Now, my question: Instead of going to the trouble of raking up thatch, why not leave it on the ground for similar beneficial reasons? After all, thatch is organic matter, so why not just leave it be? -- M.H., Camillus.
Dear M.H.: The problem with thatch is that it isn't breaking down. Organic matter is normally broken down by the microorganisms in healthy soil. Grass clippings disappear quickly as long as they aren't too long and piled up. They are mostly water. Fallen leaves decompose if they are chopped up finely; matted leaves can kill the grass underneath. Clippings and leaves are free fertilizer and help improve the soil. Putting them out for pickup makes no sense.
Thatch is not the accumulation of dead stuff that you rake out of the lawn in the spring. That debris can be left alone; it just doesn't look very good. Raking the lawn in the spring loosens matted grass, but if the raking is done too vigorously, the growing grass is pulled out by the roots, so take it easy.
Thatch is primarily a concern with intensively managed turfgrass like golf greens and sports fields. It is a tough, dry, spongy layer of stems that develops between the soil and the green parts of the grass. A thin layer of thatch is OK, but if it's too thick, it can cause problems.
Figure out how much thatch there is by making a vertical slice into the grass and looking at the cross section. If the brown part is more than an inch thick when compressed, the thatch may have to be removed. Some types of grass are more prone to developing thatch than others, notably fine fescues and creeping bentgrasses. Compacted soil, acidic soil and high nitrogen fertilization can also lead to more thatch.
What's happening is that dead organic matter is building up more rapidly than soil microorganisms can break it down. Water and air can't penetrate into the soil. The grasses begin to root into the thatch, and these shallow roots dry out easily, damaging the grass.
A combination of mechanical techniques can reduce thatch. Core aeration plus vertical mowing plus raking can be done in the late summer and early fall. Core aeration removes plugs from the soil, allowing water to soak into it. The soil cores can be raked back in to the holes. Vertical mowing cuts slits in the soil that allow water and air to penetrate. All of these can make the lawn look bad for a few days, but better in the long run.
Carol T. Bradford, of Syracuse, has been gardening in Central New York for more than 25 years. Send questions in care of Home & Garden, The Post-Standard, P.O. Box 4915, Syracuse, NY 13221, by fax to 470-2111 or by email to email@example.com. Letters might be edited for space and clarity.