Spokane County Extension

Agriculture and Natural Resources

COVER CROPS

 Introduction 

Cover crops, which are also referred to as green manures, provide many benefits in gardens and agriculture, including protecting the soil from runoff and erosion, recycling and supplying nutrients, replacing soil organic matter, and suppressing weeds. Your choice of a cover crop will depend on which benefits are most important to you. This bulletin describes the benefits of cover crops, different types of cover crops and their suitability for different situations, and how to grow and manage cover crops in your garden.

What is a cover crop?
Cover crops are plants grown to cover the soil during idle periods. Gardeners usually plant cover crops in the fall for winter cover, but some gardeners also use cover crops as part of a summer rotation. When cover crops are returned to the soil, they supply plant nutrients and organic matter.

Choosing a cover crop 
Table 1 lists benefits provided by cover crops. No one cover crop provides all of these benefits. Different cover crops offer different benefits and disadvantages. Deciding which cover crop or combination of cover crops to grow depends on which benefits are most important to you, and which cover crops best fit into your garden management.

Table 1. Benefits of cover crops

 •Replacing soil organic matter

•Reducing runoff and erosion

•Recycling nutrients

•Protecting water quality

•Supplying nitrogen (only if incorporated)

•Suppressing weeds

•Protecting soil from raindrop impact

•Reducing disease and nematodes (some    
evidence for this)

 

Specific cover crops and their traits

Common cover crops belong to one of three groups:
•Cereal grains and grass
•Legumes
•Other broad-leaved plants

Cereal grains grow vigorously and can provide quick ground cover even when the weather is cool. They yield large amounts of biomass when planted and harvested at the proper times. Legumes generally grow slower than cereal grains in cool weather, but grow rapidly when the weather is warm. They can fix nitrogen from the atmosphere, increasing the amount of soil nitrogen available for the next crop. Most other broad-leaved plants are more difficult to grow and manage than cereal grains and do not provide the nitrogen-fixing benefits of legumes. Buckwheat is a frost-sensitive broadleaf that is easy to grow in warm weather and is well suited as a summer cover crop.

Cover crops within each family are described below. Tables 2-4 list average planting dates, seeding rates, seed availability, and chance of winterkill for each crop. Planting dates, chance of winterkill, and management vary with climate. 

Table 2. Cereal and other grass cover crops

Crop

Planting dates

Seeding rate lb/1000 SQ. ft.

Depth

Type

Winterkill

Seed availability

Cereal rye

Late Aug. – Late Oct.

2-3

¾-2

WA

No

+2

Winter wheat

Late Sept. – Early Oct.

2-3

½-1½

WA

Seldom

+

Spring oats

Late Aug. – Early Sept.

2-3

½-2

CSA

Susceptible

+

Annual ryegrass

Late Aug. – Mid Sept.

0.5-1

-

WA

Seldom

+

Fescue

Fall or Spring

1-2

¼ - ¾

LP

No

+

 

Table 3. Legume cover crops

Crop

Planting dates

Seeding rate lb/1000 SQ. ft.

Depth

Type

Winterkill

Seed availability

Hairy Vetch

Late Aug. 0 Early Sept.

1.5-2

½-1½”

WA/CSA

Seldom

+2

Austrian winter peas

Early Sept. – Late Sept.

2-3

½-1½”

WA

Occasional

+

White Dutch Clover

Fall or spring

0.5-1.5

¼-½”

LP/WA

Seldom

+

Fava bean

Early Oct.

2-3

1-2”

WA/A

Seldom

-

Red Clover

Late Sept. – Early spring

1-2

½-¾

SP/B

Seldom

+

 

 Table 4. Other broadleaf cover crops

Crop

Planting dates

Seeding rate lb/1000 SQ. ft.

Depth

Type

Winterkill

Seed availability

Buckwheat

June - Early Aug.

1-2

1-2”

SA

Yes

+1

Rape

Late Aug.

1/8-1/4

1-1 ½”

WA

Seldom

+

Mustard

Late Aug.

1/8-1/4

1-1 ½”

WA

Seldom

-

 

Type; B=Biennial;  CSA=Cool season annual;  LP=Long lived perennial;  SA=Summer annual:  SP=Short lived perennial;  WA=Winter annual; 

Seed availability; 1+ means seed is usually available at feed or garden stores
       - means seed usually must be ordered

Listed below are the characteristics of the most commonly grown cover crops and green manures in the Inland Northwest

Grains and grass

Cereal rye, also referred to as winter rye, is one of the most commonly grown cover crops in the Northwest. It is vigorous, very cold hardy, and can germinate and become established in cool weather. A possible drawback of rye is it can  become weedy in fields used for growing other cereal grains.  This can be avoided by proper incorporation prior to seed head development.

Legumes

The most important benefit of legumes is their ability to fix nitrogen from the atmosphere into the plant. This is different from the grasses, which can only take up nitrogen already available in the soil. Legumes fix nitrogen in association with bacteria called Rhizobia, which form nodules on legume roots. Active nodules look pink when broken open. When the legumes are turned under and decompose, the fixed nitrogen is released for future crops.

Rhizobia are usually present in the soil, ready to inoculate legume plants and begin fixing nitrogen. Occasionally the proper Rhizobium is not present, and nitrogen fixation will not occur. In this case, root nodules will be white or green rather than pink on the inside. If you have not grown a particular legume in your garden before, you should buy the correct Rhizobium and mix it with the seed before planting. Be sure to buy a Rhizobium that is compatible with your legume. Buying Rhizobia is not always necessary, but it is an inexpensive way to ensure inoculation.

Hairy vetch and common vetch. Hairy vetch is one of the more aggressive legumes and is a good companion crop for cereal grasses. It becomes established slowly and is poor ground cover when planted by itself. When the weather warms in the spring, hairy vetch will grow quickly. Its tendrils can wrap around tiller tines, making it more difficult to turn under with a rototiller.

Austrian winter pea.   Austrian winter pea can be grown alone or mixed with cereal rye. It is a poor competitor against winter weeds, and weed competition can choke out Austrian winter pea during mild winters. Gardeners who raise animals can graze them on the Austrian winter pea before turning the crop into the soil.

Other Broadleaf’s

Buckwheat is the most widely grown summer cover crop and is well suited to the Northwest. It grows quickly in warm weather in a wide range of soils and is usually ready to turn under in 30 days. It is possible to grow several crops of buckwheat in a single summer. Buckwheat will smother weeds with its prolific growth. If allowed to grow too large, it becomes difficult to turn under. Buckwheat is sensitive to frost and should not be planted until the danger of frost has passed in the spring.

Cover Crop Mixtures

Cover crops are commonly grown as mixtures, which combine some of the advantages of each of the component species. Cover crop mixtures sometimes function synergistically, providing more benefits than the individual species grown alone.

Cover Crop Management 

Garden planning for cover crops
Cover crops are more likely to perform well if you include them in your garden plan rather than if you plant them as an afterthought. By planning ahead, you will be ready to plant and turn under the cover crops at the appropriate times.

 How to plant
 You can plant seeds either by broadcasting across the area or by using a garden planter that places seeds in rows. Most gardeners will find broadcasting more convenient. You can broadcast by hand from a perforated can or from an inexpensive hand-held crank seeder. If you broadcast, use the high end of the range of seeding rates shown for each crop in Tables 2-4.

Cover the seeds by raking at least 1/4"deep, or rototill no more than 2" deep. This provides good soil-seed contact and protection from drying, which increases germination. You won't be able to work all the seeds below the surface, so do not be concerned that some seeds remain on the surface.

You do not need to fertilize cover crops in established gardens. Enough nutrients will remain available in the garden to meet their needs. Summer and early fall plantings usually need irrigation to germinate and become established.

When to plant
You can plant cover crops by section in your garden, planting the earliest cover crops as soon as harvest is complete in the earliest parts of your garden. Many gardeners are still harvesting some parts of their garden into November or December. Since this is too late for planting cover crops, these parts of the garden are better mulched with straw or compost. If enough space is available, gardeners can plant cover crops between rows of late crops.

When to turn under
Turn the cover crop under before flowering (boot stage or earlier for grasses). Plants become woody and decompose more slowly if they grow to the flower or seed stage. If you can't turn the crop under in time, first chop or mow it, and turn it under as soon as possible. You can remove and compost the clippings, returning them to the garden as compost.

Ideally, turn the cover crop under about 3 weeks before planting the following crop. This gives time for some decomposition to occur, and for the soil to warm. You can turn your cover crop under in sections based on when you plant different parts of the garden. Avoid turning the cover crop when the soil is too wet for field work (when it collects in large clumps under your boots) because this can harm the soil's structure.

How to turn under
If the top growth is too heavy to turn under easily, cut and remove most of the top growth first. Use a rototiller, shovel, or garden fork to turn the cover crop under to a depth of 3-6 inches. You can get into the garden earlier in the season using the hand tools than using a rototiller. Before planting the garden, prepare the seed bed as usual.

Summer cover crops
Summer cover crops are grown to help improve soil quality in future garden space, to provide cover for garden space that isn't planted this year, or as part of a planned garden rotation. Buckwheat is the main summer cover crop, although legumes also can be grown.

Summer cover crops combined with winter cover crops are especially valuable in improving future garden space. Preparation for the cover crop helps break up soil compaction, and the crop helps suppress weeds. Cover crops can be used together with compost to improve the tilth of a new garden. The cycle of summer and winter crops further reduces weed problems in new garden space.

Compiled By Don Dysart. Sources include Managing Cover Crops Profitably, Sustainable Agricultural Network and Extension Publication EB1824 Cover Crops for Western Washington and Oregon  by Craig Cogger, Extension Soil Scientist; Dyvon Havens, Cooperative Extension, Skagit County; Steve Fransen, Extension Forage Agronomist, all of Washington State University; John Luna, Extension Specialist, Integrated Farming Systems OSU; Wilbur Anderson, Horticulturist, and Shiou Kuo, Soil Scientist, both of Washington State University.

Resources: 

Managing Cover Crops Profitably,  Sustainable  Agriculture Publications, UVM, Hills Building, Burlington, VT  05405-0082
Phone (402) 472-7081 e-mail SARE002@unlvm.unl.edu

ATTRA Appropriate Technology Transfer for Rural Areas  - Responding to specific sustainable practices or enterprise questions.

P.O. Box 3657 Fayetteville, AR 72702
1-800-346-9140     http://www.attra.org/

Washington State University Extension, Spokane County
N. 222 Havana St.
Spokane, WA 99202
Telephone #: (509)-477-2048
Hours: 8:30 am-5 pm
http://www.spokane-county.wsu.edu/

Washington State University Extension
PO Box 646230 (mailing)
Hulbert Hall 411 (street)
Pullman, WA 99164-6230
Phone(509) 335-2933FAX (509) 335-2926
Web http://ext.wsu.edu/

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