Grass Seed Calculator

Ask how much grass seed a lawn needs and the answer varies by a factor of sixteen depending on nothing but the species. That is not vagueness; it is a real physical difference.

A pound of Kentucky bluegrass seed contains about 2.2 million seeds. A pound of tall fescue contains about 227,000. Bluegrass seed is roughly a tenth the size, and the plant it produces is smaller and spreads by underground rhizomes, so far fewer pounds are needed to cover the same ground. Tall fescue is a bunch grass — each seed produces one clump that never moves — so the stand has to be established at the density you want it to stay at.

Centipedegrass takes this to its extreme at half a pound per 1,000 square feet, which people routinely exceed by a factor of ten because it seems impossibly small. Buffalograss goes the other way: it is sold as burs containing several seeds each, so a pound contains only 50,000 units and the rate looks high.

The practical consequence is that any seeding advice has to name a species before it means anything, and the number on a generic bag of "lawn seed" is a rate for whatever mixture happens to be in it.

Overseeding — sowing into existing turf to thicken it — takes roughly 60% of the new-lawn rate, and the temptation to use the full rate produces a measurably worse lawn.

The reason is competition. On bare soil every seed has access to light, water and space, and the rate is calculated to produce a stand at the density you want. In existing turf, most of that space is already taken. Seedlings germinating into an occupied sward are competing with mature plants that have established root systems and with each other, and the result at a full rate is a mat of thin, weak seedlings that mostly die in the first hot or dry spell.

What actually determines overseeding success is not the quantity but the contact. Seed sitting on thatch or on hard ground germinates and dies, because the root never reaches soil. Core aerating, scarifying, mowing short and removing the clippings all do more for the outcome than any adjustment to the rate — and on a badly thatched lawn, none of the seed will work until the thatch is dealt with.

Patch repair is a third case and it uses the full new-lawn rate, because a bare patch is bare soil. Measure the patches rather than the lawn.

For cool-season grasses — fescue, bluegrass, ryegrass — late summer into early autumn is the best seeding window of the year, and it beats spring by a wide margin.

Three things line up. The soil is still warm from summer, so germination is fast. The air is cooling, so seedlings are not fighting heat stress while they have no root system. And the annual weeds that dominate spring — crabgrass above all — have finished their cycle, so a seedling is not competing with them for the same bare ground.

Spring seeding inverts all three. Germination is slower in cold soil, the seedlings go into summer with a root system a few weeks old, and crabgrass germinates in exactly the same conditions that suit the grass. It also rules out pre-emergent crabgrass control, since a pre-emergent cannot distinguish between crabgrass and lawn seed.

The practical rule is to seed with about six weeks of growing weather left before the first hard frost. Later than that and the seedlings go into winter too small to survive it. Much earlier and summer heat is still a problem.

Warm-season grasses run on the opposite schedule, and on soil temperature rather than the calendar: seed once soil is reliably above 65°F, in late spring to early summer, so the plants have the full season to establish before their first winter.

Grass seed is sold by weight and bought by hope, and the seed tag is the one document that lets two bags be compared.

Purity is the percentage by weight that is actually the named species. Germination is the percentage of that seed that will grow, from a test whose date is printed alongside. Multiply them and you have pure live seed, which is what you are really buying: a bag at 95% purity and 85% germination is 81% pure live seed, and a fifth of its weight is doing nothing.

Below those two lines are the ones worth reading before anything else. "Other crop seed" is agricultural seed that is not what you bought — under 0.5% is normal, and more indicates a cheap bag. "Weed seed" should be under 0.5%, which still amounts to thousands of seeds. And "noxious weeds" should read "none found"; anything else means the bag contains a species the state regulates, and no price makes that worth spreading on a lawn.

Germination falls with age, so a tag test date more than a year old is worth discounting. Seed stored cool and dry keeps reasonably well; seed that spent a summer in a garage does not.