lower than the standard tillage treatment.

None of the alternative practices proved to be economically viable alternatives for improving soil health in market vegetable production, primarily due to difficulties controlling weeds in the reduced-tillage systems. However, crimson clover slowed the loss of soil organic matter, and has potential for use with transplanted vegetables if stand establishment can be improved. Over 300 people attended the twilight meetings and workshops featuring results of this project. In April 2014 a web-based survey was sent to 127 attendees who provided email addresses; 32 people completed the survey. Sixteen farmers indicated that they had increased their total acreage of cover crops as a result of this project, and 12 farmers added new species to their mix. Seven of these farmers had both increased their acreage and added new species, and six farmers were using intercropping. Eight farmers had tried zone tillage, and five were using permanent beds to reduce tillage.


This project was developed in response to the needs of direct-market vegetable producers in southern New England. Southern New England leads the nation in the importance of direct market production to the agricultural economy, but land is very limited. The vegetable production systems are intensive and farmers rely on tillage to incorporate residue, prepare seed beds, and control weeds; high crop diversity discourages use of herbicides. Organic matter is key to soil health, and repeated tillage depletes soil organic matter. The standard winter rye (Secale cereale) cover crop does not always fit into the vegetable cropping system, and even when it does, biomass production is insufficient to maintain soil organic matter levels. Longer cover crop rotations may not make economic sense on high value peri-urban land.

Performance Target:

Twenty vegetable producers (10% of Rhode Island total) will adopt a combination of cover crops and reduced tillage to control weeds and improve soil quality on a total of 500 acres (25% of RI vegetable acreage). Quarterly twilight meetings and workshops will attract 200 growers over three years; participants will learn about new vegetable varieties and production methods they can adopt on their farms. Fifty University of Rhode Island students pursuing careers in agriculture will receive hands-on learning opportunities by assisting with cover crop trials as part of the vegetable production class or as summer interns.


Click linked name(s) to expand
  • Emily Cotter
  • Heather Faubert
  • Ruth Hazzard
  • Jeff Pieper
  • Andy Radin
  • Carl Sawyer
  • Tim Sherman
  • Mina Vescera


Materials and methods:

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.


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 17th and 21st 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 27th 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.


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.

Research results and discussion:

  • 20-30 growers will attend each Twilight Meeting at the URI research farm. This milestone will be met repeatedly in years 1-3.

We conducted 10 twilight meetings over four years. Attendance ranged from 15 to 50 growers, and averaged 29.6 growers per meeting.

  • At least 50 growers will attend each winter workshop. This milestone will be met repeatedly in years 1-3.

Winter workshop attendance was 20 in year 1, 59 in year 2, and 43 in year 3. Attendance was lower than expected due primarily to difficulties in communicating and coordinating schedules between RI, CT, and MA. We relied on extension staff in MA and CT to advertise to their growers; this did not happen in year 1 so only RI growers attended the meeting. In year 2 we drew few RI growers, but many from CT and MA. In year 3 attendees were from all three states, but was low because our workshop was too close in time to a cover crops workshop sponsored by Cooperative Extension in CT.

  • 15 URI undergraduate students will participate in the cover crop research each year as part of the vegetable production class.

Students participated in the project in years 1-3, and the target was exceeded each year. In total 60 students participated in the project as part of their course work, and 5 graduate students, 15 undergraduate students, and 2 high school students assisted with the project during the summers.

  • By the end of the second year 50 growers will have contacted us for more information on cover crops/reduced tillage and individual assistance in implementing these practices on their farms.

We do not have good data on the number of growers who contacted us for more information, due to changes in extension staffing and the complete absence of a system for recording extension contacts at any level above the individual extension staffer. It is unlikely that the milestone was reached, in part because by the end of year 2 it was apparent to everyone that none of our methods for integrating cover crops into vegetable crops were effectively managing weeds. However, 25% of the growers who returned the final survey indicated that they had tried zone tillage, and 15% had tried permanent beds. We loaned zone tillage equipment to four growers, none of whom returned the survey.

  • The field research will generate new information on the relative efficacy of the four cover crop + tillage treatments for improving soil quality, controlling weeds, and maintaining profits.

The research was successful in generating new information on the relative efficacy of the four treatments. Research was completed in July 2013. Results have been published in a thesis and presented at scientific meetings; manuscripts are in preparation.

Participation Summary


Educational approach:

Over the course of the project we held ten twilight meetings and three full-day winter workshops. Eight of the twilight meetings were held at the research farm, and two were held at grower farms. Our typical twilight meeting lasts 3-4 hours, and includes a farm tour, a guest speaker, and dinner. Meetings were held in April, June, and August, with the meetings at grower farms being June meetings. The winter workshops were held in February of each year at the Old Sturbridge Village Education Center in Sturbridge, MA. Each workshop included 3-4 speakers and lunch. Attendance varied widely, but the growers and service providers who attended found the meetings worthwhile, and covercrop use does appear to be increasing. The April meetings seemed to be the most popular.

The research portion of this project has been published as an MS thesis, and peer-review journal articles are in preparation. None of the methods researched are really ready for farmer adoption at this time, so we have not released any extension publications.

Additional Project Outcomes

Project outcomes:

Impacts of Results/Outcomes

Twenty vegetable producers (10% of Rhode Island total) will adopt a combination of cover crops and reduced tillage to control weeds and improve soil quality on a total of 500 acres (25% of RI vegetable acreage). Quarterly twilight meetings and workshops will attract 200 growers over three years; participants will learn about new vegetable varieties and production methods they can adopt on their farms. Fifty University of Rhode Island students pursuing careers in agriculture will receive hands-on learning opportunities by assisting with cover crop trials as part of the vegetable production class or as summer interns.

Our performance target has three sections. Section 1 refers to adoption of new practices by growers. We met our target for getting growers to increase their use of cover crops, but not for tillage reduction, and we did not even come close to meeting the target for acreage. The reason this target was not met is that none of the tillage reduction methods trialed were effective at controlling weeds or compatible with plasticulture. As a result, they were not adopted by our larger growers, for whom soil health is secondary to economic sustainability.

Sections 2 and 3 of the performance target refer to education of growers and future growers. These targets were met and surpassed. We held 13 educational events, with a total attendance of over 400 people. The majority of event attendees were farmers or farm workers, but we also had many ag service providers and a few agriculture students attend. Sixty undergraduate students participated in the project as part of their course work in vegetable production, and 5 graduate students, 15 undergraduate students, and 2 high school students assisted with the project during the summers. Approximately half of the students were majoring in agriculture, with the others coming from a wide variety of majors. This project was also extremely effective at jumpstarting the sustainable agriculture extension program at URI, and making RI an active partner in the regional vegetable production extension effort. When this project was submitted in 2009 our entire extension program consisted of part of one person, whose time was funded by competitive grant dollars and who served as our state PDP coordinator as well as the extension partner for grants. We now have a person who is funded entirely on cooperative extension funding to serve as the extension partner on grants, a second person who serves as state PDP coordinator, and part of a third person who is grant funded, and we are working towards adding another grant-funded position.

Economic Analysis

Our plan was to collect data on inputs and labor required for the different systems, to determine if the reductions in costs for tillage and cultivation were sufficient to offset any reduced yields. However, after the first year it was clear that input and labor costs were similar across all the systems except zone till, and zone till yields were completely unacceptable. Thus we stopped collecting economic data, and focused on yield and soil quality.

Farmer Adoption

Farmers in Rhode Island are definitely interested in cover crops. Many of them would like to use organic no-till, but we have not been able to identify a system that works under our conditions. There is less interest in reduced tillage, and no interest in herbicide-based no-till. Most growers rely on a roto-tiller or a disc harrow for most of their tillage. Some growers have tried zone tillage, but none have actually adopted it as a practice because of issues with weed control, seedbed preparation, and access to equipment.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

More research is needed on intercropping cover crops with vegetables, particularly on seeding rates and strategies for controlling weeds.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.