Quick quiz: Which element of nature is most damaging to a Northwest garden? If you said rain, you'd be in good company. In the fall, you...

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Quick quiz: Which element of nature is most damaging to a Northwest garden? If you said rain, you’d be in good company. In the fall, you can greatly reduce the damage to your vegetable garden’s topsoil and nutrients by planting “cover crops.”

Winter plants, such as fava beans and cereal rye, will build your garden’s soil, smother weeds and generate free fertilizer until spring, when your cover crop can be chopped down for another vegetable-growing season.

“It’s a deeper level of organic gardening,” explains Kathy Dang, a garden coordinator at Seattle Tilth. “You’re growing something just for the soil, as opposed to all the plants we grow for us.”

What to sow

Cover crops are primarily legumes (such as fava beans, field peas, crimson clover and vetch) or cereal grains (like rye, barley and winter wheat). Your choice could be dictated by taste or practicality.

Crimson clover has beautiful flowers and is easy to handle. Hairy vetch offers “extrafloral nectaries,” nectar openings on its stems which will feed beneficial insects well before most flowers are open. To support the vetch’s vining habit, grow it with rye or wheat.

Another consideration is the garden’s next crop. “If you’ll be growing peas or beans, maybe don’t use a legume cover crop,” says Dang. “But legumes could be followed by corn or squash, because those are heavy feeders.”

Timing might limit your choice. Some cover crops are most productive if sown in early fall. But “the summer vegetables tend to be not quite done when you should be planting cover crops,” says Kathleen Glasman, perennial buyer at City People’s Garden Store. Seeds such as fava beans, field peas and rye “can germinate in very cool weather, so you can procrastinate.”

How they help

The primary benefits from cover crops happen below ground: the soil gains nitrogen, a better structure and greater biological activity.

“If you grow legumes, they will ‘fix’ nitrogen in the soil, and you can reduce the amount of fertilizer in the spring,” explains Craig Cogger, soil scientist for Washington State University’s Research and Extension Center in Puyallup. “You may be able to reduce by one-third to one-half the amount of nitrogen you need to add.”

Legumes attract soil-dwelling bacteria that attach to the plant’s roots and pull atmospheric nitrogen out of the air and soil, storing it on the roots as white nodules. When the plant is cut down and chopped up to decompose in the garden bed, that nitrogen remains in the soil to feed the leafy growth of other plants.

The impact of winter rain is reduced, too. Nitrogen and other nutrients can leach from the soil into groundwater and wastefully drain away. Compaction from rain can reduce soil air space and break up the good soil structure needed for healthy plant roots.

Some ward off these problems by laying compost or mulch, but Dang says “cover crops have an advantage because they have extensive root systems that do more to build the soil.” The tap roots of legumes will break up clay soil, whereas grains have a root system that will help bind up sandy soils, she says.

“I’ve noticed that the structure of the soil has improved,” says Thomas Hargrave, a P-Patch gardener at Sand Point Magnuson Park. “You’re adding a lot of air space, making it easier for a root to penetrate into it.”

From sowing to chopping

All those benefits, yet cover crops require very little care. Rake the bed clear of previous crops. Sow by hand and rake it in. Water to help them germinate, unless nature takes that job.

If sown early enough (see chart on back page), cover crops will gain a few inches of growth before winter. In early spring, they will take off in a blaze of growth. Crops like clover, fava and vetch will begin to flower. About that time, chop them down, triggering another benefit to your soil: the addition of your vegetative fertilizer, known as “green manure.”

“I start at the top, using pruning shears to chop it down in two-inch segments until I can see the plant roots,” says Hargrave. “After that, it’s really easy to roll it in with a shovel.”

Early spring is the best time to dig the cover crop into the soil, says Dang, because that dose of green “biomass” is at its peak before the plants mature. After a few weeks of decomposition, your energized soil is ready for vegetable seeds and starts.

Bill Thorness is a freelance garden writer in Seattle: bill@thorness.com