Plant Cover Crops This Week: Free Nitrogen by Spring

My summer beds are officially done. The tomatoes came out last weekend, the peppers are limping, and every bed is sitting there bare — which is exactly how soil loses nutrients and structure over winter. This is the week I plant cover crops, and if you garden anywhere in zones 6 through 8, it should be your week too.

A cover crop is just a plant you grow to feed the soil instead of yourself. Sown now, it holds the bed together through winter, smothers weeds, and — if you pick a legume — fixes nitrogen from the air that your spring vegetables get for free. Here is what to plant, how much seed you need, and how to get it in the ground before the window closes.

The clock is ticking — here are your windows

Not every cover crop can go in this late. University of Delaware's September 2026 guidance for Zone 7 lays out the reliable establishment windows clearly:

• Cereal rye: September 21 to November 15 — your safest bet right now.
• Crimson clover: reliably August 15 to September 20, with decent odds through October 10.
• Mustard (for spring biofumigation): September 20 to October 12.
• Forage radish: August 10 to 31 — that ship has sailed for this year.

The University of Georgia's advice matches: wait until the soil cools in late September or early October, then plant. UGA's Bob Westerfield and Julia Gaskin both recommend getting seed down before nighttime temperatures consistently drop below 40°F, and Gaskin puts a hard target of end of September for the mountains and Piedmont, Halloween for the Coastal Plain.

Translation: if you are reading this the week of September 29, you are in the sweet spot for rye and still fine for clover.

Pick your crop by your goal

Free nitrogen → crimson clover. Gaskin calls crimson clover "the workhorse legume cover crop in the Southeast," and for good reason: as a legume, it partners with soil bacteria to pull nitrogen from the air. Plant it now, kill it in spring, and that nitrogen feeds your tomatoes. Hairy vetch does the same job and tolerates more cold — a good pick for zone 6.

Weed suppression and biomass → cereal rye. Rye grows through winter when nothing else will, produces a mountain of biomass, and its residue smothers spring weeds. It is the most forgiving option if you have never planted a cover crop.

Disease suppression → mustard. Mustard planted in the September 20 to October 12 window can be chopped and tilled in spring as a biofumigant — the decaying residue suppresses some soilborne pathogens, per Delaware's guidance.

Best of both worlds: UGA recommends pairing a small grain with a legume — cereal rye plus crimson clover is the classic combo. The rye gives you winter growth and weed control; the clover gives you nitrogen.

How much seed a bed actually needs

This is where most gardeners overbuy. Extension seeding rates are quoted per acre, but your bed is not an acre. Penn State's fall cover crop rates, converted to pounds per 1,000 square feet, are eye-opening: a 4×8 bed needs a few ounces of clover seed, not a 50-pound bag.

Figure: Seeding rates per 1,000 sq ft, converted from Penn State Extension's per-acre recommendations. Source: Penn State Extension.

For a typical 4×8-foot bed (32 sq ft), that is roughly 1.3 ounces of cereal rye or a tenth of an ounce of clover. Buy seed by the pound from a farm supply store, not the pre-packaged garden-center bags, and you will spend a few dollars to cover every bed you own.

How to sow it this weekend

The technique is simple, per UGA's Westerfield:

1. Clear the bed of crop residue and weeds.
2. Rake the surface rough — seed needs soil contact.
3. Broadcast the seed evenly by hand. For tiny clover seed, mix it with sand so you can see where it lands.
4. Rake lightly to cover — about 1/4 to 1/2 inch deep. Deeper for rye, shallower for clover.
5. Water it in. Fall rain usually takes over from here.

One extra step for legumes: inoculate the seed with the right rhizobium bacteria before planting. Gaskin and Westerfield both flag this — without inoculant, clover and vetch fix far less nitrogen. A packet of inoculant costs a few dollars and treats pounds of seed.

The spring payoff (and the one myth to drop)

Here is the part people get wrong: clover does not feed your soil nitrogen while it is growing. The nitrogen payoff comes when you kill the cover crop and the residue decomposes. Plan to chop-and-drop or till it under two to three weeks before planting spring vegetables.

I terminate mine with a string trimmer, let the residue lie as mulch for a week, then turn the top few inches. By planting time, the bed is darker, looser, and visibly alive with worms.

FAQ

Is it too late to plant cover crops? Not for cereal rye — Delaware's window runs to November 15 in Zone 7. Crimson clover is borderline but reasonable through about October 10.

Do I really need inoculant for clover? If legumes have not grown in that bed recently, yes. The bacteria that fix nitrogen do not live in every soil, and uninoculated clover is mostly just a pretty ground cover.

Can I just leave the beds bare over winter? You can, but you will lose. Bare soil erodes in winter rains, leaches nutrients, and wakes up compacted. A $3 bag of rye seed prevents all of that.

What if I want to plant garlic or fall vegetables instead? Do both — garlic gets its own bed, and cover crops take the beds that would otherwise sit empty. They compete with bare dirt, not with your crops.

Will the cover crop become a weed next spring? Only if you let it go to seed. Terminate rye and clover before they flower and they will not volunteer.