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A crew member mows strips of annual ryegrass/Dutch white clover interseeded between peppers in plastic mulch. Sogn Valley Farm received a SARE Farmer Rancher Grant to experiment with interseeding cover crops. Photo submitted

Interseeding cover crops in vegetable fields gives them head start before winter

By Dana Jokela

On our farm in southern Minnesota, we struggled with getting a late-season cover crop established prior to the snow. We would finish the final pepper harvest of the season in early October from plants already singed by frost. We’d hustle to get the field ready for winter rye by removing plastic mulch and mowing and tilling in crop residue. A couple days later, we’d drill the rye, hoping for a gentle rain shortly after to set the stage for some nice late-fall growth to keep the soil protected during winter and spring, while building soil organic matter before our next cash crop.

After a couple of weeks, I’d check on emergence. The field would be covered with a carpet of purple-green rye seedlings, fresh out of the ground and starting to make chlorophyll. Cue the early winter snowstorm that would dump five inches of heavy snow. Growth would grind to a halt. Even when the snow melted soon after, the nights would become colder, the days shorter, and that cover crop’s soil-building potential would be severely hindered.

This was the challenge we faced as we scaled up our pepper production to nearly a quarter of our total acreage. How could we avoid leaving these fields bare over the winter? 

Two years ago, we got curious about the feasibility of interseeding a cover crop into the pepper fields earlier in the summer. While this was not a new concept, it was not widely practiced. We couldn’t find many recommendations for cover crop species, rates, and planting dates for interseeding in vegetable cropping systems.

With funding from a Farmer Rancher Grant from North Central-SARE, we conducted a trial to see if interseeding could help us with our post-pepper harvest cover-cropping conundrum. 

Our main concern was the potential for the interseeded cover crop to compete with our crop and decrease yield. We hoped to mitigate competition by delaying establishment of the cover crop until after the critical weed-free period. That led us to the primary question to answer with the experiment: how long should we wait after transplanting peppers to interseed a cover crop?

We opted to seed a mixture of annual ryegrass and Dutch white clover at rates of 20 lbs/A and 10 lbs/A, respectively. In the relatively scant literature on interseeding, these two species were described as well-suited to interseeding. 

We grow peppers in two different production systems—cultivated bare ground and plastic mulch. We tested three different seeding dates in each of these two production systems. 

In the plastic mulch system, where the cover crop would be seeded only in the pathways, we figured the spatial segregation of cash and cover crop plus the physical barrier of the plastic film would lead to less competition. As such, we tested fairly early interseeding dates ranging from three weeks before transplanting to three weeks after. We believed that earlier interseeding would lead to greater ecological benefits by protecting the soil through the heavy rainfalls we see in June and July. We replaced early pathway cultivation with mowing of the cover crop.

Annual ryegrass and Dutch white clover emerge among maturing pepper plants in the bare-ground plot at Sogn Valley Farm in Minnesota. The farmer found that seeding 7 weeks after transplanting peppers minimized competition and allowed the pepper plants to produce acceptable yields. Photo by Dana Jokela

In the bare ground system, we decided to broadcast the cover crop seed across the full field width, leading to cover crop seedlings co-mingling with our pepper plants. To limit competition, we looked at later interseeding dates ranging from 3-7 weeks after transplanting. We were looking for the sweet spot that gave our pepper plants competitive advantages for water, nutrients and light, while still allowing the cover crop seeds to establish beneath the canopy. 

Green bell peppers were harvested weekly beginning August 1st and marketable fruits were weighed and counted. We observed some successes, some failures, and plenty in between. Since treatments were not replicated in this production-scale trial, our results should be interpreted as anecdotal. 

In both production systems, there was a slight yield drag from interseeding in most cases. Interseeding decreased yield by 14-20% in the plasticulture systems and by 10-12% in the bare ground system. The outlier was the earliest interseeding date in the bare ground system, where heavy competition was evident very early and yields were diminished by over 60%.

While a 10-20% yield reduction may not be a great selling point, I would argue that it’s not a reason to dismiss this approach. First, yields across all plots (with that one exception) were quite good based on our historic averages without use of interseeding, ranging from 20,000 to 30,000 lbs/A of marketable bell peppers. Second, there are a host of other benefits to interseeding, including reduced erosion from summer rainfall, cleaner fruit from less soil splash, and mud-free harvesting even right after heavy rainfall. Plus, all interseeding dates achieved the original goal of ending the season with a vigorous cover crop rather than spindly rye cotyledons.

With the caveat that this is still a new and evolving practice for us, we can share some general recommendations.

When cover crops were interseeded between beds in the plasticulture system, there was a reduction in yield, ranging from a 22% loss at the Mid seeding date to 13% loss at the Late date.

Plots without cover crops had somewhat higher yields than the Mid and Late cover crop plots, but in this case there was only a 10-12% difference.

First, let’s discuss interseeding pathways in the plasticulture system. We didn’t see any benefit to interseeding weeks before transplanting peppers. Our experience indicates that the longer one waits to interseed, the lower the chance that yield will be reduced from competition. The tradeoff is that the pathways must be weeded thoroughly up until the point of interseeding. Since we have decent tools for mechanically cultivating pathways, we’ve chosen to interseed 6-7 weeks after transplanting. The added benefit of delaying this long is that we don’t need to mow the pathways.

When interseeding the full field width in a system without plastic mulch, we typically follow the same schedule as in plasticulture and interseed 6-7 weeks after transplanting. We keep our fields cultivated with our fingerweeder and do a quick hand-weed to clean up escapes prior to interseeding. The combination of shade from the mature pepper plants and trampling from harvesting seems to keep the cover crop in check during late summer and fall. After mowing crop residue at season-end, the cover crop puts on a nice flush of growth with its newfound access to sunshine.

There are a few challenges that should be addressed. First, since cover crop seeds are surface sown, it’s critical that they receive moisture from rain or irrigation until they germinate. We once interseeded an acre of plasticulture pathways (which don’t get any water from adjacent drip-irrigated pepper rows) only to enter a 6-week drought. Almost none of that cover crop germinated. Plan to time seeding ahead of a forecast rain event, or have irrigation ready.

We aren’t especially happy with annual ryegrass in this system, as it goes to seed fairly rapidly. The obvious problem with that is its potential to become a weed the next season, and its seeding habit creates much taller plants that start to obstruct the pepper plants. Low-growing cover crops are ideal for interseeding. We are hoping to find an alternative grass species, or perhaps just use a heavy rate of straight clover. 

Interseeding is no miracle practice. It has its trade- offs. Effectiveness will vary from region to region, season to season. There are undoubtedly circumstances where it could be counterproductive, possibly exacerbating disease by reducing airflow or harboring insect and rodent pests. It may also pose a challenge in dry climates or on farms without irrigation, since the cover crop will use water. But if you’re looking for a way to keep your fields in living cover for more of the year, it might be worth giving interseeding a shot.

Dana Jokela owns and operates Sogn Valley Farm,
a diversified, organic farm near Cannon Falls, Minnesota. See more details about this grant project.

From the March | April 2021 Issue


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