How to Grow Cover Crop Mixes in Australia

Cover crop mixes blend legumes such as field pea (Pisum sativum, Fabaceae) with cereals such as oats (Avena sativa, Poaceae). Many add buckwheat (Fagopyrum esculentum). Sown to cover resting soil and then cut back into it, the legumes add nitrogen and the cereals add organic matter. Where gardeners go wrong is season: each mix suits a soil temperature, and the wrong one stalls.

Three mistakes that matter

1. Sowing the wrong mix for the season. Summer species such as soybean, mung bean and buckwheat sit still in cold soil and die in frost. Cereals and ryegrass slow down in summer heat. Match the mix to the soil temperature in Choosing a mix.
2. Letting it set seed. Once a cover crop has fully flowered, its nitrogen content falls, and ripe seed comes up as weeds through the next crop. Ryegrass, millet and early-seeding buckwheat are the ones to watch. Cut at first flowers, as Cutting and Digging In describes.
3. Sowing seed straight into the dug-in crop. Fresh green material breaks down fast, and for a few weeks that activity can hold back germinating seed. Cutting and Digging In gives the wait, and a way round it.

When to plant

Soil temperature decides which mix goes in. Cool-season mixes want soil between about 8°C and 20°C, so autumn is their season, and Anzac Day sits inside the southern window. Summer mixes need soil above 15°C to 18°C, after the last frost. One bed can carry a cool mix from autumn to spring, then a summer mix until autumn.

Late in autumn, choose a cereal-based mix, because all three cereals tolerate frost. In hot, dry spells, water a summer mix daily for its first week.

Site, soil and setup

Any resting vegetable bed, a new garden area or the bare ground under fruit trees can carry a mix.

Sowing Seeds

Every mix is broadcast straight onto the bed and raked in: no trays, no transplanting.

Choosing a mix

Three mixes are built to stay rather than be dug in. Living Mulch, Erosion Control and the Pollinator mix settle into lasting clover and lucerne cover. Single-species cover crops, lablab, tillage radish, soybean, buckwheat and millet, each have a guide of their own, as does the Chicken Forage Mix. The two biofumigation blends belong with Biofumigant Mustard, and brassica biofumigation explains the method.

Sowing direct

• Mix fine seed with dry sand so it spreads evenly, then scatter at the rate in the table.
• Rake it in 1 to 2 cm deep, the most its finest seed will take; Triple Cereal, all cereals, can go 2 to 3 cm.
• Water in, and keep the surface moist for 10 to 14 days while the slower species come up.
• Expect buckwheat and mung bean in 3 to 5 days, peas in 5 to 10, and clover and lucerne in 10 to 14.

Legumes fix nitrogen only with their matching rhizobia, soil bacteria that form root nodules. A bed that has not grown that legume for four years or more may lack them. A legume inoculant for the right group, coated on just before sowing and kept out of the sun, supplies them. Peas, clovers, lucerne and the summer beans each take a different group, so a mix can need more than one.

Care by growth stage

Moisture in the first fortnight decides a mix. After that most cover crops look after themselves.

Leave Living Mulch unmown for its first 12 weeks, and Erosion Control for 8 to 12 weeks, so the perennials root. If a cereal mix stalls on poor sandy soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched in can help.

Companion planting

With a cover crop, the companions that matter are the crops that come before and after it.

Grows well with

• Tomatoes, corn, pumpkins and potatoes, hungry crops that follow a legume mix well.
• A legume mix after the Triple Cereal, to make up the nitrogen the cereal straw ties up.
• Fruit trees over Living Mulch, sown with 30 cm kept clear around each trunk.
• Vegetable beds edged with the Pollinator mix, which feeds beneficial insects while it flowers.

Keep apart

• Broccoli, cabbage and other brassicas straight after a radish, rape or mustard mix, which can carry their pests through.
• Carrots and other small seed sown straight after a cereal mix is dug in.
• Ryegrass mixes beside vegetable beds or bushland, because perennial ryegrass persists for years.

Cutting and Digging In

Cut when the first flowers open, before any seed sets; cereals are cut at boot stage, just before the heads emerge from the stem. In a mix, cut before the quickest species, often buckwheat, sets seed. Most annual mixes reach this 6 to 16 weeks after sowing.

• Slash or mow the crop low, then chop it finely so it breaks down faster.
• Dig it into the top 10 to 15 cm, or leave it on the surface as mulch.
• In a no-dig bed, cover the cut crop with a tarp until it has died, about 2 weeks for Quick Flip, and check before planting.
• Wait 2 to 6 weeks before sowing seed into a dug-in bed.
• If the bed is needed sooner, leave the tops as surface mulch and plant through them.
• Mow Living Mulch to 10 to 15 cm once it passes 30 cm. Cut ryegrass before it flowers near anyone with hay fever or asthma, as its spring pollen can trigger both. Let the Pollinator mix flower, then cut it back as the flowers fade, before seed ripens.

Cut tops not needed on the bed make mulch around the rest of the garden. Keep leftover seed dry, cool and dark.

Weed lists, state by state

No species in these mixes is a declared weed in any state or territory, and Western Australia lists every one as permitted. Several are naturalised, though, some are on environmental weed records, and Gardening Australia warns lablab may be one in some areas. Cutting before seed set keeps them in the bed.

Common problems

Seed gone before it sprouts. Birds pick broadcast seed off the surface, leaving bare patches. Lay shade cloth over the bed after sowing and lift it once seedlings appear.

Only buckwheat and millet showing. The fast nurse species come up first and shade the ground while clover, lucerne and ryegrass root below. Give a lasting mix three months before judging it.

Pale, slow legumes. Few or no nodules on a lifted root means the rhizobia did not take (see Sowing Seeds). The legumes then draw on soil nitrogen instead of adding it.

Yellow, slow crop after a cereal mix. Microbes breaking down carbon-rich cereal straw tie up soil nitrogen for a while. Cut cereals younger, leave them as surface mulch, or follow with a legume mix.

Pests and diseases

Seedlings in their first three to five weeks carry most of the risk, and southern autumn sowings meet earth mites.

Redlegged earth mite (Halotydeus destructor) and blue oat mite (Penthaleus species). Velvety black redlegged earth mites, about 1 mm long with eight orange-red legs, silver the leaves of clover, lucerne, peas and cereals. Blue oat mites carry a red mark on the back and feed singly. Both feed from autumn to late spring in southern Australia, and heavy numbers kill seedlings as they emerge.

Lucerne flea (Sminthurus viridis). A light green-yellow springtail up to 3 mm long, often with darker mottling, that rasps clear windows in clover and lucerne leaves. It is worst in moist southern winters and on loam or clay.

Common garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They graze seedlings overnight after rain, leaving slime trails. Hand-pick them after dark. Read more

Common armyworm (Mythimna convecta). Striped, smooth caterpillars up to 40 mm long that hide at the base of plants by day and strip cereal leaves at night. Barley, oats and wheat are among its hosts. Read more

Oat aphid (Rhopalosiphum padi) and cowpea aphid (Aphis craccivora). Colonies suck sap from cereals and legumes, though they rarely do real damage and predators usually clear them. Hose off heavy clusters. Read more

Cutworms (Agrotis species). Caterpillars that hide in the soil by day and cut young seedlings off at or below ground level at night.

Prevention first

• Clear weeds, especially capeweed, well before sowing, because mites, lucerne fleas and cutworms build up on them.
• Check seedlings in the morning or on overcast days, when mites are easiest to see.
• Remove boards, pots and debris where slugs and snails hide by day.
• Avoid broad-spectrum sprays, which kill the predators that keep aphids and armyworms down.

Tips

1. Before sowing seed kept from an earlier year, test a few on moist tissue in a small container and watch for roots.
2. Sow straight after clearing a finished crop, because bare soil dries out, loses structure and lets weeds in.
3. On a slope, sow at the higher rate and rake downhill. A 1 to 2 cm layer of straw over the seed helps stop rain washing it away.

Due to quarantine restrictions and our shipping policy, we cannot send cover crop mixes to Western Australia or Tasmania.