Cover Your Soil for the Winter: A Plain Guide to Cover Crops

When the last tomato is picked and the beans are done, most garden beds sit empty until spring. Rain packs the soil down. Weeds move in and set seed. The nutrients that the summer crop left behind wash away. A cover crop fixes all of that with a handful of seed and a few minutes of work.

A cover crop is simply a crop you grow to feed the soil instead of to eat. You sow it into a bed that would otherwise be bare, let it grow through the fall, and then cut it down before spring planting. The plant turns into "green manure," and your soil ends up richer than it would have been if you had just left the bed empty.

It is one of the cheapest things you can do for a garden, and it is forgiving enough that a beginner can do it right the first time.

Why Cover the Soil at All

Bare soil in fall and winter is not "resting." It is being worked over. Here is what happens to an uncovered bed:

• Rain compacts the surface. Heavy fall rains beat down the top inches of soil, and the structure you worked so hard to build gets flattened.
• Weeds take over. Open ground is an invitation. Winter weeds sprout, go to seed, and give you more pulling to do in spring.
• Nutrients wash out. The soluble nitrogen and other good stuff left in the soil get carried away by rain and melting snow.
• Soil life loses its food. The worms and microbes that feed your plants need something living in the ground to eat through the cold months.

A cover crop solves each of these in one move. The plants shade out weeds, catch the rain, hold nutrients in their roots and leaves, and feed the soil life.

Two Kinds of Cover Crops

Cover crops fall into two broad families, and it helps to know the difference before you buy seed.

Grasses and grains. This group includes winter rye, oats, winter wheat, and barley. They grow fast, make a lot of leafy material, and are excellent at suppressing weeds and holding soil. They also put down roots that improve soil structure. Winter rye is the hardiest of the lot and can be planted later than almost anything else.

Legumes. This group includes crimson clover, hairy vetch, red clover, and field peas. Legumes team up with bacteria in the soil to pull nitrogen out of the air and store it in their roots. When the plant breaks down in spring, that nitrogen becomes available to your vegetables. They add fertility, but they grow more slowly and do not smother weeds as well as the grasses.

The classic move is to combine the two: a fast, sturdy grass to cover the soil and beat the weeds, plus a legume to add nitrogen. Winter rye with hairy vetch or crimson clover is a well-tested pairing.

Winter-Kill or Winter-Hardy?

This choice matters, and it depends on how cold your winters get.

• Winter-kill crops (oats, field peas, most annual clovers grown in cold zones) die when hard cold arrives. They leave their residue on the surface all winter, and by spring they are soft and easy to turn under. No effort needed to kill them.
• Winter-hardy crops (winter rye, hairy vetch, crimson clover in milder areas) survive the winter and grow again in early spring. They hold more soil and add more organic matter, but you have to actually kill them before planting, or they will turn into a weed problem and steal your spring.

If you want the easiest possible first experience and you get cold winters, oats are a strong pick. You sow them, they grow, they die over winter, and in spring you turn in a soft, mostly-dead mat. If you want maximum soil benefit and do not mind doing one extra step in spring, go with a rye and vetch mix.

How to Sow

Timing is the part most people get wrong, and it is simple: sow as soon as the bed is free, not when you get around to it. University extension services are blunt that you should plant cover crops immediately after harvest. Every week you delay gives weeds a head start and cuts the growth your crop can put on before cold weather.

The steps:

1. Clear out spent plants. Pull the old crops and any big weeds. Do not leave a jungle for your seed to compete with.
2. Loosen the top inch or two. A quick rake or a few passes with a hoe is enough. Cover crop seed is small and just needs contact with soil.
3. Scatter the seed. For most home beds, a generous handful per ten square feet is plenty. Read the rate on the bag, but heavier is usually better than too thin for a cover crop.
4. Rake it in lightly. You want the seed covered by a thin layer of soil, about a quarter of an inch, not buried deep.
5. Water and keep it moist until the seed sprouts. A dry spell right after sowing will stall germination.

That is the whole planting. In a few weeks you will have a low green mat, and in frosty areas the rye will stand through the winter as a living mulch.

What Happens Over Winter

The crop will do different things depending on your climate and the seed you chose.

• In mild winters, a winter-hardy mix may keep slowly growing.
• In cold winters, a rye crop goes dormant but stays green at the base and holds the soil all season.
• A winter-kill crop like oats browns out and lies down as a natural mulch.

Either way, you are ahead of where you were with bare soil. The ground is covered, the nutrients are held, and the soil life has food.

Turning It Under in Spring

This is the step that turns a cover crop into a green manure. The goal is to kill the crop and let the plant material break down in the soil before you set out your vegetables.

1. Cut it down before it goes to seed. If you let a rye or vetch crop set seed, you have just made next year's weed problem. Cut when it is still growing and lush, in mid to late spring, before flowering sets seed.
2. Mow or chop it close to the ground. A string trimmer, a scythe, or even a mower set high enough not to scalp the soil all work. Chopping it into smaller pieces helps it break down faster.
3. Turn it into the top few inches if you till, or if you are no-till, leave it as a surface mulch and plant right through it.
4. Wait about two weeks before planting so the decomposing material has time to settle and stops robbing nitrogen from your new seedlings. The plant matter takes up a little nitrogen as it rots, and giving it a head start avoids competing with your young vegetables for a week or two.

If you planted a winter-kill crop like oats, spring is even easier: the mat is already soft and mostly dead, so you just turn or push through it.

Mistakes Beginners Make

• Planting too late. The number one reason cover crops fail is sowing weeks after the bed emptied. Sow right away.
• Sowing too thin. A thin cover crop loses to weeds. Scatter generously.
• Burying the seed too deep. Small seed dies if planted too deep. Rake it in lightly, do not plow it under.
• Letting a winter-hardy crop go to seed. That becomes a weed patch. Kill it in time.
• Planting too soon after turning it in. Give the green manure a couple of weeks before putting in transplants.

Where to Start

If you are new to this, keep it small. Pick one bed that is empty right now, buy a small packet of oats or winter rye (both are cheap and easy to find), sow it this week, and watch what it does. Next fall you can add a legume to the mix and cover a second bed.

Cover crops are not fancy. They are one of the oldest tricks in farming, and they work because they keep the ground alive instead of leaving it bare. A handful of seed, ten minutes of raking, and your soil is working for you all winter.

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— C. Steward 🥕