Cover cropping means sowing a crop not to eat, but to look after the soil. When a vegetable bed is empty — after the summer harvest, over winter, or between plantings — bare soil loses nutrients in rain, dries out in sun, erodes in wind, and fills with weeds. A cover crop, also called a green manure, covers that soil with living roots and leaves, holding it together, feeding soil life, and adding organic matter when it is dug in.
Cover crops are one of the cheapest soil improvements there is: a packet of seed costs little, the crop does the work, and the next vegetables you plant in that bed grow noticeably better. They suit every garden, from a single raised bed to a full allotment, and they are especially valuable on light sandy soils that lose nutrients quickly and on heavy clay that needs organic matter to open up. If you are unsure which cover crop suits your soil and climate, check your local extension service before you buy seed.
How cover crops help your soil
Cover crops help in several ways at once. Their roots hold the soil together and stop rain washing nutrients and topsoil away, which matters most on slopes and in winter. Their leaves shade the soil, keeping it moist and cool and stopping weeds from germinating. When dug in, they add organic matter, which improves both drainage in clay and water-holding in sand. Legume cover crops such as clover, vetch and field beans go further: they work with bacteria in their roots to fix nitrogen from the air, leaving the soil richer for the next crop. Deep-rooted types such as phacelia and winter rye break up compacted layers and bring nutrients up from deeper down. The result is soil that is darker, softer, full of worms and easier to work — the kind of soil vegetables love. See our cover crops for soil guide for the soil-science background, and composting basics for combining cover crops with compost.
Best cover crops by season
Choose by the season and by what the bed needs next. For autumn sowing, winter rye and crimson clover are reliable: rye is very hardy and produces a lot of bulk, while crimson clover fixes nitrogen and has pretty red flowers that bees love in spring. Field beans sown in autumn overwinter well and add nitrogen. For late summer, after early crops finish, phacelia is a fast grower that bees adore, and buckwheat grows in six to eight weeks and smothers weeds before a late planting. For spring, sow white clover as a living mulch between rows, or mustard, which grows fast but must be dug in before it sets seed. For a bed that will hold hungry brassicas next year, a legume such as vetch is the best choice. For a bed that will hold root crops, choose a non-legume such as rye or phacelia, because fresh manure and high nitrogen can cause forking in carrots — see growing carrots for why that matters.
How to sow a cover crop
Sowing is simple. Clear the bed of old crop debris and weeds, rake the surface level, and scatter the seed evenly — roughly a handful per square metre for most types, following the packet rate. Rake lightly to cover the seed, firm it with the back of the rake, and water if the soil is dry. Most cover crops germinate within a week in warm soil. There is no need to feed them, and only the slowest benefit from watering in a dry spell. Sow autumn cover crops by early autumn so they establish before cold weather, and sow summer cover crops as soon as a bed is cleared. If you are sowing into a bed that still holds a crop, scatter seed between the rows a few weeks before harvest so it is already growing when the crop comes out. Keep a note of what you sowed where, because it affects what you plant next — legume cover crops mean less nitrogen feed is needed for the following crop.
When and how to dig cover crops in
Dig a cover crop in two to three weeks before you want to plant the next crop, so it has time to break down. Cut or strim the growth first, let it wilt for a few days, then dig or fork it into the top 15 centimetres of soil. Do not dig it in too deep, or it will rot slowly without air. If the growth is very tall, chop it and add some to the compost heap instead of digging it all in — too much fresh green matter at once can temporarily lock up nitrogen as soil microbes break it down. An easier method on no-dig beds is to cut the cover crop at ground level, leave the roots in the soil to rot, and lay the cut tops on the surface as mulch. Winter rye should be dug in while it is still young and soft, because mature rye is tough and slow to rot. Our no-dig gardening guide explains the cut-and-leave method in more detail.
Cover crops in rotation
Cover crops fit naturally into a rotation plan. After a hungry crop such as tomatoes or brassicas, sow a legume to restore nitrogen. After potatoes, which leave the soil loose and weedy, sow rye to hold it together over winter. In a four-bed rotation, rest one bed each year under a cover crop while the others produce — see our crop rotation guide for the full plan. Cover crops also let you use ground that would otherwise sit empty: the strip beside the shed, the bed waiting for spring, the patch where the sweetcorn stood. On an allotment, a winter cover crop also signals that the plot is cared for, which matters on sites where untidy plots are noticed. If you grow in containers, a cover crop is less practical, but you can still rest a large pot with a small sowing of clover, or simply empty it, refresh the compost and cover it with mulch over winter.
Mistakes and timing traps
The main trap is letting a cover crop set seed: mustard, phacelia and buckwheat self-seed freely, so cut or dig them in before they flower and seed, or you will be weeding their seedlings for years. The second trap is sowing too late in autumn, so the crop barely grows before winter and does little good — sow by early autumn in cold areas. The third is digging in a huge mass of fresh growth the day before planting, which can check the growth of seedlings while microbes break it down; always allow two to three weeks. The fourth is choosing the wrong family: mustard is a brassica, so do not follow it with cabbage or broccoli in a rotation, and do not sow it where clubroot is a problem. Ask your local extension service if you are unsure which cover crops are recommended for your region, and buy seed from a garden supplier rather than using unknown saved seed.
Common mistakes to avoid
The biggest mistake is sowing a cover crop and then forgetting to dig it in, so it flowers, sets seed and becomes a weed problem of its own. The second is sowing too late, so the crop never establishes. The third is digging in a massive amount of fresh growth right before planting, which starves seedlings of nitrogen for a few weeks. The fourth is growing a brassica cover crop such as mustard before brassica vegetables, which breaks the rotation. The fifth is expecting a cover crop to fix badly compacted or waterlogged soil on its own — it helps, but compacted ground also needs forking and organic matter, as explained in our soil drainage improvement guide. Finally, do not skip watering at sowing time in dry weather: seed scattered on dry soil will not germinate, and a failed sowing wastes a whole season of soil building.
Frequently asked questions
What is the easiest cover crop for a beginner?
Phacelia is one of the easiest: it germinates quickly, grows fast, smothers weeds, and bees love its flowers. Crimson clover is also forgiving and adds nitrogen. Both are sown by scattering seed on raked soil. Start with one of these before trying rye or vetch, which need more careful timing.
Do I need to dig a cover crop in, or can I leave it?
On no-dig beds, cut it at ground level and leave the roots to rot in place, using the cut tops as mulch. On dug beds, dig it into the top 15 centimetres two to three weeks before planting. Either way, do not leave it standing where you want to plant — it will compete with seedlings for light and water.
Will a cover crop help my clay soil?
Yes, over time. The roots break up the soil and the dug-in growth adds organic matter, which opens clay up and improves drainage. It is a slow fix, not an instant one, so combine cover cropping with adding compost each year. See our clay soil improvement guide for the full approach.
Can I sow a cover crop in summer?
Yes — buckwheat and phacelia are fast summer cover crops that grow in six to eight weeks and can fill a gap between spring and autumn plantings. Sow them as soon as a bed is cleared, and dig them in before they set seed so they do not self-sow.
Are cover crops safe around children and pets?
Most cover crops are safe, but field beans and some vetches are not meant to be eaten, and mustard seed can irritate skin. Keep seed packets out of reach, and teach children that cover crops are for the soil, not for eating. If you are unsure about a particular species, check your local extension service.