Terms: Green Manure and Cover Crop

A cover crop is simply high numbers of plants, usually specific annual, biennial, or perennial grasses and/or legumes, growing and covering the soil surface.  When the cover crop is tilled into the soil it is referred to as a green manure crop.  These two terms are often used interchangeably. 

Why is Cover Cropping Beneficial?

Cover crops can protect the soil from wind and water erosion, suppress weeds, fix atmospheric nitrogen, build soil structure, and reduce insect pests.

Erosion protection – The primary erosive force for Colorado is wind.  Winter winds are especially destructive, carrying away small surface particles of topsoil.  A thick stand of a cover crop protects the soil surface from wind erosion and its roots can hold soil in place against water erosion during heavy downpours.

Weed suppression – Cover crops left in place for part or all of a growing season can suppress annual and some perennial weeds.   Among the grasses, annual rye has alleopathic properties that prevent weed seeds from germinating and suppress weed seedlings around the root zone of the rye.

Nitrogen fixation – Legumes, inoculated with their specific rhizobium bacteria, will take nitrogen out of the air (present in the soil) and store it in their plant tissues via nodules on the roots of the legume.  Some of this nitrogen is available as roots die, but the majority becomes available when the legume is tilled under (green manure).

Soil Structure Creation – Plant roots exude a sticky substance then glues soil particles together, creating structure.  Grasses are exceptional in their ability to do this.

Insect Pests Reduction – Cover crops encourage beneficial insect populations, often minimizing or eliminating the need for other insect control measures.

Why is Green Manuring Beneficial?

Green manuring enhances soil fertility and soil structure by feeding soil organisms and gluing together soil particles into aggregates. 

Soil Fertility – When fresh plant material breaks down in the soil, its carbon to nitrogen ratio is low, allowing the nitrogen to be easily released into the soil chemistry by bacteria.  Nitrogen accumulated is greater in legumes with rhizobium growing on roots (forming pink nodules), as shown in Table 1.  Notice the lower figure for rye.

Table 1.  Nitrogen accruement of selected cover crops

Cover Crop	Nitrogen Accruement*
Hairy vetch	3.2 lbs/1000 ft2
Crimson clover	2.6 lbs/1000 ft2
Austrian winter pea	3.3 lbs/1000 ft2
Winter (annual) rye	2.0 lbs/1000 ft2

* Nitrogen accumulated in growing crop prior to tilling under
Source: ATTRA: Overview of Cover Crops and Green Manures

Table 2 shows values of nitrogen fixation for legumes.  Rates vary due to activity of rhizobium.

Table 2.  Potential nitrogen fixation rates of selected legumes for Colorado

Legume Crop	Pounds N per 1000 ft2
Crimson clover	1.6-3.0
Field peas	2.0-3.4
Hairy vetch	2.0-4.6
Medics	1.1-2.8
Red clover	1.6-3.4
Sweet clover	2.0-3.9
White clover	1.8-4.6

Source: Managing Cover Crops Profitably, Sustainable Agriculture Network

Soil Structure – Microorganisms decomposing plant material and the plant material itself produce substances that glue soil particles together.  These include slime, mucus and mycelia with the gums, waxes, and resins that aggregate soil particles enhancing the tilth, porosity, and water holding capabilities of soil.

Basic Recipes for Cover Crops and Green Manure Crops in a Garden

Spring Planted
Most gardeners do not have enough space to forfeit to a cover crop for an entire growing season.  However, if you do a spring seeded clover would give your soil a great boost.  Some seed companies will “rhizo-coat” seed with the specific rhizobium for you.  If not, apply rhizobium as specified on the bag.  Rhizobium comes in a black powder specific to the species of clover.  It also has a definite shelf life, so check the expiration date.  Broadcast the seed/rhizobium mix at a specified rate after the last frost with a hand held broadcaster (often used with pelleted fertilizer) into a loose seedbed and incorporate shallowly and water until germinated.  Monitor water as you would in a lawn.

Till under at least two weeks prior to planting.  Decomposing plant material consumes soil oxygen and can create plant health problems if not tilled in ahead of time.  More than one tilling may be necessary to get an acceptable kill of the clover.

Fall Planted for Spring Till
Most will opt for a fall cover crop tilled under as a spring green manure.  Seeding dates should be by mid-October at the latest.  Mid-September is ideal on the Front Range.  In mountain elevations, plant in August or earlier.  A rye/Austrian winter pea or rye/hairy vetch mixture will overwinter in Colorado.  Hairy vetch is hardier than winter pea.  Rye is extremely winter hardy.  Prepare as above and broadcast at the rates in Table 3. 

Table 3.  Seeding Rates for Selected Winter Cover Crops

Cover Crop	Ounces per 100 square feet	Pounds per 1000 square feet
Winter Rye	4 – 6	2.5 – 3.75
Austrian Winter pea	4 – 6	2 – 4
Hairy Vetch	2 – 3	1 – 2

Source: Managing Cover Crops Profitably, Sustainable Agriculture Network

Over-wintered cover crops become a veritable salad-bar to geese and deer.  A cover crop that is well established prior to winter temperature extremes should rebound from wildlife grazing in late winter/early spring.

Till the cover crop in mechanically or turn it under with a spade a month before you plan to plant/seed into that area.  Decomposing plant material consumes soil oxygen and can create plant health problems if not tilled in ahead of time.

Landscape Uses

Bare soil presents erosion and aesthetic issues for homeowners.  During droughty periods watering restrictions and natural precipitation may make turf establishment difficult or impossible.  A temporary cover crop or long term xeric grass may be the answer.

In this scenario, the homeowner has to understand that a cover crop will not look or feel like a healthy Kentucky bluegrass lawn, but should satisfy the need to cover the soil.

Species Selection

Annual Species Options
These are cool season grains that should be broadcast at 2-3 pounds per 1000 square feet in February or March.  Natural precipitation may be sufficient to get them established.  They are suited for non-traffic areas, as they will grow to 2 feet tall and brown-out in the heat of summer.  The Sterile Triticale will not produce viable seeds so may be a good idea for areas that will eventually be put into turf or garden space.  Winter rye seeds can be a weed problem in seeded turf grass and gardens.  [Table 4]

Table 4.  Annual Species

Name	Bunch or Sod	Cool or Warm Season	Annual or Perennial	Turf?	Reseed?
Winter rye	Bunch	Cool	Annual	No	Yes
Pioneer Sterile Triticale	Bunch	Cool	Annual	No	No

Perennial Species Options
These are non-native grasses often used on roadsides for stabilization and cover.  They are perennial and will be persistent (i.e. – difficult to kill) once they are established.  Water requirements for both are 9-10 inches of precipitation per year.  Streambank wheatgrass has a slightly higher water requirement but is tolerant of very clayey soils, unlike Crested wheatgrass.  Broadcast in February or March at 3-5 pounds per 1000 square feet.  [Table 5]

Table 5.  Perennial Species

Name	Bunch or Sod	Cool or Warm Season	Annual or Perennial	Turf?	Reseed?
Streambank Wheatgrass	Sod	Cool	Perennial	Yes	Some
Crested Wheatgrass	Bunch	Cool	Perennial	Yes	Some

Native Species Options
These have the lowest water requirements at 8 inches of precipitation per year and should be considered for areas of a landscape that are being converted to xeric management.  This is a long-term management decision as the price of these seeds is more than the other options.  These will not feel like Kentucky Blue grass and will brownout like other cool season grasses.  Seed as per perennial species options specifications.  Seed for native species will be available from local seed sources, such as Pawnee Buttes Seeds.  [Table 6]

Table 6.  Native Species

Name	Bunch or Sod	Cool or Warm Season	Annual or Perennial	Turf?	Reseed?
Indian Ricegrass	Bunch	Cool	Perennial	No	Some
Squirreltail Bottlebrush	Bunch	Cool	Perennial	No	Some

Establishment and Care

Before Seeding – Prepare a seedbed for fine grass seed, ideally amending the soil with compost and tilling as deeply as possible.  If possible, fence off the area from traffic.

Seeding – Water area prior to seeding if possible to establish ample soil moisture levels.

Broadcast the correct amount of seed per area onto a loosely tilled, fine (no soil pieces bigger than 1/4 inch) seedbed.  Shallowly incorporate seed with garden rake (not a leaf rake) to a depth of ¼ to ¾ inch deep.

For larger areas consider hydromulching the seed.  This will save time and increase germination of seeds.

After seeding – Consider laying a thin layer (<1” deep) of seed free straw to hold in moisture and increase germination and survival of grass seedlings.  Bird netting over the straw fastened to the ground with landscape fabric staples will keep the straw from blowing away.

Check moisture levels in the upper inch of soil at least every other day (soil should feel as moist as a wrung out sponge) and water if necessary (and if possible).

Mowing – If necessary, mow as high as possible.

Removing cover crops – For Winter Rye, either till under, mow and mulch heavily, or spray herbicide before it goes to seed.  A seed bank can be sodded over or watered, germinated and killed.  Perennial grasses can be either mowed and mulched heavily prior to sodding or sprayed with herbicide and sodded or sprayed with herbicide, tilled and seeded.

Additional Information – CMG GardenNotes on Soils, Fertilizers and Soil Amendments:

#211 The Living Soil
#212 Earthworms
#213 Managing Soil Tilth
#214 Estimating Soil Texture
#215 Soil Compaction
#216 Mulching with Wood/Bark Chips, Grass Clippings and Rock
#217 Soil Drainage
#221 Soil Tests
#222 Soil pH
#223 Iron Chlorosis
#224 Saline Soils
#231 Plant Nutrition
#232 Understanding Fertilizers
#233 Calculating Fertilizer Rates
#234 Organic Fertilizers
#241 Soil Amendments
#242 Using Manure
#243 Using Compost
#244 Cover Crops and Green Manure Crops

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