Why a heavier seeding rate gives you a thinner lawn

Tall fescue goes down at about 7 lb per 1,000 square feet for a new lawn. Kentucky bluegrass goes down at 2.5. Both are grass, both are sold in similar bags, and the gap between them is nearly three to one.

It is not a quality difference or a marketing position. It is seed size, and once you know that, the rest of the rate table stops looking arbitrary.

The rate is a seed count wearing a weight disguise

What a lawn needs is a certain number of seeds landing per square inch of soil. Too few and it comes up patchy. Too many and they fight each other.

Seed is sold by weight, so the published rate has to convert that target count into pounds, and the conversion depends entirely on how big the seed is. Bluegrass seed is tiny, so a pound of it holds an enormous number of seeds. Tall fescue seed is much larger, so a pound holds far fewer, and you need more pounds to put the same number of seeds on the ground.

That is the whole explanation for a table that otherwise looks like a list of opinions.

What over-seeding actually does

Doubling the rate feels like insurance. It is the opposite.

Seedlings that come up shoulder to shoulder compete immediately for light, water and nutrients. They grow tall and thin reaching past each other rather than putting energy into roots, and a shallow-rooted lawn is the one that browns off first in July.

The other half is disease. A dense carpet of seedlings holds moisture against the soil surface and around the crowns, which is exactly the condition damping off needs. Losing patches of a brand new lawn to a fungal problem caused by generosity is a demoralising way to learn this.

Overseeding is roughly half

Thickening an existing lawn uses around half the new-lawn rate, because the living turf is already occupying most of the ground and you are filling gaps rather than starting from bare soil.

Ryegrass is the usual choice there because it germinates in five to ten days, so it establishes before the existing grass crowds it out. Bluegrass takes two to four weeks to show, which is a long time to keep bare soil damp. The grass seed calculator carries each species rate and halves it automatically in overseed mode, along with the starter fertiliser that goes down at the same time.

Where the real losses happen

Rate is not the thing that usually decides whether a seeding works. Three others matter more, and all three are free.

Seed has to touch soil. Broadcast onto thatch or onto an unraked surface, most of it never reaches the ground and simply feeds birds. Raking lightly, or running a core aerator before overseeding, does more for the result than any adjustment to the rate.

It has to stay damp. Not wet, damp, and continuously, from sowing until the seedlings are up. A new seeding that dries out for one hot afternoon at the wrong moment is largely a write-off, and that single factor accounts for more failures than everything else combined.

And it has to be the right species for the site. Sowing a sun-loving ryegrass under a mature maple at any rate produces a thin lawn, and the usual response, more seed next spring, produces a thin lawn again.

Bare patches want a different approach

For a patch rather than a lawn, the rate hardly matters. What matters is contact and moisture, so loosening the top inch of soil, scattering seed, pressing it down with a boot and keeping it damp will outperform a carefully measured rate scattered on hard ground.

One honest exception

There is a case for going slightly heavy, and it is a narrow one.

Seeding late in the season, when you are racing the first frost, a modestly heavier rate buys insurance against the portion that will not have time to establish. The same applies on a slope where some seed is going to wash regardless of what you do. Both are deliberate hedges against a known loss, which is a different thing from doubling the rate because the bag was nearly empty anyway.