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Using Cover Crops: The Right Fit

If you have kept up with the past several articles, a few cover crop species might have caught your eye. Before you rush out and order several pounds of hairy vetch (tempting, I know), stop and think about not just where but when you want to plant this cover crop. For example, choosing hairy vetch for nitrogen addition requires two seasons of growth—uh oh, what if you have your garlic in that area in the fall?  Throwing vetch in an already occupied bed can be trouble, even if you limit them to the rows in between your bulbs/garlic.

Find Gaps in your Garden Plan

It may seem like a chore, but recording where and how long your vegetables grow in the garden is key to using cover crops effectively. If you had made a note showing garlic occupied those beds from September to July, you should have caught yourself before clicking “order now” on a pound of hairy vetch. I should know, I think I have a couple of pounds of vetch stored away in my garage.


One way you can keep track (and avoid stockpiling vetch seeds) is to make a timeline with each month on the bottom, your beds on the left, and lines where you have crops growing. By doing this, you will see gaps forming on the timeline. Those gaps many seem like nothing is going on in your garden, but the truth is more unsettling. If you have those gaps in the summer, weeds are germinating with no shade to stop them. During fall to spring gaps, rain will strip nitrogen and other minerals right out of your garden, and may even carry off with a pound of soil or two. Here is a quick guide to common times you may have little growing and what species may work best.

Summer Gaps

Generally, summer gaps occur after fall planted garlic or leafy green harvest. During that time, there may be a few months (June to August) where you may have nothing growing. In those situations, heat-loving annuals such as buckwheat or phacelia are planted to provide quick growth. As “Pollinator Hotels”, both species provide a long blooming time, attracting pollinators and other beneficial insects. Phacelia is more expensive and a little harder to come by than buckwheat, but can tolerate sandier soil and drier conditions. A word of caution with that cheaper buckwheat; make sure to cut the plants down before they start setting seed, they can be an annoying weed if the seeds survive winter.

Fall to Spring Gaps

Many traditional garden favorites wind up causing this gap, especially tomatoes or sweet corn. This far north, planting cover crops by early September (at the latest) is your best bet to see roots stay alive in the soil over winter and get a head start in spring. Many of your winter hardy legumes such as hairy vetch and winter rye thrive in this environment. You can also experiment with mixes in this gap period as well. For example, I have found success by planting a 60-40% mix of hairy vetch and oats together. The oats grow quick and suppress weeds, but die in the winter, and the spring vetch uses them as a scaffold to increase growth in the spring.


Winter rye’s reputation for being extraordinarily tough is perfect in this situation. If weather turns sour in fall and you cannot plant the cover crop until late September/early October, rye may be your only candidate left. In the spring, you can terminate the rye once it reaches about 6 to 8” tall. Rye can be left to grow taller (and add more organic matter), but if you are concerned about lack of spring rainfall that 6-8” termination height may be preferable. Unlike many of the cover crops, winter rye can put out certain chemicals that can suppress grass seed growth. This process is allelopathy; black walnut is another plant that can also do this. This may be great if you deal with giant foxtail as a persistent weed, but may be less great if you plan on growing sweet corn (which is also a grass). Because of that possible danger, avoid planting vegetables for 2 weeks after killing the rye to avoid allelopathy risks. This waiting time can also help you avoid nitrogen issues in your main crop as well, as microbes that may have held onto the rye’s nitrogen tightly would eventually release it into the soil and to your growing crop.

Spring Gaps

This may be an issue in late fall/early winter harvested crops such as parsnips. You only have a short growing period before May, and this may limit your cover crop options. Choosing fast annual grasses that thrive on cooler weather like oats may be your best option here. If your main crop is a legume such as soybeans (aka edamame) that can handle being planted in June instead of May, that extra month of oats can go a long ways to building soil organic matter and suppressing weeds.

Works Cited

• Clark, A. 2007. Managing cover crops profitably. College Park, MD: SARE.
• Fortier, J. M., and M. Bilodeau. 2014. The Market Gardener: A Successful Grower's Handbook for Small-scale Organic Farming. New Society Publishers.

Contacts

Shane Bugeja
Extension Educator, Ag Production Systems
(507) 304-4325
sbugeja@umn.edu
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