Cover crops are plants grown not to eat, but to improve the soil: sown on a bed that would otherwise stand empty, they protect the surface, feed soil life, fix nitrogen, break up compaction, and suppress weeds, and then they are cut down and dug in or left as mulch. A bed under a cover crop is working even when nothing is being harvested from it, and the next crop planted there grows better for it. This guide explains what cover crops do, which ones to choose for your situation, when to sow and when to dig in, and the mistakes that waste a season.
Cover crops are the oldest soil improvement method there is, and they are the cheapest: a packet of seed, a bare bed, and a few weeks of growth. They are also the method most often skipped, because the benefit is invisible — it shows up in the next crop, not in the cover crop itself. But a garden that uses cover crops every winter builds soil steadily year after year, while a garden that leaves beds bare all winter loses nutrients to rain and organic matter to the air.
What cover crops do for soil
Cover crops work in five ways at once. First, they protect the surface: their leaves shield the soil from heavy rain, which would otherwise break the surface structure and wash nutrients away, and from wind, which dries and erodes bare ground. Second, their roots hold the soil together and open it up, leaving channels for water and air when they rot. Third, legume cover crops such as clover, vetch and field beans fix nitrogen from the air into the soil through bacteria on their roots — free fertiliser, built by the plant itself. Fourth, they feed soil life: living roots leak sugars into the soil that feed bacteria and fungi all winter, keeping the soil food web alive. And fifth, they suppress weeds by covering the ground so weed seeds cannot germinate, and by competing for light and water.
The effect on the next crop is real and measurable in most gardens: better soil structure, more nitrogen where legumes were grown, fewer weeds, and more moisture held in the ground. But the effect depends on choosing the right cover crop for the job and managing it properly — a cover crop left to set seed becomes a weed, and a cover crop dug in too late becomes a woody mat that robs nitrogen as it breaks down.
The best cover crops
| Cover crop | Type | Best for | Sow |
|---|---|---|---|
| Crimson clover | Legume, annual | Fixing nitrogen, bees, light soils | Late summer to autumn |
| White clover | Legume, perennial | Living mulch, paths, nitrogen | Spring to early autumn |
| Field beans | Legume, annual | Heavy clay, nitrogen, deep roots | Autumn |
| Winter vetch | Legume, annual | Nitrogen, smothering weeds | Late summer to autumn |
| Winter rye | Grass, annual | Weed suppression, holding nutrients, cold hardy | Autumn |
| Phacelia | Annual | Bees, light soils, quick cover | Spring to late summer |
| Buckwheat | Annual | Fast summer cover, bees, phosphorus | Late spring to summer |
| Daikon radish | Brassica, annual | Breaking compaction, deep roots | Late summer |
| Mustard | Brassica, annual | Quick cover, biofumigation | Spring to late summer |
| Annual ryegrass | Grass, annual | Holding soil, quick cover | Spring to autumn |
Crimson clover is the best all-rounder for a vegetable garden: it fixes nitrogen, flowers beautifully for bees, is easy to dig in, and suits most soils. Winter rye is the toughest and most reliable for cold winters and weed suppression, but it must be cut down early in spring or it becomes woody and hard to dig in. Daikon radish, often called tillage radish, is the best choice for compacted soil — its thick root pushes through hard layers and rots to leave a deep channel. Phacelia and buckwheat are the fastest summer covers, filling a gap between crops in six to eight weeks. Field beans suit heavy clay and cold winters. Mustard is traditionally said to help suppress some soil pests when dug in, though the evidence is modest — treat it as a possible bonus, not a reliable treatment.
When to sow cover crops
Sow cover crops whenever a bed becomes empty and will stay empty for six weeks or more. The main sowing is in late summer and autumn, after summer crops finish: sow crimson clover, vetch, winter rye or field beans from August to October, depending on your climate, so they establish before winter. Check your planting dates on our zone pages, such as when to plant tomatoes in zone 7, and work backwards from your first frost. In spring and summer, sow fast covers such as phacelia, buckwheat and mustard into gaps between crops, or on beds that will be empty for a few weeks. Scatter the seed evenly, rake it in lightly, and water if the soil is dry. Cover crop seed is cheap, so sow generously — a thin, patchy cover does little good.
Do not sow a cover crop from the same family as the crop that will follow it in that bed. Mustard and daikon radish are brassicas, so do not follow them with cabbage, broccoli or other brassicas, or you risk carrying clubroot and other brassica diseases. Legume covers are the safest choice before most vegetables, and grass covers such as rye are safe before almost anything.
When and how to dig cover crops in
Cut or dig in the cover crop before it sets seed and before it becomes woody. For autumn-sown covers, that usually means cutting them down two to four weeks before you want to plant the next crop, in late winter or early spring. Cut the growth down with shears or a strimmer, leave the cut material on the surface for a few days to wilt, then fork it lightly into the top 15 cm of soil. The two-to-four-week gap matters: freshly dug-in green material temporarily uses nitrogen as it breaks down, and planting straight into it can starve young seedlings. Leave the gap, and the cover crop becomes a net gain instead of a temporary loss. This is the same principle as green manure, which is the dug-in form of a cover crop.
For a no-dig approach, cut the cover crop at ground level and leave the roots in the soil to rot, then plant through the cut material or cover it with compost. The roots still feed the soil and leave their channels, and the top growth becomes a mulch. White clover can be left as a living mulch between crops, mown occasionally, and it keeps fixing nitrogen all season. Winter rye should be cut early — if it grows tall and woody, it is hard to dig in and can regrow from the base, so cut it while it is still soft and green.
Cover crops for problem soils
For heavy clay, field beans and daikon radish are the best choices: beans fix nitrogen and their roots open the soil, while daikon's thick root breaks compaction. See our clay soil improvement guide for the full clay programme. For sandy soil, crimson clover and winter rye are best: clover adds nitrogen, rye holds the soil and its nutrients against winter rain, and both add organic matter that sandy soil consumes quickly. See our sandy soil improvement guide. For compacted soil, daikon radish and alfalfa have the deepest roots and do the most to open the ground; see our soil compaction guide. For a bed that will be empty all winter, winter rye or field beans are the toughest and most reliable. For a quick summer gap, buckwheat or phacelia fill the space in weeks and flower for bees.
Common mistakes
The most common mistake is letting the cover crop set seed, so it becomes a weed in the next crop — cut it down before it flowers and seeds. The second is digging it in too late, when it is woody, so it robs nitrogen for weeks. The third is planting straight into freshly dug-in green material without the two-to-four-week gap. The fourth is sowing too thinly, so weeds grow through the gaps. The fifth is choosing a cover crop from the same family as the next crop, which carries disease. And the sixth is sowing too late in autumn, so the cover does not establish before winter — sow in time, and water if the soil is dry.
Safety notes
Wear gloves when handling seed and when cutting down cover crops, and wash your hands afterwards. Some cover crops, such as mustard and rye, can cause skin irritation in sensitive people, so wear long sleeves when handling large amounts. Do not eat cover crops unless you know the variety is edible — field beans are edible, but most cover crop varieties are grown for soil, not food. If you are unsure which cover crop suits your soil and climate, check your local extension service for advice.
Frequently asked questions
What is the best cover crop for a vegetable garden?
Crimson clover is the best all-rounder: it fixes nitrogen, is easy to dig in, flowers for bees, and suits most soils. Winter rye is the toughest for cold winters and weed suppression, and daikon radish is the best for compacted soil. Choose the one that matches your main problem.
When should I sow cover crops?
Whenever a bed becomes empty for six weeks or more. The main sowing is late summer to autumn, after summer crops finish. In spring and summer, sow fast covers such as phacelia or buckwheat into gaps between crops.
Do I need to dig cover crops in?
You can dig them in, or cut them at ground level and leave the roots to rot while the top growth becomes mulch. Either way, leave two to four weeks between cutting down and planting the next crop, so the green material breaks down and does not rob nitrogen from seedlings.
Will cover crops become weeds?
They can, if you let them set seed. Cut them down before they flower and seed, and they will not become a problem. Winter rye can also regrow if it is cut too late, so cut it while it is still soft and green.
Can I grow cover crops in a small garden?
Yes — even a single bed benefits, and a packet of crimson clover seed costs little. In a small garden, use fast summer covers in gaps between crops and an autumn cover on any bed that will be empty over winter. Even a few square metres of cover crop improves the soil.