Final report for LNE22-451R
Project Information
This project investigated planting cover crops into standing late season vegetables, also known as interseeding. Leaving soils exposed over winter risks excessive sediment and nutrient loss from farm fields, and degradation of soil aggregates. Results from a 2021 survey about cover cropping practices on Maine farms (n= 27), showed that 77.8% of respondents (n=21) are limited in their ability to plant cover crops due to “late season cash crops coming out too late”. 74.1% of respondents (n= 20) stated that research-based data about interseeding in the Northeast would help them with decision making about late season cover cropping.
Jason Lilley from the University of Maine Cooperative Extension and Dr. Rachel Schattman of the University of Maine School of Food and Agriculture designed and managed a research project to investigate the feasibility of interseeding into late season vegetable crops in the Northeast. A grower advisory committee provided project input and guidance based on past experiences with cover crop species selection, timing of seeding, and planting methods in various vegetable crops. Based on that grower input, Lilley, Schattman, and PhD candidate Gladys Adu Asieduwaa researched 1) the optimal timing of interseeding cover crops into sweet corn and cabbage crops in the Northeast, 2) the suitability of distributing the cover crop seed with various methods, 3) the suitability of various grass/legume cover crop mixes for interseeding, and 4) on-farm logistical constraints to the practice.
These research questions were addressed through 2 seasons of research-farm based replicated trials including A) a multifactorial trial looking at cover crop seeding timing in relation to the cash crop development stage (main plot), and seeding methods of broadcasting, broadcasting and incorporating, or drilling seed (subplot) in both sweet corn (A1) and cabbage (A2). We conducted 2 field seasons of a B) cover crop species trial in both sweet corn (B1) and late season cabbage (B2). We also collaborated with four farmers to trial the practice in ways that best suited their operations. These collaborations provided invaluable data on logistical constraints and opportunities for adoption of the practice.
We identified that upright crops like sweet corn are more suitable for interseeding than lower growing crops. However, when seeded 30 days after transplanting (DAT) or later, interseeding into late season brassica crops can be successful. Incorporating seed increased cover crop germination, particularly in dry years. A mix of annual ryegrass and crimson clover (25 lb per acre (60%:40%)) germinated quickly and remained relatively low growing, limiting competition with the cash crop. The farmer collaborators we worked with had success using drop spreaders or broadcasting cover crop seed just ahead of their last cultivation. The last cultivation frequently aligned with the 30 DAT stage for brassica crops.
Our team shared project results with farmers, agricultural service providers, and academic audiences. Findings from this research were shared to over 575 individuals at 11 regional grower meetings/ conferences including the 2023 Northeast Cover Crop Council’s Annual Conference, the Maine Fruit and Vegetable School, the 2025 Vermont Vegetable and Berry Association Annual Conference, and the Maine Organic Farmers and Gardeners Association’s Farmer to Farmer conference, and two years of the New England Vegetable and Fruit Conference. Project outcomes include a graduate thesis, three drafted/submitted peer-reviewed publications, and a farmer facing comprehensive guide on interseeding, which is hosted at UMaine Cooperative Extension publications. While publication and dissemination of results is on-going, we have confirmed that at least 20 farms have utilized our project results to support increased cover cropping and soil protection on the combined 695 acres that they manage.
This project will develop recommendations for interseeding cover crops into late season vegetables in the Northeast to improve overwintered soil coverage. The goals of the proposed research are to investigate key management considerations including: timing of planting the cover crop in relation to the cash crop, methods of seeding, and how these factors affect crop nutrient availability, pests, and yields. We will also investigate cover crop species performance. By sharing these research results with conventional and organic vegetable farmers, we will support farmers with the information they need to protect more acres overwinter with cover crops.
A 2021 survey of Maine vegetable farmers who use cover crops, conduced by our team, found that 78% (n= 21) of respondents view “late season cash crops coming out too late for establishment of a cover crop” as the primary barrier to planting more cover crops. These fields left bare over winter are susceptible to soil erosion, nutrient leaching, soil structure degradation, and winter-annual-weed seed production (Pimentel et al. 1995; Sainju and Singh 1997; Sarrantonio and Gallandt, 2003; Williams et al. 1998). Climate change forecasts relevant to the northeast suggest that our region is likely to experience longer spring wet periods in coming decades, making bare spring soils increasingly vulnerable (Wolfe et al. 2018).
Interseeding cover crops in standing grain crops has proven successful in warmer regions, like the Mid-Atlantic (Curran et al. 2018; Caswell et al. 2019). However, investigations of interseeding in vegetable crops suggest timing, species selection, and cover crop placement are all key variables for avoiding weed growth and water and nutrient competition in the system (Brainard et al. 2004; Pfeiffer et al. 2015; Brainard and Bellinder 2004; Vanek et al. 2005).
To address the suitability of interseeding cover crops in late season vegetables in Northern New England, our team conducted two seasons of field trials investigating the timing of seeding cover crops in relation to cash crops, and the method of seeding (broadcasting, vs. drilling, vs. broadcasting and incorporating). In other field trials we are investigating cover crop species selection for interseeding. Sweet corn and storage cabbage were the crops used in these trials, meaning that we conducted 4 replicated trials across 2 growing seasons.
Our team leveraged additional funds to support a graduate student to complete a sociological investigation of Northeast farmers' perceptions of and experiences with interseeding in vegetable crops. We will also enhance the outreach component of our original proposal with support of these funds.
Additionally, we have worked with our farmer advisory committee to trial this practice on their working farms. Those trials have shown great promise for this practice, as well as some approaches that should be avoided. While one cooperating farmer has implemented this practice by hand on their smaller scale farm, 3 others have spent time and money modifying equipment to make interseeding efficient on their operations. Those farmer experiences are published in the case studies section of the UMaine Extension bulletin "Interseeding Cover Crops in Late Season Vegetables: Covering Ground to Build Soil Health".
Goal setting for the UMaine Cooperative Extension Aggies team is accomplished initially through reviews of existing and priority need positions in the organization. While the Extension administration and leadership team has the final say on which positions they will push for, faculty and agriculture focused program staff have a say in what those priority needed positions are.
Beyond the hiring and staffing process, areas of focus are largely left to program area staff and the constituent groups that we support. For example, our small fruit and vegetable specialist know the small fruit and vegetable community best, so it is left to that specialist to determine what work would best support that group. For faculty programming staff, it is a requirement of our promotion and tenure process to demonstrate clear implementation of regular and on-going needs assessments.
Needs assessments are done through a variety of means including informal logging of client inquiries or conversations with growers, formal surveys of producer groups, conversations with industry association groups, or based on current events such as the release of the Food Safety Modernization Act, or PFAS.
Our Aggies team meets once per year to share project outcomes, and to discuss any needed adjustments to our work. Our fruit and vegetable team subgroup meets on a monthly basis to discuss grower needs and projects. All voices are welcomed and encouraged in all meetings.
Cooperators
Research
In this project, we ask the following research questions:
- How does interseeding cover crops in late season cabbage and sweet corn affect crop productivity?
- What planting methods work best to establish interseeded cover crops?
- What planting dates (based on crop growth stage) optimize both cover crop biomass development and cash crop productivity in the Northeast?
- What are equipment, timing, labor and other barriers to establishing a cover crop in an established sweet corn or fall brassica crop in Northern New England?
- What cover crop species are best suited for late-season establishment in our region?
METHODS:
This proposal included two distinct experimental trials, referred to in the following sections as Trials A (Seeding Time and Methods) and B (Cover Crop Species Trials).
Trial A:
Research plot treatments: Two research project investigated interseeding timing and seeding method in late season plantings of sweet corn (2022 and 2024) and (separately) storage cabbage (2022 and 2023). Treatments in both crops were the same. (a) timing of seeding (main-plot treatment), and (b) cover crop seed incorporation method (sub-plot treatment). The interseeded cover crop in all treatments was a mix of annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A.
A replicated split plot design replicated four times was used to evaluate trial A (timing/method) in both the sweet corn and cabbage trials.
Sweet Corn Trial A:
The sweet corn trial treatment plots included 4 rows of corn (var. Montauk) planted 30” apart, which included 2 data rows, and 2 buffer rows. Treatment plots were 15' long. The cover crop treatments were planted in the 3 between row zones; between and on either side of the 2 data rows (7.5’ wide). Each main plot treatment (timing) was planted in the between bed zones along 45 bed feet (3 plots long), with subplots (seeding method) being 15’ long within that. Those blocks were randomized and replicated four times
Past field corn trials rely upon crop maturity stage for initiating cover crop seeding (Caswell et al., 2019; Curran et al., 2018). Based on this, our (main plot) timing treatments for planting cover crop mix in the sweet corn trials were v3, v5, and v7, with a "no cover crop" treatment as our control. These timing treatments were testing planting dates that are earlier, later, and at a hypothesized optimal planting date that has been informed by published literature from other regions.
Subplot seeding methods treatments tested the effects of different interseeding planting methods in Trial A. While broadcasting seed is the easiest method for distributing seed when interseeding a cover crop, there is significant evidence that incorporation of the seed will significantly decrease time to germination and increase germination rate (Baker and Griffis, 2009; Barnett and Comeau, 1980; Brennan and Leap, 2014). Therefore, we tested; a) broadcasting by hand, b) broadcasting by hand and incorporation, and c) drilling the seed between crop rows with an earthway seeder calibrated to use the "beet/ swiss chard/ okra" plate. We hypothesized that drilled seed will result in the optimal time to germination, cover crop stand density, and long term cover crop biomass.
Storage Cabbage Trial A:
In the cabbage trial, each replicated block consisted of one data bed from which we collect data, with a buffer bed between each data plot. The cabbage (var. Storage #4) was spaced on each bed-top, 18" in row and 18" between rows on bed centers (2 rows per bed) with beds 5.5' on center. Cover crop treatments (timing and planting method) were planted over bed tops and in between bed zones, flanking the crop rows in the data plots. Each main plot treatment (timing) was planted along 45 bed feet (3 plots), with subplots (seeding method) being 15’ long within that. Those blocks were randomized and replicated four times. The full cabbage trial field was 225 bed feet, by 54’ wide (9 beds).
Past research on interseeding and weed management in brassica crops has utilized a days-after-transplanting (DAT) approach for determining the timing of seeding (Brainard and Bellinder 2001; Brainard et al, 2004; Weaver, 1984). Based on the literature, our timing treatment were intended to be 14, 21, and 30 DAT and post-harvest as a control. Due to weather and staffing issues the planted dates ended up being 17, 23, and 31 DAT in 2022, and 12, 22, and 31 DAT in 2023. Both years included a control plot with no-cover crop.
Our subplot seeding methods treatment tested; a) broadcasting by hand, b) broadcasting by hand and incorporation, and c) drilling the seed between crop rows with an earthway seeder calibrated to use the "beet/ swiss chard/ okra" plate.
Trials B:
There is minimal data available about what cover crop species will work best for this practice in northern New England. To address this question, the two Trial B experiments tested cover crop species planted at the hypothesized optimal seeding time (v5 for sweet corn, and 21 DAT for cabbage). All treatments were seeded using the broadcast and incorporate method (running a hard rake over the seed). The sweet corn trial was conducted in 2022 and 2023, while the cabbage trial was conducted in 2023 and 2024.
The species treatments in both crops included 1) annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A, 2) and oats and peas broadcasted and incorporated at 100lb/A (50%:50%) and 3) winter rye and hairy vetch (67%:33%) at 55 lb/A, all broadcasted and incorporated.
Crop spacing and plot layout mirrored that used for Trial A above.
Data Collection and Analysis:
Trial A:
Soil nitrate testing (Vanek et al., 2005) occured at the first cover crop planting date, and again at crop harvest. All soil tests were analyzed at the UMaine Soil Analytical Services Lab. Soil moisture data was collected hourly starting at cover crop planting, with centralized Irrometer Watermark soil moisture sensors. Cabbage yields was assessed at harvest by counting the number of heads per plot, weight and diameter per head, and marketable vs. diseased or damaged yields. Sweet corn yields were assessed at harvest by counting the number of ears, and assessing the kernel count per ear by counting filled kernels down an ear, and around the ear at the mid-point. Cover crop and weed biomass yields were collected by harvesting and separating above ground biomass from 0.25m sq. quadrants, just prior to the first major snow fall.
Trial B:
Data collected in trial B mirrored that of trial A with the exception of the frequency of collecting soil moisture levels. Soil moisture data was collected in each plot weekly starting at cover crop planting, with Watermark soil moisture sensors and a handheld meter. We collected this data using a different system than Trial A for cost efficiency, and due to the relative lower level of complexity in this trial.
Statistical Design and Analysis. For both Trial A experiments and years, a split-plot factorial design was used, with seeding timing being the whole plot treatment and seeding method being the subplot treatment. Independent variables of yields, soil moisture, soil nitrate, weed density, and cover crop biomass were analyzed using the R Core Team (2020).
For Trial B, statistical design for the cover crop species comparison will be a randomized complete block design, to analyze the same variables as Trial A. Normality and homogeneity of variance were assessed using the Shapiro-Wilk test and Levene’s test, respectively. Based on these assessments, either a one-way ANOVA or a Kruskal-Wallis test was applied. For datasets that violated the assumptions of ANOVA, the non-parametric Kruskal-Wallis test was used. Statistical significance was assessed at α = 0.05.
ON-FARM RESEARCH
We collaborated with 4 commercial farms to install small plots of interseeded cover crop. This included 2 organic, and 2 conventional production farms. We worked with each farm individually to determine the timing, cover crop species, and equipment that would fit into their systems. The farmers helped with seeding, and joined our research team for a field walk at least once prior to harvest. In these plots, we were mostly focused on assessing and reporting on logistical considerations and hurdles to adoption from farmers' perspectives. Qualitative assessments of pest damage, weed densities, and differences in water movement in the fields were recorded.
Bumbleroot Organic Farm-
We trialed interseeding in a densely planted cabbage plot, with 3 rows per bed with plants spaced 12" in-row. In 2022 we trialed 25' bed length plots of the following treatments; 1) annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A broadcasted and incorporated over the entire beds, 2) annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A broadcasted and incorporated just in the walkways, 3) and oats and peas broadcasted full width and incorporated at 100lb/A 50%:50%). All treatments were planted 30 days after transplanting. In 2023, we seeded either annual ryegrass and crimson clover, dutch white clover, or medium red clover in a variety of slightly wider spaced brassica crops at 21 days after transplanting.
Belanger's Farm-
In 2022, Belanger's Farm trialed annual ryegrass and crimson clover, as well as winter rye, either drilled with an Earthway Seeder, or broadcast and incorporated over bare soil cabbage and in a plasticulture cauliflower field. In 2023, they trialed broadcasted annual ryegrass and crimson clover in bare soil cabbage. The seed was incorporated with their Kult Kress finger weeder system at last cultivation.
This farm has a Gandy Orbit Air fertilizer sidedress unit, which they have retrofitted to deliver cover crop seed to deflection plates to distribute seed over 6 rows of corn when the corn is at knee height. The seeder has attached liliston cultivators for weed management and to work the seed into the soil. Separately, the farm has a tool bar with 2 x 12" wide rotovators, intended for renovating strawberries. This piece of equipment works well for incorporating cover crop seed over beds, including over plastic mulch.
Goranson Farm-
A farm manager at Goranson Farm collaborated closely with our team to install large scale plantings of annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A broadcasted and incorporated over whole blocks of sweet corn in 2022. The seed was spread with a human powered chest spin spreader, just prior to final cultivation in the corn at the V5 stage. We also trialed interseeding in their bare soil cabbage fields with beds 6' on center, with two rows of cabbage planted 18" between rows and 18" in rows. 25' bed length plots were installed to, 1) annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A broadcasted and incorporated just in the walkways, 2) and oats and peas broadcasted and incorporated at 100lb/A 50%:50%) and 3) annual ryegrass and crimson clover (60%:40%) drilled at 25lb/A with an Earthway seeder.
In 2022, ’23, and ’24, Goranson Farm used a chest spin spreader to broadcast medium red clover, and crimson clover seed over their winter squash fields, just prior to vine run.
In 2023 the farm interseeded annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A over sweet corn at the v5 stage, and a block of late storage cabbage. Additionally they seeded medium red clover over winter squash at vine run again.
Jordan Farm-
Their 2022 sweet corn field was selected for broadcasting annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A over a block of their corn while sidedressing with urea. The seed was mixed into the 3-point hitch tractor mounted spin spreader with the fertilizer. A final cultivation happened after spreading the seed to control weeds and to work in the seed.
That same cover crop mix and rate was applied between plastic in a pepper field early in the season, at planting of the peppers. A narrow mulching riding mower (blocked discharge shoot) was used to keep the cover crop and weeds low throughout the season.
In 2023, interseeding was used on a large portion of the 65 acre farm. Crops including onions, peppers, brassicas, and sweet corn all had annual ryegrass and crimson clover planted, or dutch white clover. In 2023 the farm mostly used a Gandy box gear driven sidedress hopper for calibrating and precisely distributing the seed between rows of crops. This unit was attached to their cultivation tractor for single pass seed distribution and cultivation.
Statewide, the weather across the three seasons that we conducted these trials varied. 2022 included drought conditions and high heat, 2023 being the 3rd wettest June-July-August on record, and more average precipitation over 2024 despite high heat conditions during crop and cover crop planting times. These extremes resulted in vastly different interseeding outcomes. In dry conditions, we observed a slower cover crop germination and growth rate. In 2023, the wet conditions made it difficult to access fields for seeding on the dates that we had planned for and made for difficult weed management. When seeding was able to happen, the availability of water resulted in quick germination and fast cover crop growth rates, along with rapidly rerooted weeds.
Seeding Method
In these trials there was significantly more biomass when cover crop seed was broadcast and incorporated into standing sweet corn, compared to other seeding methods. In both years of the TRIAL A sweet corn trial (2022 and 2024), there were very dry conditions during the seeding period. Covering seeds with soil decreased the risk that seeds would dry out and die. Drilling seed also resulted in uniform cover crop germination in some plots, yet it did not work well in compacted soils.
Surprisingly, incorporation of seed yielded a similar cover crop biomass compared to the other seeding methods in the TRIAL A cabbage trials. This was particularly true in the extremely wet season of 2023, where it is likely that seeds had adequate moisture whether they were in the soil or on the surface. That said, incorporation of seed with a cultivator simultaneously kills weeds while improving conditions for seed germination. This gives cover crops a leg up on weeds and increases their chance for success.
This suggests that small-scale farmers do not necessarily need specialized equipment for interseeding. Simple methods like hand broadcasting or using chest-mounted spin spreaders can be effective and affordable, making the practice accessible even for those with limited resources. Additionally, both small- and large-scale farmers can modify existing farm tools to suit their interseeding needs. These modifications reduce the need for costly new machinery and can make interseeding viable with no additional passes. These efficiencies make interseeding a practical and budget-friendly option across different farm sizes.
Seeding Timing
Timing is critical for successful interseeding. Early interseeding may lead to more biomass production but also more competition with cash crops. Early seeding also limits weed management activities; strategies like mechanical cultivation cannot be used after cover crops have been seeded. If cover crops do not develop sufficient canopy cover quickly, weeds may grow and set seed. While seeding cover crops later in the season reduces the risk of competition with the cash crop and allows for more weed control opportunities, it is possible to wait too long. When a cash crop is too mature, it may be physically impossible to enter the field. Additionally, if the soil is too shaded by the cash crop, a cover crop is unlikely to germinate.
Sweet corn at the V3 stage corresponds to the period when the corn plants are still relatively small and haven’t yet fully shaded the ground between rows. By V7, the corn was too tall to access the field with equipment. TRIAL A sweet corn results showed that the V5 stage allowed for equipment access to the fields, adequate cover crop germination, and reduced weed growth as compared to both the V3 and the V7 growth stages. From a practical perspective, this maturity stage often corresponds with the sidedressing and the final cultivation of a sweet corn crop. This suggests that interseeding into either sweet or field corn could integrate well into existing management calendars. While logistically, V5 was favorable, all timings across both years of the trial yielded the same as the control treatment, meaning that the cover crop did not compete with the crop for resources.
Our sweet corn trials demonstrated that a higher end of season cover crop biomass correlates to a lower weed biomass.
TRIAL A cabbage found the timing of interseeding to be even more critical to avoid crop damage. Early interseeding, at 10 DAT, allowed cover crops to establish while the
cabbage was still small. This timing lead to excessive growth and competition for water, nutrients, sunlight, and airflow to the cabbage crop in all years of our trial. The 21 DAT treatments also proved to be too early in a year with record-setting wet conditions (2023). This was true in our replicated research trials, and the on-farm trial sites. In the drier year of 2022 our 30 DAT treatments yielded the same as the control treatment. For farms that have a wider between row spacing with exposed soil throughout the season, our research results suggest seeding at 30 DAT to avoid crop competition. In the case of a year with extremely favorable growing conditions for the cover crops and weeds (heavy precipitation), having a backup strategy (mowing or heavy cultivation) would be prudent.
SPECIES SELECTION
High biomass is often viewed as a key indicator of success when using cover crops. However, interseeding goals are more nuanced. In an interseeded planting, it is better to have low biomass early in the planting to avoid competition with the cash crop. Cover crops that stay small while cash crops are in the ground, but add biomass after cash crops have been harvested, are desirable. Cover crops that are planted too early, or that have robust upright or vining growth habits, are likely to compete with cash crops for water and nutrients. They may also reduce air flow around the cash crop, increasing the risk of disease.
TRIAL B Sweet Corn demonstrated the importance of considering cash crop growth habit when interseeding. The upright growth of the corn allowed for a higher level of competitiveness. The corn plants successfully shaded the three cover crop mixes (Annual Ryegrass+Crimson Clover; Oats+Field Peas; Winter Rye+Hairy Vetch). The vining habit of the vetch and peas was minimal as those species were suppressed under the cash crop. Sweet corn yields, and ear tip fill were equivalent among all treatments.
TRIAL B Cabbage
Both coverage and growth habits of cover crops and cash crops are important factors to consider. Cabbage and other brassicas have a relatively quick growth habit and large leaves, however they are low growing. In the dry season of 2022, we observed that when peas and hairy vetch were used in interseeding mixes, they vined on top of and through the cabbage plants. This was concerning due to reduced airflow and increased risk for disease. It also increased the difficulty of cabbage harvest. The annual ryegrass/crimson clover was much lower growing, yet still provided uniform soil coverage.
In the extremely wet 2023 season, seeding early (21 days after transplanted and earlier) resulted in apparent crop competition at all of our research and demonstration sites. While the demonstration sites did not show visible yield reduction, the cover crop was taller than the brassica crops at harvest leading to difficulty with harvest. The risk of disease issues in the wet season was only exacerbated by the tall cover crop holding moisture against the crop. That said, annual ryegrass and crimson clover maintains a much lower growth habit than the oats+peas, and winter rye+hairy vetch mixes. We recommend annual ryegrass and crimson clover over the other mixes due to its lower growth habit.
ON-FARM RESEARCH
We collaborated with 4 commercial farms to install small plots of these treatments.
Bumbleroot Organic Farm-
Due to the very tight plant spacing in 2022, the crop closed in and covered the soil shortly after seeding. These resulted in next to no cover crop biomass being present at harvest. In 2023 we planted earlier (21 DAT) and focused on the walk rows. Given the extra time to establish prior to canopy closure, the farm collaborators were pleased with the results. They reported the cover crop increasing the ease of access into these plots and cleaner crops as the cover crop reduced the mud in the trial plot areas. This annual ryegrass and crimson clover cover crop would have also reduced soil splashing in this wet season, reducing the risk of disease.
Belanger's Farm-
In the 2022 bare soil cabbage field, the winter rye experienced very poor germination. The grass that successfully germinated had clear signs of herbicide damage. The farm confirmed that a mix of residual herbicides had been applied to suppress grass weeds. The annual ryegrass and crimson clover showed no visible damage. Broadcast and incorporated plots showed the most uniform and thickest coverage.
There was significant deer damage in the bare soil cabbage field. The growers are concerned that the clover may be more of a draw to their already very troublesome deer pressure. We are consulting with the farmers to identify a cover crop mix that might work as a deer "trap crop" to draw the deer away from their crop fields.
A field of plasticulture cauliflower was seeded to annual ryegrass and crimson clover 21 days after seeding the clover. The plastic mulch reduces the interaction of the crop with the cover crop. The walkways between beds were pitched downhill, yet no visible evidence of between row erosion was observed. Interseeding between plasticulture beds appears to be a lower risk approach. If weeds or excessive cover crop growth did occur, the farms we worked with had plans for mowing between the beds to reduce growth.
This farm has a Gandy Orbit Air fertilizer sidedress unit, which they have retrofitted to deliver cover crop seed to deflection plates to distribute seed over 6 rows of corn when the corn is at knee height. The seeder has attached liliston cultivators for weed management and to work the seed into the soil. Separately, the farm has a tool bar with 2 - 12" wide rotovators, intended for renovating strawberries. This piece of equipment works well for incorporating cover crop seed over beds, including over plastic mulch.
In 2023, the bare soil cabbage had annual ryegrass and crimson clover seed incorporated with finger weeders at last cultivation (21 DAT). The seed germinated quickly and out competed the weeds very effectively. At the same time, the cover crop grew quickly and was as tall as the cabbage crop at harvest. This quick growth was attributed to the very wet conditions of that season. There was some minor purpling of the outer leaves observed in the interseeded plots at a higher rate than the control, however, no difference in yield or incidence of disease was observed. The growth and potential competition was concerning to the farm managers.
Goranson Farm-
The 2022 block of annual ryegrass and crimson clover (60%:40%) seeded at 25lb/A broadcasted and incorporated over whole blocks of sweet corn was very successful. The other half of that succession of corn was not cover cropped until after harvest. Winter Rye was seeded on that half of the field on October 20th, and was barely germinated (about 1" of growth) before winter dormancy. The interseeded half had a lush stand of mostly crimson clover with some annual ryegrass.
The cabbage trial at this farm had mixed results. Due to the size of the cabbage at harvest, and the amount of crop residue left on the soil surface at harvest, the cover crop was largely smothered going into the winter. Any exposed areas were well covered by cover crop, however that was a small percent of the total area in that field.
Additionally, Goranson Farm used a chest spin spreader to broadcast medium red clover, and crimson clover seed over their winter squash fields, just prior to vine run. The field was left as very uniform mat of clover as coverage going into the winter, post-harvest. A pass with a mower set high after harvest would have cut back escaped weeds.
Jordan’s Farm-
This 2022 sweet corn field had excellent germination. After a heavy rainfall event, we observed significantly less erosion late season, prior to harvest, in the interseeded blocks as compared to the neighboring bare soil plots.
In the full season mowed cover crop in the plasticulture peppers, there was no observed negative effects to the crop, and better access into the field for harvest.
In 2023, the Gandy sidedress hopper was used widely to distribute seed on many crops across the farm. Doing this at final cultivation took no additional time, but had great benefit for improving harvest access in a very wet season.
Interseeding is an effective way to increase cover crop acreage on diversified vegetable farms, however there are tradeoffs. Though interseeding results in soil coverage and protection during the growing season and after the harvest of late season crops, it also increases weed management challenges compared to full width cover cropping. When a crop will be harvested early enough for good post harvest cover crop establishment, consider the tradeoffs between interseeding, vs. more uniform and less weedy cover crop stands from post-harvest seeding. Interseeding is one tool in the cover cropping toolbox available for vegetable farms in Northern New England. Whether or not to use it depends on farm cropping mix and farmer management goals.
The timing of interseeding cover crops into vegetables is important. Seeding cover crops at last cultivation or while sidedressing (about 30 days after planting/transplanting the cash crop) works well for many crops. Planting too early can result in excessive cover crop growth and eliminates the ability to manage weeds early in the season. While interseeding reduces weed density, weeds that get through the cover crop can easily set seed. This is most likely to happen if interseeding cover crops occurs too early in the season. Conversely, seeding too late results in low cover crop establishment due to shading from the cash crop. It may also be difficult to complete field work due to the maturity and size of cash crops.
The method of seeding a cover crop can be important. This is especially true during dry spells. Our research showed that lightly working the soil after seeding results in earlier and more uniform cover crop emergence, as well as significantly higher end-of-season cover crop biomass compared to other seeding methods. This approach also kills weeds at cover crop planting time. As a result we recommend seeding just before or during the last cultivation of the cash crop.
The cover crop species for interseeding should be carefully selected. Cover crops that aggressively compete with the cash crop for water, nutrients, and sunlight should be avoided. Potential disease and insect pest hosts should also be avoided. Tillage radish could serve as a disease reservoir on farms that grow brassica cash crops. We found that a mix of annual ryegrass and crimson clover performed well due to the low growth habits and the crimson clover’s ability to reduce nitrogen demands for both species. Note that both of the species overwintered in Maine in some, but not all winters.
Choosing cash crops that are suitable for interseeding. While interseeding has been successfully used in many cash crops, some are not well suited to this practice. Cash crops should have some ability to compete with the cover crop either through quick upwards growth, robust shading of the soil, or being planted in plasticulture systems. Less aggressive crops such as bare soil onions, carrots, and radishes should be avoided. Also, quick season crops like lettuces, beans, and radishes would have better outcomes from being harvested, mowed, disked, and full-width cover cropped as opposed to interseeded.
Education & outreach activities and participation summary
Participation summary:
We have shared the results of our research through the following channels. Many of these outlets have drawn on research findings from related past research projects in the region, including past Northeast SARE projects.
Presentations and Webinars
- Field Tour/Workshop at the UMaine Roger's Research Farm, Old Town, ME. 11/4/22. At this program we described the project and allowed farmer attendees to walk through the fields to observe the cover crops stands of our various treatments. We also discussed soil health more generally, and highlighted other research taking place at Roger's Farm including late season W. Rye seeding assessments.
- Presentation- MOFGA's Farmer to Farmer Conference in Belfast, ME. 11/6/22. Innovative Cover Cropping Systems. This included a detailed explanation of the role of cover cropping in nutrient and fertility management, and an overview of preliminary findings of the Covering Ground, interseeding trials.
- Presentation- The New England Fruit and Vegetable Conference in Manchester, NH. 12/13/22. Graduate student Gladys Adu Asieduwaa presented on Interseeding in Late Season Vegetables.
- Presentation- The Mid-Atlantic Fruit and Vegetable Conference in Hershey, PA. 2/1/23.
- Presentation- Northeast Cover Crop Council Conference in Portland, ME. 2/16/23. Interseeding in Late Season Vegetables in the Northeast
- Presentation- UMaine Cooperative Extension's Fruit and Veg School (Conference). Interseeding in Late Season Vegetables in the Northeast
- Field Day- UMaine Roger's Farm Field Day in Old Town, ME. 7/26/23.
- Presentation- Common Ground Country Fair in Unity, ME. 9/23/23. Innovative Cover Cropping Approaches. This talk was targeted towards smaller scale farmers and gardeners and highlighted several considerations and approaches to cover cropping, including the concept and practical considerations for interseeding.
- Presentation- The New England Fruit and Vegetable Conference in Manchester, NH. 12/10/24. Graduate student Gladys Adu Asieduwaa presented on Interseeding in Late Season Cabbage
- Presentation- The New England Fruit and Vegetable Conference in Manchester, NH. 12/11/24. on Interseeding in Late Season Sweet Corn.
- Webinar- Interseeding Research Updates and Discussion Session- UMaine Extension event: 8/14/24 : Presentation and grower discussion.
- Presentation- Maine Small Fruit and Vegetable Growers Association meeting at the Maine Ag Trades Show in Augusta, ME. 1/15/2025: Interseeding in Late Season Sweet Corn: Research Updates
- Zoomed in Presentation- Vermont Vegetable and Berry Growers Association Annual Meeting: Fairlee, VT. 4/9/25. Interseeding in Late Season Brassicas: Research Update
- Webinar- UMaine Extension Soil Health Lunch and Learn Series: 6/24/25. Incorporating Interseeding into your Vegetable Operatio
Educational Resources
- University of Maine Cover Crop Estimator and Budgeting Tool
- UMaine Cooperative Extension Bulletin #XXXX : Interseeding Cover Crops in Late Season Vegetables: Covering Ground to Build Soil Health: A guide of research findings and real-life case studies about best practices for interseeding cover crops in mixed vegetable systems.
Peer Reviewed Publications
Under Review
- Asieduwaa, G.A., Lilley, J.M., Schattman, R.E. 2026. Evaluating Planting Methods and Dates for Interseeding Annual Ryegrass and Crimson Clover into Fall Cabbage in Northern New England, USA. Journal of Soil and Water Conservation.
Ready for Submission
- Asieduwaa, G.A., Lilley, J.M., Schattman, R.E. 2026. Assessing Cover Crop Species Suitability for Interseeding in Fall Sweet Corn and Cabbage in the Northern New England. Hort Science.
- Asieduwaa, G.A., Lilley, J.M., Schattman, R.E. 2026. Annual Ryegrass and Crimson Clover Cover Crops Interseeded into Fall Sweet Corn: Impacts on Yield and Soil in Northern New England, USA. Agriculture Ecosystems and Environment.
Honors Thesis
- Cooper, C. 2024. "Assessing the Influence of Interseeded Cover Crops on Beneficial Arthropod Abundance
in a Northeastern Agroecosystem"Honors College. 873. https://digitalcommons.library.umaine.edu/honors/873
Learning Outcomes
Farmers and service providers have noted knowledge gain in the following areas:
- Understanding of limitations of this practice.
- Crop productivity risks associated with interseeding cover crops too early, or using cover crop species with aggressive growth habits.
- That interseeding is not a substitute for full width cover crop planting. When there is time to incorporate cash crop residue post harvest and seed a full width cover crop, that will often result in fewer weeds and more uniform soil coverage.
- Farmers with very intensive tight plant spacing have decided against interseeding in those plantings. The lack of sunlight reaching the soil decreases the germination of the cover crop. The tight crop spacing results in reduced airflow and increased disease potential. The presence of a cover crop in that system would further increase that disease risk.
- The use of residual herbicides can negatively affect cover crop germination and growth. Farmers must adjust active ingredients in fields that will be interseeded, or select active ingredients and cover crop species that work together.
- Understanding optimal timing for interseeding.
- This includes the realization that the optimal timing often aligns with final cultivation, or sidedressing of crops.
- Farmer participants have been seeding cover crops in the same pass as fertilizer sidedressing applications and/or cultivation passes, resulting in added soil health benefits with no additional labor.
- Brassica farmers have expressed the intentional delay in sidedressing and/or final cultivation in order to reach the 30 days after transplanting seeding date that our team recommends.
- Understanding the benefit of seed incorporation for interseeding success.
- At least four farmers made equipment modifications for cover crop seed distribution and cultivation in the same pass. In dry years, farmers have noted a significantly better germination rate with incorporation of the cover crop seed.
- As a result of our research results, farmers target increased cover crop seed germination rates with the aim of achieving better weed suppression.
- Cover crop species selection.
- At least 7 farmers had never used annual ryegrass as a cover crop prior to consultations with our group. Their adoption of this cover crop species, often mixed with crimson clover, has increased the diversity of cover crops on their farms. The use of these new species on their farms has increased the diversity of scenarios where they can successfully incorporate cover crops to their crop rotation.
Project Outcomes
As a result of the of the USDA Natural Resources Conservation Service funding that built on this project's deliverables, our team has accomplished the following outcomes.
- Conducted a survey to assess the interseeding implementation approaches that farmers are using, as well as the and logistical and cultural barriers to to initial adoption. Results will be published in the spring of 2026.
- The Cover Crop Cost Estimator Tool, developed by the University of Maine, is an interactive decision-support calculator designed to help farmers evaluate the financial impacts of cover cropping within vegetable production systems. By allowing users to compare historical and projected enterprise budgets under both conventional and organic practices, this tool empowers data-driven decision making. Users can tailor inputs such as crop type, enterprise size, and market assumptions in order to model cost and return scenarios that reflect their unique operation. The web-based tool is ideal for producers planning or assessing sustainable cropping strategies.
Unanticipated outcomes of NESARE project funding.
- An undergraduate research assistant on this project took the opportunity to conduct an Honor's Thesis project within our existing field trials. Charlie Cooper implemented an assessment of beneficial arthropods within all of our established plots to assess if the protection provided by the interseeded cover crop might attract predatory and weed seed eating arthropods.
- Cooper, C. 2024. "Assessing the Influence of Interseeded Cover Crops on Beneficial Arthropod Abundance in a Northeastern Agroecosystem". Honors College. 873. https://digitalcommons.library.umaine.edu/honors/873
One of our farm collaborators from Southern Maine has always struggled to get fall cover crops planted on more than 60% of their land. They produce a wide diversity of crops including a lot of late season brassicas. Between those late harvests, much of the farm being on wet soils that stay wet after harvest, and the business of the harvest season, cover cropping in time to get germination was often not feasible. After trialing interseeding on one sweet corn succession in 2022, the farmers spent the winter retooling fertilizer spin spreaders and drop spreaders to make distribution of cover crop seed more efficient. They also ensured that the size of hoppers was sufficient to carry fertilizer and cover crop seed. This allowed for them to distribute cover crop seed and fertilizer at the same time, costing them no extra time beyond the labor of sidedressing. Over the winter of 2023/'24, they spent more time modifying equipment to increase efficiency and the uniformity of seed rates and distribution In 2024, they interseeded 75% of their land and were able to traditionally plant cover crops on the majority of the remaining open ground.
While replicated research farm trials are essential for reliable research results, there are limitations to the relevance of these findings for on-farm practice adoption. We know that practice adoption on farms is largely based on peer to peer learning and skill sharing. Our approach combined replicated trials to answer key research questions, while simultaneously supporting farmers to trial the practice in logistically relevant ways on their farms. To support the on-farm trials, our team provided current literature reviews and preliminary research results. This approach created a 2-way feedback loop confirming that our approach to the replicated research projects were relevant on working farms, and educating our research team on what approaches are or are not feasible on farms.
This systems level project had many confounding variables. There are many aspects of this work that could further inform best practices when interseeding in vegetable crops. For example, further research is needed to understand;
- How soil texture and structure influence cover crop germination,
- And, how various weed seed banks influence the success or failure of interseeding in vegetable crops.