Thatch Over Half an Inch Will Waste Your Grass Seed

People spread a lot of good seed on a lawn that was never going to take it. The reason is often hiding right under the grass: thatch.

What thatch is

Thatch is a layer of dead and living stems, roots and runners between the green grass and the soil. A little is normal and even helpful. It cushions the lawn and holds some moisture.

Trouble starts when it builds up past about half an inch. Then it acts like a sponge: it holds water near the surface and dries out fast on top, and roots grow in the thatch instead of the soil.

Why it wastes seed

Seed needs to touch soil. Seed sitting on top of a thick thatch layer:

• Dries out between waterings
• Sprouts, but the roots can't reach the soil
• Dies in the first hot or cold snap

That's money and a season gone.

How to measure it

Cut a small wedge out of the lawn with a knife or a trowel, a couple of inches deep. Look at the side:

• Green grass on top
• A brownish, spongy layer next
• Soil at the bottom

Measure the spongy layer. Under half an inch, you're fine. Over that, deal with it before you seed.

What to do about it

• Core aeration pulls plugs of soil and thatch, opens the surface, and gives seed pockets of soil to land in. It's the best prep for overseeding. See aeration, topdressing and vertical mowing.
• Dethatching or vertical mowing slices through the thatch and pulls some of it out. It's rough on the lawn, so do it when the grass is actively growing.
• Topdressing with a thin layer of compost or soil helps break thatch down over time. See compost topdressing rates and timing.

Why thatch builds up

Too much nitrogen, too much water, mowing too low, and some grass types (Kentucky bluegrass spreads by rhizomes and can build more thatch). Healthy soil life breaks thatch down; heavy pesticide use can slow that.

Timing

Fall is a good time to aerate in the northern US. If you're past the seeding window, aerate now anyway, and seed next August or dormant seed later. See is it too late to overseed in October?.

On the golf course, controlling thatch was a constant job on greens and fairways. At home, a yearly check with a trowel is usually enough.

Sources (1)

Penn State Extension: Lawn establishment

Photo: Rasen vor dem Vertikutieren by Andreas Goerrings (user Klugo), Public domain.

Related: Understanding Rhizomatous, Stoloniferous, and Bunch-Type Grasses · Your Fall Seeding Browned in the Cold. Does It Need More Fertilizer?