Key to cover crop success: Kill on time 

By Thomas Björkman, Professor of Horticulture Emeritus, Cornell AgriTech 

A note from Anya: cover crops can be a wonderful asset for site preparation. I urge anyone planning to install a new planting of berry crops to read this article. 

It is time to think about how and when overwintering cover crops will meet their end. Termination timing is as important as planting timing. The right timing depends on the management goals for the cover crop. 

The most common management goals are weed suppression, nitrogen fixation, erosion control and organic matter production. The first step is to decide which management goal is most important. That priority may have changed from fall. 

Weed suppression
Overwintering cover crops mainly suppress cool-season weeds, and therefore reduce seed rain in the spring. If weeds in the cover crop flower, pull the plug on the cover crop because it is no longer doing the job. If the stand is clean, terminate based on another goal. Don’t let the cover crop grow so long that it will suppress your vegetables. 

Nitrogen fixation
Overwintering legumes, like hairy vetch and clover, do most of their nitrogen fixation until early flowering. Terminate as soon as you see flowers. 

Erosion control
Rye is widely used to keep the soil in place over the winter. It can be terminated soon after green up. That timing provides an optimal seed bed and good pre-plant management of spring weeds. Bigger crowns or stems often require more tillage for field prep, undoing the soil-health benefits of the cover crop. 

Biomass production
To add soil organic matter, getting the most vegetative growth is ideal. Lignified, high-carbon material is valuable. Try for at least three tons of dry matter per acre, which is a lot. If the stand is too weak to get that kind of biomass, terminate earlier and optimize a different management goal. You only want vegetative growth. Once seeds have been initiated the value starts to drop. 

Nitrogen carryover
Winter cover crops can scavenge left-over soil nitrate and make some available to the following crop. Terminate when there is some spring growth but still mostly leaves rather than stems. The Carbon:Nitrogen ratio rises quickly once stems are a large part of the biomass, which ties up a lot of N.  

Preventing Volunteers
Cover crops that go to seed leave an undesirable legacy of volunteer seedlings. Terminating no later than early flowering is best. 

Will it die?
Regardless of the management goal, the cover crop needs to die completely from the termination technique. Herbicides are more effective on smaller plants, and they generally need a minimum temperature to be effective. Cultivation works if the plants are small enough that the roots can be unearthed or severed from the shoot. Mowing works if the plant is mature enough that the buds below mowing height are unable to sprout. Roller-crimping works if the stand is thick and all the stems are lignified enough to be brittle. Winterkill works if the minimum temperature was low enough. If not, manage the escapes as soon as possible.  

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Still not sure which cover crop is right for you? Check out the Cover Crop Decision Tool website.