Winter Cover Crops for Garden Beds: What to Sow and How Much Seed

Farm guides and many seed bags list cover crop rates in pounds per acre. Your bed is 32 square feet. This guide does the conversion for you, bed by bed.

It also answers the question that worries small-bed gardeners most. Will it go wild in spring, and can you kill it with just a spade?

What a winter cover crop does for an empty bed

A cover crop is a plant you grow for the soil, not for the kitchen. You sow it after the last harvest and let it grow through winter.

Its roots hold the soil in place when rain and snow hit bare ground. They also soak up leftover nitrogen that would wash away. In spring, the plants break down and feed the soil, per University of Maryland Extension.

Some cover crops are legumes, like clover. A legume teams up with soil bacteria to pull nitrogen out of the air. When it dies, that nitrogen goes back into your bed.

Winter rye, crimson clover, or oats?

Pick by how much spring work you want. Rye is the toughest. Clover adds the most nitrogen. Oats die on their own, so there is nothing to kill in spring.

The rates below are broadcast rates, meaning seed thrown by hand. All of them come from University of Missouri Extension, converted from pounds per acre. These are the same rates the cover crop calculator uses on its Broadcast setting.

Annual ryegrass is not in the calculator. Its amount comes from Missouri's broadcast rate of 20–25 lb per acre. The mix row uses 60% of the rye and clover rates, per Maryland Extension.

Crimson clover's winter survival depends on where you live. SARE says it often dies over winter in zone 5 and colder. It "often can survive" the colder part of zone 6, and it overwinters dependably in zone 8.

Pick winter rye if you want sure winter cover

SARE's cereal rye profile calls it the best cool-season cover for soaking up leftover nitrogen. It sprouts at temperatures as low as 34°F, so you can sow it later than the others.

The catch is spring. Rye grows fast once the weather warms. In a small bed worked by hand, that takes a plan.

Pick crimson clover if you want free nitrogen

Crimson clover can add 70–150 lb of nitrogen per acre by mid-May in the Deep South, per SARE's crimson clover profile. That works out to about 1.6–3.4 lb of nitrogen per 1,000 sq ft. Expect less where winters are colder and spring comes later.

It is also easy to kill. One cut after the flower buds form finishes it.

Pick oats if you want no spring work

Oats die in hard winter cold, per SARE's oats profile. They winter-kill in zone 6 and colder and in much of zone 7.

In spring you find a flat, brown mat. Rake it aside or plant right through it. Maryland Extension suggests oats for your first try at cover crops.

How much cover crop seed for a 4x8 bed

A 4x8 bed needs 1.2–1.4 oz of winter rye, 0.2 oz of crimson clover, or 1.4–1.6 oz of oats. A kitchen scale is the easiest way to weigh that out.

Here is how the seed amounts scale by bed size. Each range comes from the broadcast rate in the table above. The calculator shows the same low–high range, plus one number from the middle of it.

Use the low end if you sow on time. Use the high end if you sow late in fall. SARE also advises more seed for late broadcast sowing.

The formula (so you can use any seed bag)

An acre is 43,560 square feet. Divide any per-acre rate by that number, then multiply by your bed size.

lb for your bed = lb per acre × bed sq ft ÷ 43,560
oz for your bed = lb for your bed × 16

Here is a 4x8 bed of winter rye at the low rate of 100 lb per acre.

100 × 32 ÷ 43,560 = 0.073 lb
0.073 × 16 = 1.2 oz

For 1,000 square feet, just divide the per-acre rate by 43.56. So 20 lb per acre of crimson clover is about 0.46 lb per 1,000 sq ft.

Use the calculator above to convert any lb/acre rate to your exact bed size.

How a home-garden chart compares

University of Maryland Extension's garden chart lists 3–4 oz of winter rye or oats and 1–2 oz of crimson clover per 100 sq ft. The rye amount runs a bit under Missouri's rate. So does the oat amount. The clover amount is about 1.5 to 3 times the farm rate.

That extra clover is fine in a small bed. Hand-sowing is patchy, so a bit more seed helps fill the gaps.

When to sow by your first frost date

Count back from your average first frost date. Clover and oats need the most time before cold weather. Rye needs the least.

• Crimson clover: 6–8 weeks before your average first frost. Near the northern edge of its range, that is early to mid-August.
• Oats: at least 40–60 days (about 6–8 weeks) before the first hard frost.
• Annual ryegrass: at least 40 days before the first hard frost in zone 5 and colder.
• Winter rye: late summer to mid-fall in zones 3–7, and fall to midwinter in zone 8 and warmer.

Maryland Extension gives August 15 to October 1 as the ideal window for fall cover crops in that state. Adjust for your own frost date.

Don't sow crimson clover too early, though. SARE warns it can flower and go to seed in fall, then fail to regrow in spring.

How to sow a cover crop by hand

Clear the bed, scatter the seed, and press it into the soil. These steps follow Maryland Extension's home-garden method.

1. Pull out spent plants, weeds, and summer mulch.
2. Loosen the soil with a garden fork.
3. Rake the surface smooth.
4. Weigh out the seed for your bed size from the table above.
5. Mix the seed with a few handfuls of soil or compost so it spreads evenly.
6. Scatter it by hand or with a hand-held spreader, ideally right before rain.
7. Rake lightly so seed ends up just under the surface. Keep rye no deeper than 2 inches, per SARE.
8. Walk over the bed to press seed against the soil. Seed must touch soil to sprout.

For clover, you can coat the seed with a legume inoculant before sowing. That is a powder of the bacteria clover roots team up with to pull nitrogen from the air.

You can also sow between plants that are still producing. Pull back the mulch, rake the soil, and scatter the seed around them.

Will a cover crop go wild in spring?

Winter rye can, if you ignore it. Penn State Extension notes that rye's "aggressive growth in the spring" can make it hard to manage. It gets tall and thick fast once the weather warms.

Crimson clover is much easier to handle. One cut after the flower buds form kills it.

Oats will not go wild at all in cold climates. They are dead by spring.

The bigger risk with any cover crop is letting it go to seed. Ripe seed drops into the bed and comes back as weeds. Kill every cover crop before its seed ripens.

How to kill a cover crop by hand in spring

Cut it down at the soil line, then cover it or dig it in. Farmers call this "terminating" the cover crop. In a garden bed, a spade, string trimmer, or mower does the job, per Maryland Extension.

Timing matters more than the tool. Here is when and how to end each crop.

Winter rye: pick early or late

Rye gives you two choices. Each one has a trade-off.

Cut late, at flowering. SARE says mowing works best once rye has begun to flower. One cut kills it.

The downside is the straw. By flowering, Penn State says rye has a carbon-to-nitrogen ratio (C:N) of about 40:1. Soil microbes borrow nitrogen from your soil to break down anything above about 20:1. That can leave your next crop short on nitrogen for a while.

Cut early, before it heads out. Young rye is soft and low in carbon, about 14:1 per Penn State. But it is still alive at the roots. Penn State says rye cut at the boot stage, just before the head pokes out, usually regrows. So cut it and cover it.

To smother it, cut the rye low and lay a heavy tarp over the bed. Weigh down the edges. UMass Extension advises leaving a tarp on for at least 3 weeks. Maryland Extension notes that cutting first, then tarping, works well together.

To dig it under, slice the rye low and turn it with a spade or fork. SARE says tilling usually stops regrowth, except when rye is under 12 inches tall. Short rye may send up a few new shoots, so pull those as you see them.

Crimson clover: wait for the flowers

Clover is the easy one. SARE says mowing after the early bud stage kills it. It will not grow back from the roots.

For the most nitrogen, wait until late bloom. SARE says killing it about a month early can cost up to 50 lb of nitrogen per acre. Just cut it before the seed heads ripen and drop.

Leave the cut clover on top as mulch, or fork it into the soil.

Oats: rake or plant through

If oats died over winter, there is nothing to kill. Maryland Extension says to treat the dead mat as mulch, plant through it, or fork it into the soil.

If a mild winter spared your oats, cut them down like young rye.

How long to wait before planting after a cover crop

Give the bed 2–4 weeks after you kill the cover crop. The exact wait depends on the crop and on what you plant next.

After winter rye, wait 3–4 weeks before sowing small seeds like carrots or onions, per SARE. Rye residue gives off natural chemicals that slow seed sprouting. SARE says those effects usually fade after about 30 days. University of Minnesota Extension also advises about 3 weeks after rye so nitrogen is not tied up.

After crimson clover, SARE advises 2–3 weeks before planting seeds. Maryland Extension gives about 2 weeks for most cover crops to break down.

After winter-killed oats, you can plant right away.

Getting the bed ready after the cover crop

Once the residue starts to break down, your bed is ready for spring. Too much to handle? Maryland Extension says you can pile the extra on the compost heap.

A cover crop adds organic matter, but it does not replace compost. Spread a thin layer, 1/4 to 1 inch, if the soil looks tired. Our guide on how much compost you need gives the bag count for any bed.

If your raised bed has sunk over winter, top it up first. The raised bed soil guide covers what mix to add and how deep.

Cover crops aren't the only fall job on the calendar. If you're also putting in spring bulbs, how to plant tulip bulbs and when to plant tulip bulbs run on a similar soil-temperature clock.