Green Manures: Which Cover Crops to Grow

Green manure and cover crops are vital to the success and sustainability of any garden.

Credit: iStock / purplevine

Green manure and cover crops are vital to the success and sustainability of any garden

Sheena’s best cover crops for adding nutrients to soil:

Red Clover

(Trifolium pratense) – hardy to zone 5 Winter-hardy, short-lived and a superb nitrogen fixer – this is considered one of the best green manures. Sow it in early spring through September. Attractive with its traditional cloverleaf and honey-scented flowers, it reaches a mature height of only 30 cm (12 in.), with a taproot three times as long. A food source for bees and shelter for beneficial ground beetles. This clover is a useful cover crop in summer or winter. If grown in an apple orchard, farmers report the trees will produce tastier fruit. Do not grow red clover with camellias or gooseberries because it harbours a mite that can cause fruit drop in the gooseberries and premature budding in the camellias.

Tips for Using Cover Crops

• For your first spring plantings, when the tilth is quite rough, choose plants that cope well in those conditions, such as potatoes, broccoli, Asian greens, Brussels sprouts and cauliflower. • Decide in fall where you will plant the following spring’s root crops and sow a cover crop with a large taproot to help break up the soil. Consider a combination of alfalfa and fava beans. The root crops will appreciate the open soil texture and added phosphorus. • Never let green manures go to seed. • Grow a crop of buckwheat during the summer, cut it down and mulch it into next season’s tomato, cucumber and squash beds. Sow a winter cover crop over top. The calcium lovers will reward you next season with a bumper crop. • For next summer, consider covering any bare areas with buckwheat. This green manure not only attracts beneficial insects such as hoverflies and bumblebees, it also is an excellent source of phosphorus and calcium. It helps to break up compacted soil, prevents wind erosion and will compete with weeds. This one can be left to flower for one week, and then it should be tilled in.

Fava Bean

(Vicia faba) – hardy to zone 7 or 8 This is an excellent nitrogen fixer with an extraordinarily long taproot useful for breaking up clay or compacted soils. When tilled in, the leaves decompose rapidly; however, the fibrous stem will loosen heavy soils. It may also be cut and composted, leaving the nitrogen-fixing roots in the soil. Plant in early fall or early spring, as it thrives in cooler temperatures. For extra winter hardiness, seek out the Banner variety. As with all legumes, be sure to rotate crops within the garden. Pull the plants or till them in before they form pods.

Fall Rye

(Secale cereale) – hardy to zone 3 This cover crop suppresses weeds and prevents erosion and soil compaction. It grows well from fall through spring, when it should be tilled in. It is quite fibrous and should be tilled in at least three weeks before planting. It is an excellent soil amender and supplier of nutrients, particularly phosphorus.


(Medicago sativa) – hardy to zone 5 Alfalfa fixes nitrogen and is an excellent weed suppressor. Its roots can go down 1.2 m (4 ft.) to reach nutrients deep in the earth and break up the subsoil. It can be grown year-round; it will die back in severely cold winters but should resprout in spring, when it can grow until it is time to till it in. It should be double-tilled to prevent resprouting. Alfalfa does not like water-logged or acidic soil.

Austrian Winter Pea

(Pisum arvense) – hardy to zone 6 This very hardy pea is an excellent nitrogen fixer. Sow in fall or early spring. The wiry stems can be tilled in, as they compost quickly. The crop provides a home for many beneficial insects, such as ladybugs and predatory mites. Till, or cut and compost, when flowers appear. This one can be companion-planted with fall rye.

Hairy Vetch

(Vicia villosa) – hardy to zone 4 This is an excellent weed suppressor and supplier of nitrogen. Winter-hardy and tolerant of poor soil conditions, this can grow where no other cover crop can survive. Sow in fall or spring and till or dig in at the first sight of blooms.