Annual and short-lived perennial clovers are frequently used alone or in mixes with grasses as cover crops.

Annual Legumes
In mild environments, annual legumes can be sown in fall or spring. In cold-winter regions, annuals are planted in the spring after danger of hard freeze. Winter annuals (like subterranean clover) should be planted in the autumn.

Black Medic
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(See http://www.sarep.ucdavis.edu/cgi-bin/ccrop.EXE/show_crop_8)

Crimson Clover
Trifolium incarnatum
Type: winter annual or summer annual legume
Roles: N source, soil builder, erosion prevention, reseeding interrow ground cover, forage
Mix with: rye and other cereals, vetches, annual ryegrass, subclover, red clover, black medic
Information from "Managing Cover Crops Profitably"
(See http://www.sarep.ucdavis.edu/cgi-bin/ccrop.EXE/show_crop_16)

Subterranean Clover
Trifolium subterraneum, T. yanninicum, T. brachycalcycinum
Also called: Subclover
Type: reseeding cool season annual legume
Roles: weed and erosion suppressor, N source, living or dying mulch, continuous orchard floor cover, forage
Mix with: other clovers and subclovers
Information from "Managing Cover Crops Profitably"
(See http://www.sarep.ucdavis.edu/cgi-bin/ccrop.EXE/show_crop_39)

Perennial Legumes
Short-lived perennial legume species are sometimes used as cover and green manure crops.

Red Clover
Trifolium pratense
Also called: medium red clover (multi-cut, early blooming, June clover); mammoth clover (singlecut, late blooming, Michigan red)
Type: short-lived perennial, biennial or winter annual legume
Roles: N source, soil builder, weed suppressor, insectary crop, forage
Mix with: small grains, corn, soybeans, vegetables, grass forages
Information from "Managing Cover Crops Profitably"
(See http://www.sarep.ucdavis.edu/cgi-bin/ccrop.EXE/show_crop_13)