Cover Crop Termination for Organic Growers

Cover Crop Termination for Organic Growers

A Hensley's Seed Co. guide

Termination is one of the most overlooked parts of a cover crop plan. It's easy to focus on the big biomass, weed suppression and erosion protection a good stand provides, yet those same strengths can cause trouble if you don't plan ahead. Mature winter rye standing 6' tall, for example, makes huge amounts of biomass, crowds out weeds and holds soil firmly. But if you intend to till that mature residue in, it will be a headache. Better to change when or how you end it, fitting the tools and method to the crop's growth stage. Handled well, all that biomass becomes organic matter that feeds the soil and the crops that follow.

Methods of Cover Crop Termination

Many tools and methods can end a cover crop, and most work at more than one scale. The equipment may differ, a tractor flail mower versus a push mower or string trimmer, but the principle is often the same. Which method you choose depends mostly on your tools, the size of the area and how far along the cover crop is.

Growth stage matters: some methods only work well once the crop has moved past leafy (vegetative) growth into the reproductive stage, when it flowers.

Mowing

Mowing a spring green manure blend at a trial farm. In smaller farms and gardens, a push mower or string trimmer can do the same job.

Mowing at the right time is an efficient, low-disturbance way to end a cover crop. Usually the crop must have moved from leafy growth into flowering for mowing to kill it, and even then some species shrug off repeated mowing.

Knowing whether a crop is an annual, biennial or perennial helps predict whether mowing will kill it. Most annuals, such as grains and some broadleaves, die if mowed while flowering. Perennials and biennials, such as some clovers and alfalfa, regrow after several mowings. That's useful if you want a durable living mulch, but a problem if you need to switch to another crop.

Mowing alone, without tillage, leaves residue lying on the surface. That mulch can be helpful, but it gets in the way if a small-seeded crop such as carrots or lettuce comes next. To speed breakdown, chop the residue as finely as you can; a flail mower makes smaller pieces than a string trimmer or rotary mower. If you'd rather keep a lasting mulch, a scythe, sickle-bar mower or string trimmer leaves stems mostly whole so they decompose more slowly.

Tillage

Tillage covers many tools and methods, all aimed at working cover crop residue into the soil. The more aggressive the tillage, the more finely the residue is chopped and buried and the faster it breaks down, but that usually means larger equipment and, if misused, harm to soil life.

As with mowing, the tool has to suit the growth stage. A tiller or disc harrow may chop and bury young, leafy wheat easily, but the same wheat in flower is likely to wrap and clog those tools.

For most cover crops, tillage works best while the crop is still leafy, and a more aggressive tool needs fewer passes to finish the job. These rules have exceptions, so learn each crop's habits and test different approaches to see what fits your situation.

Smothering/Tarping

A silage tarp works well for smothering a cover crop.

Covering a cover crop with a tarp is an effective no-till way to end it. With no light, the plants can't photosynthesize and run out of the sugars they need to keep growing.

Since the plants must exhaust their reserves before they die, tarping is slower than other methods; depending on the crop, a full kill can take weeks, which is awkward if you need the bed soon. Timed well, though, it's a gentle and very effective method.

Like mowing, tarping leaves residue on top of the soil. Left in place for a long time, a tarp keeps the soil moist and warm, which speeds decomposition, especially of lush, leafy cover crops.

Bulk is often the biggest hurdle. Tall grains may need mowing or crimping first so the tarp lies flat. Tarps must also be weighted down well with sandbags, blocks or rocks, especially in wind, and storing them can be tricky where space is tight. Still, tarps are an excellent choice for small plantings and no-till systems, ending many kinds of cover crops with no soil disturbance.

Crimping

Plants move water and nutrients through vascular tissue, and if it's cut or blocked, they die. Crimping uses a tool with blunt ridges to flatten the crop and crush that tissue, leaving the dead crop lying on the soil as a mulch that protects the surface and suppresses weeds. Crimping works best during flowering, when plants are putting their energy into flowers and seed and can't easily bounce back. Crimped while still leafy, many crops simply push out new growth and recover.

Purpose-built roller-crimpers usually have a drum with flat steel blades in a chevron pattern. The chevrons let the drum roll smoothly while focusing pressure on the blades to crush the stems. The aim is to crush each stem in several places without cutting through it, so the residue breaks down slowly and gives the best mulch.

Ending a cover crop with a roller-crimper.

At small scale, everyday items work too, such as a T-post or two 2x4s screwed together in a T. Tie a rope to each end as a handle, lay the post or boards on the crop, and have one or two people step on it to flatten and crimp the stems. Lift, move ahead 8–12", and repeat.

Winter Kill

Where winters are cold, many frost-tender cover crops die on their own. Sown in summer or fall, they grow until the cold kills them. The big plus is how little work this takes, and many leave only light residue that's easy to work in come spring. Crops with heavy biomass or woody stems, such as sudangrass or sunflowers, may still need mowing or tilling before the next planting.

To judge whether a cover crop will winter-kill where you live, check the USDA plant hardiness zone map along with the cover crop comparison chart, which lists each cover crop's minimum hardiness zone.

Stacking Techniques

Often the best approach combines two or more methods in sequence, both to make sure the crop dies and to leave the bed in the best shape for what comes next.

Typical combinations:

• Mow, then till: chopping the residue first makes it easier to work into the soil
• Crimp, then tarp: flattening the crop first makes it easier to cover
• Mow before or after winter-kill: chopping the standing crop around the time cold kills it makes spring incorporation easier

Termination Efficacy by Cover Crop

Use the chart below to find the best method, or combination of methods, for each cover crop. It rates methods separately for the leafy (vegetative) stage and the flowering (reproductive) stage.

Recommended Termination Methods by Cover Crop
Key	Poor: Not effective; very hard to manage because of size or persistence.		Fair: Limited; hard to manage because of size or persistence.		Good: Fairly effective; workable with the right tools and timing.		Best: Effective; easy to manage.
	Till, leafy stage	Till, in flower	Mow, leafy stage	Mow, in flower	Crimp, in flower	Tarp, leafy stage	Tarp, in flower	Winter-kill
Summer alfalfa	Good	Poor	Poor	Poor	Poor	Best	Best	Poor
Barley	Good	Poor	Poor	Best	Best	Good	Fair	Good
Buckwheat	Best	Best	Good	Good	Good	Good	Good	Best
Berseem clover	Best	Best	Fair	Best	Fair	Good	Good	Best
Crimson clover	Good	Fair	Poor	Fair	Fair	Best	Best	Fair
Mammoth red clover	Fair	Fair	Poor	Poor	Poor	Best	Good	Poor
Medium red clover	Fair	Fair	Poor	Poor	Poor	Best	Best	Poor
New Zealand white clover	Fair	Fair	Poor	Poor	Poor	Best	Best	Poor
Sweet clover	Fair	Poor	Poor	Fair	Poor	Best	Fair	Poor
Fall green manure blend	Fair	Poor	Poor	Poor	Poor	Good	Fair	Poor
Spring green manure blend*	Good	Poor	Poor	Fair	Fair	Best	Fair	Good
Millets	Fair	Poor	Poor	Best	Best	Fair	Poor	Best
Mustards	Best	Good	Fair	Best	Fair	Best	Fair	Good
Common oats	Good	Poor	Poor	Best	Good	Best	Fair	Best
Hulless oats	Good	Poor	Poor	Best	Good	Best	Fair	Best
Pea and oat blend	Good	Good	Poor	Best	Fair	Best	Good	Best
Field peas	Good	Good	Good	Best	Fair	Best	Good	Best
Oilseed radish	Good	Good	Fair	Best	Poor	Best	Good	Best
Winter rye	Fair	Poor	Poor	Best	Best	Best	Fair	Poor
Annual ryegrass	Fair	Fair	Poor	Good	Poor	Best	Good	Fair
Sudan grass	Poor	Poor	Poor	Good	Poor	Fair	Poor	Best
Sunflowers	Fair	Poor	Fair	Best	Poor	Fair	Poor	Best
Sunn hemp	Fair	Poor	Fair	Good	Poor	Fair	Fair	Best
Teff grass	Fair	Fair	Poor	Poor	Poor	Good	Good	Best
Hairy vetch	Poor	Poor	Poor	Good	Best	Best	Fair	Poor
Spring wheat	Good	Poor	Poor	Best	Best	Best	Fair	Good
Key	Poor: Not effective; very hard to manage because of size or persistence.		Fair: Limited; hard to manage because of size or persistence.		Good: Fairly effective; workable with the right tools and timing.		Best: Effective; easy to manage.
*the vetch survives frost; the peas and oats do not

Good termination is key to getting the full value from cover crops. Match the method to the crop's growth stage, your tools and your goals, and termination can be smooth and simple. With good timing, careful management and some willingness to experiment, cover crops can anchor a sustainable growing system.