Cover Crops: How to Feed Your Soil Over Winter

A cover crop is a fast-growing plant you grow to protect and feed the soil rather than to harvest. For most Australian gardens, sow it in late summer to autumn so the bed is covered through winter, then cut it down and dig it in, or leave it as mulch in a no-dig bed, a few weeks before spring planting. A mix of a legume and a cereal, such as field peas and oats, gives you nitrogen-rich green growth, root structure and organic matter.

An empty winter bed looks tidy, but it is quietly going backwards. Bare soil loses nutrients to winter rain, washes and compacts, and gives weeds an open run. There is a better way to use a resting bed, and it costs little more than a packet of seed.

What a cover crop, or green manure, does

A cover crop, also called a green manure crop when it is grown to be dug back into the soil, is something you grow not to harvest, but to look after the soil. You sow it, let it grow, then cut it down and turn it in, or chop it and leave it on the surface if you garden no-dig, a few weeks before spring planting. As it breaks down it becomes organic matter, the dark, crumbly material that holds moisture and feeds the crops that follow. Along the way it shields bare soil from heavy rain, holds it together with its roots, and crowds out weeds.

Choosing what to sow

A good winter cover crop usually combines two types of plant, and it helps to think about what each one brings.

Legumes, such as field peas, faba beans, vetch, clover and lupins, work with soil bacteria called rhizobia to fix nitrogen in small root nodules. Much of that nitrogen is then held in the plant's roots and leafy growth, becoming useful to the next crop as the plant breaks down.

Cereals and grasses, such as oats, barley and cereal rye, give fast ground cover, fibrous roots and plenty of organic matter. They also scavenge leftover nutrients that would otherwise wash away in winter rain. Some deep-rooted covers can help with compaction, but cereals alone are not a cure for it.

A simple mix of a legume and a cereal gives you the best of both: nitrogen-rich green growth, strong soil cover, fibrous roots and plenty of organic matter. The cereal also helps capture leftover nutrients in the soil before winter rain can wash them away. For beginners, field peas and oats are one of the easiest combinations: the peas add nitrogen-rich growth, while the oats give bulk, roots and weed suppression. Oats are simple to manage. Cereal rye gives heavier winter cover but can be harder to cut down, and it may slow small-seeded crops if the residue is still fresh.

One tip for legumes: if you are sowing peas, beans or vetch into a bed where that type of crop has not grown before, a suitable legume inoculant can improve nodulation and help the plants fix more nitrogen. It is optional, but worth knowing.

Where mustard fits

Mustard is useful when you want fast growth and strong weed suppression. It belongs to the brassica family, and when chopped finely and dug in it may help suppress some soilborne pests and diseases, though results vary, so it is best not to rely on it as a cure. It does not fix nitrogen the way peas, beans, vetch and clover do.

One rotation note: avoid using a mustard or other brassica cover crop right before brassica vegetables such as broccoli, cabbage, cauliflower, kale or radish, and keep them within your brassica rotation group, so you are not building up shared diseases.

Getting the timing right

For most Australian gardens, sow a winter cover crop in late summer to autumn, once the worst heat has passed, a bed has finished cropping, and there is enough warmth and moisture for seed to germinate. In mild areas, early winter sowing can still work, but growth will be slower. The aim is simple: get the bed covered before the coldest, wettest part of the year.

Let the crop grow through winter, then cut it down two to four weeks before spring planting. If the crop is thick, woody or heavy in cereal rye, allow a little longer before sowing small seeds such as carrots, onions or lettuce, because fresh grass residues can briefly tie up nitrogen while microbes break them down.

Cut the crop down before it sets seed, ideally as it begins to flower. This gives you all the soil benefit without leaving behind seed that turns into a weed problem of its own. If you garden no-dig, cut at soil level and leave the tops on the surface as mulch, and leave the roots in the ground to feed soil life as they decay.

The payoff

Come spring, the bed that spent winter under a cover crop comes up looser, darker and more alive than the one beside it that sat bare. When your tomatoes, corn and cucumbers go in, they go into ground that has been quietly worked on for months.

A resting bed does not have to be an idle one. A cover crop turns the quiet season into the season your soil gets stronger. For more on making the most of the cold months, see our guide to winter gardening jobs.

Frequently asked questions

What is a cover crop? A cover crop is a fast-growing plant grown to protect and feed the soil rather than to harvest. Sown so it covers the bed through winter and dug in before spring, it adds organic matter, suppresses weeds and shields bare soil.

Can I sow a cover crop in winter? In mild areas, yes, but late summer to autumn is usually better. Cover crops need warmth and moisture to germinate and establish before winter slows their growth.

What is the easiest winter cover crop mix for beginners? Field peas and oats are a reliable starting point. The peas add nitrogen-rich growth, while the oats give fast cover, roots and organic matter.

Do cover crops add nitrogen to the soil? Legumes such as field peas, faba beans, clover and vetch fix nitrogen, which becomes available as the plants break down. Cereals and mustard add bulk and suppress weeds but do not add nitrogen.

Do I need to inoculate legume cover crop seed? Not always, but it can help. Legumes fix nitrogen with the help of rhizobia bacteria, and if the right bacteria are not already in the soil, a suitable inoculant can improve nodulation.

When should I dig in a cover crop? Two to four weeks before spring planting, and before the crop sets seed. Allow a little longer for thick or woody growth, especially cereal rye, before sowing small seeds.

Can you grow a cover crop in a no-dig garden? Yes. Cut it down at soil level and leave the tops on the surface as mulch, and leave the roots in the ground to break down and feed the soil.

Is cereal rye the same as ryegrass? No. Cereal rye is a grain crop and the one usually meant by winter rye for cover cropping. Ryegrass is a different plant.

Ready to plant? Browse our Vegetable Seeds.