How Much Cover Crop Seed Do I Need?

The rate comes from your bag

Seeding rates differ enormously, and there's no honest single figure to publish.

Seed size varies by an order of magnitude between species — a fine clover and a cereal rye are nowhere near each other by weight for the same ground coverage. Rates differ again between suppliers, between mixes, and by purpose: a crop grown for nitrogen is sown differently from one grown for biomass.

Your bag, your supplier's catalogue and your extension service all carry the figure for what you actually bought. That's the only one that describes your seed.

Watch the units — they're 43.56 apart

An acre is 43,560 square feet, so rates quoted per acre and per 1,000 sq ft differ by a factor of 43.56.

If your bag quotes per acre, divide by 43.56 before entering it. And check yourself against The same rate per acre, which converts back — 2 lb per 1,000 sq ft is about 87 lb per acre.

That check is worth doing. Mixing the units produces a number so far out it's usually obvious, but only if you look at it.

Broadcasting needs more seed than drilling

Scattered seed has to survive things drilled seed doesn't. Birds take a share, some dries out before its roots reach moisture, and some lands where it simply can't establish — on a clod, in a crack, on the path.

Seed placed into the soil at a consistent depth avoids most of that.

Extra for broadcasting is an input because the right figure depends on your soil, your weather and what you do afterwards. Set it to 0 if you're drilling or covering the seed. Your supplier will suggest a number for their species.

Seed if you were drilling instead sits alongside it so you can see what scattering costs — 5 lb against 4 lb at the defaults.

One thing that genuinely helps: rake or roll after broadcasting. It cuts the loss substantially and is worth the effort on anything but the smallest patch.

Buy whole bags, expect leftover

Seed comes in fixed sizes and you rarely need a round number of them. Bags to buy rounds up, Cost is what you actually spend, and Seed left over is what's in the shed afterwards.

Store it cool, dry and sealed and it keeps reasonably to the following season. Germination falls with age rather than stopping suddenly, so an older bag still grows — just a lower proportion of it, which means sowing a little more thickly for the same stand.

A shed that gets hot in summer is not a good store.

Two things this tool won't tell you

When to sow. A cover crop needs enough growing time to establish before winter, and the window depends on your climate and the species. Sown too late it never develops the top growth or root to do the job; sown too early some species run to seed, which creates a weed problem rather than solving one. Your extension service publishes windows for your area.

The rate here is correct regardless of date. The date decides whether the crop actually works.

How to kill it. Decide that before you sow, because it's part of choosing the species. Some cover crops winter-kill and collapse on their own. Others need cutting, mowing, digging in or smothering — and a few are hard enough to kill that they become the weed problem they were meant to prevent.

Read the termination line in the supplier's description before the one about nitrogen fixing. A cover crop you can't remove has cost you more than it gave.

Can you sow it too thickly?

Yes, though the margin is wide.

A very thick stand competes with itself, produces thinner weaker plants, and in a wet season holds enough moisture at the surface to encourage disease. It also costs more in seed for no gain.

Erring slightly heavy on a broadcast sowing is normal practice, and that's exactly what the broadcasting increase is for. The gap between a sensible margin and a genuinely excessive rate is large — and staying with your bag's own figure keeps you well inside it.