Cover Crop Seed Calculator

Cover crops are among the cheapest ways to improve garden soil, and fall is the season to sow them. A thin planting of rye, oats or clover over an empty bed holds the soil through winter, crowds out weeds and adds organic matter when you cut it down in spring. This cover crop seed calculator tells you how many pounds or ounces of seed to buy for the area you want to sow, using typical broadcast seeding rates for five popular fall cover crops. It works in square feet and pounds, like the other tools on the site.

The question is usually how much seed to buy, because seed is sold by the pound and the rate is quoted per 1,000 square feet or per acre. The calculator converts those rates to the size of your garden, adds a spare amount for hand sowing and shows the same rate per acre if you are comparing with a seed catalog.

How to use the cover crop seed calculator

1. Enter the area to sow in square feet. Multiply length by width for each bed and add them up. Our garden size calculator can help if you are working from a plan.
2. Choose your cover crop. The list covers winter rye, oats, crimson clover, hairy vetch and Austrian winter pea. The sections below explain how they differ.
3. Read the results. You get the seed needed, the amount with 10% extra for sowing by hand, the seeding rate used and the same rate per acre.

The rates are typical broadcast rates, which means seed scattered on the surface and raked in. If your seed bag gives a different rate, follow the bag, and if you sow in rows or with a seeder the rate may be lower.

The cover crop seed math

Seeding rates are given per 1,000 square feet, so the calculation is a simple scale-up from the size of your garden.

Seed needed (lb) = area (sq ft) ÷ 1,000 × seeding rate (lb per 1,000 sq ft)
Seed with spare (lb) = seed needed × 1.1
Rate per acre (lb) = rate per 1,000 sq ft × 43.56

One acre is 43,560 square feet, which is where the 43.56 comes from. Amounts under a pound are shown in ounces, at 16 ounces to the pound.

Worked example: a 1,000 square foot vegetable garden

You want to sow winter rye over a 1,000 square foot garden after the last harvest. The rate is 2.5 pounds per 1,000 square feet, so you need 1,000 ÷ 1,000 × 2.5 = 2.5 pounds. With 10% extra for hand sowing, that is 2.75 pounds, so a 3 pound bag covers it.

Worked example: a small raised bed with clover

A 400 square foot area is sown with crimson clover, which has a much lighter rate of 0.4 pounds per 1,000 square feet. The seed needed is 400 ÷ 1,000 × 0.4 = 0.16 pounds, or about 2.6 ounces. Clover seed is small and sold in small packs, so a quarter pound bag is plenty.

Worked example: a rye and vetch mix

Many gardeners sow rye and hairy vetch together, so the rye supports the vetch and the vetch adds nitrogen. When you mix two crops, a common approach is to sow each at roughly half to two-thirds of its full rate. For a 2,000 square foot garden, half rates work out to 2.5 pounds of rye (half of 2.5 pounds per 1,000 square feet, times 2) and 0.8 pounds of vetch (half of 0.8, times 2). Run the calculator once for each crop and halve the results.

Typical seeding rates

These are the rates the calculator uses, shown per 1,000 square feet and per acre:

• Winter rye: 2.5 pounds per 1,000 square feet, about 109 pounds per acre
• Oats: 2.5 pounds per 1,000 square feet, about 109 pounds per acre
• Crimson clover: 0.4 pounds per 1,000 square feet, about 17 pounds per acre
• Hairy vetch: 0.8 pounds per 1,000 square feet, about 35 pounds per acre
• Austrian winter pea: 1.6 pounds per 1,000 square feet, about 70 pounds per acre

Rates vary a little between seed companies and extension guides, since they depend on seed size, sowing method and how thick a stand you want. Use these as a shopping guide and follow the instructions on your bag.

Choosing a cover crop

The right cover crop depends on what you want it to do and how cold your winters are. In broad terms, grasses such as rye and oats build organic matter and suppress weeds, while legumes such as clover, vetch and peas add nitrogen.

Winter rye

Winter rye, which is a grain and not the same plant as perennial ryegrass lawn seed, is the toughest option. It germinates in cool soil, survives hard winters and can be sown later in fall than most cover crops. It grows a dense root system that holds soil and breaks up compaction, and it suppresses weeds well. It also grows tall in spring and needs cutting before it sets seed. If you are sowing a vegetable garden, rye is a good default.

Oats

Oats grow quickly in fall and usually die over winter where winters are hard, leaving a soft mat of residue that is easy to plant into in spring. That makes them popular with gardeners who want a cover crop without having to cut it down. In mild climates oats may survive winter and need to be cut in spring like any other cover crop.

Crimson clover

Crimson clover is a legume that adds nitrogen to the soil. It needs a little time to establish, so sow it early in fall, and it is less hardy than rye or vetch. In spring it produces bright red flowers that pollinators like, which makes it a good choice for a bed you will leave until late spring.

Hairy vetch

Hairy vetch is a very hardy legume that grows well through winter and produces a lot of nitrogen-rich growth in spring. It is often sown with rye. If left to flower and set seed, it can self-sow, so cut it before the seed pods mature.

Austrian winter pea

Austrian winter pea is a cool-season legume that grows quickly in fall. It tends to die in harsh winters, which makes it easy to manage, and in milder areas it grows through winter. Its tender shoots are edible, and some gardeners use them as a spring green.

When to sow cover crops in fall

Timing depends on the crop and on your first frost date. Cover crops need time to germinate and put on growth before the ground freezes, though some are more forgiving than others.

• Rye: the most forgiving. It can be sown a few weeks before the ground freezes and will still establish, although later sowings grow less.
• Legumes (clover, vetch, peas): sow earlier, ideally several weeks before the first hard frost, so they can establish a root system and begin to grow.
• Oats: sow early enough for the plants to make good growth before cold weather ends it.

If you are not sure of your date, check your first fall frost with the frost dates by ZIP code tool and count back from there. The latest planting date before fall frost calculator gives you a similar countdown for crops. If your window has passed, rye is the one most likely to work for a late sowing, and you can also sow a cover crop in early spring and cut it before planting.

How to sow a cover crop

1. Clear the bed. Pull spent plants and large weeds, and remove thick mulch. Leave crop roots in the soil, since they are food for soil life.
2. Loosen the surface. Rake or lightly fork the top inch or two so the seed can make contact with soil. There is no need to dig deeply.
3. Broadcast the seed. Scatter it as evenly as you can. Mixing the seed with a little dry sand or soil makes it easier to spread evenly on small seed such as clover.
4. Cover lightly. Rake the seed in so it is covered by roughly 1/4 to 1 inch of soil, with bigger seed such as oats and peas going deeper than clover.
5. Water if dry. A good watering gets germination going. After that, fall rain is usually enough.
6. Protect from birds. A light covering of straw or shredded leaves keeps birds off the seed and holds moisture. Our leaf mulch calculator can help you work out how much you need.

Inoculating legume seed

Legumes make nitrogen with the help of bacteria that live in nodules on their roots. These bacteria are often present in garden soil, but if the legume has not grown in a bed before, the right bacteria may be missing. Garden suppliers sell a small packet of inoculant, a powder containing the right bacteria, that you shake onto the seed just before sowing. It is cheap, takes a minute and improves the nitrogen your legume cover crop produces. Use the type of inoculant marked for your crop and follow the package directions.

Mixing cover crops

A mix combines strengths. Rye with vetch gives you biomass and nitrogen, and oats with peas gives you a quick fall cover that winter-kills in cold areas. Because each crop competes with the others, mixes are sown at reduced rates, and a common starting point is about half to two-thirds of each crop’s solo rate. Some seed companies sell premixed blends with a rate on the bag.

Cutting down or turning under cover crops in spring

In spring, the cover crop must be stopped before it competes with your vegetables or goes to seed. There are two main approaches.

• Cut and leave: mow or cut the plants close to the ground and leave the residue on the surface as mulch. Plant through it, or rake it aside for seeds. This works well with no-till beds. If you want to try that approach, our no-till garden bed calculator shows how much compost and mulch to add.
• Turn under: dig or till the plants into the soil, and wait a couple of weeks before planting so the residue can start to break down.

Timing matters. Cut grasses such as rye before they form seed heads, and cut legumes when they are flowering, when their nitrogen content is highest. Rye residue can slow germination of small seeds, so many gardeners wait two to three weeks after cutting before sowing carrots, lettuce and other small-seeded crops. Transplants of larger plants, such as tomatoes, are generally less affected.

Where cover crops fit in the garden

The easiest place to start is any bed that is empty by fall, which includes beds where you have pulled summer crops. Sow the cover crop as soon as the bed is cleared. If you plan to plant garlic or fall greens in some beds, leave those out and cover only the rest. Rotate cover crops along with your vegetables, and if you are planning next year’s layout, the garden planner shows where each crop goes. Our fall garden prep guide puts cover crops in the context of everything else to do before winter, and the beginner’s guide to composting covers the other great soil builder.

Common cover crop mistakes

• Sowing too late for legumes. Clover and vetch need time to establish. Late sowings may not grow enough to matter.
• Letting the crop go to seed. Cut rye and vetch before seed heads and pods mature, or they can become weeds.
• Buying lawn ryegrass instead of rye grain. Annual and perennial ryegrass are different plants. Check the label for cereal rye or winter rye.
• Skipping inoculant on legumes. Without it, nodulation may be poor in beds that have never grown that legume.
• Sowing seed on the surface only. Seed that is not raked in or covered is easy pickings for birds and dries out quickly.
• Planting small seeds right after rye. Give residue time to break down, or use transplants.
• Over-sowing. Too much seed gives a crowded, spindly stand with no benefit. Follow the rates.

Frequently asked questions