Excessive thatch is a common problem. Thatch is a layer of organic matter, consisting of tightly woven, living and dead grass stems, roots, and crowns, that forms between the grass blades and the soil line. These parts of the grass plants are high in lignin, an organic material that breaks down slowly. In most healthy lawns, those parts do break down. Frequent, shallow watering; over fertilization; infrequent, high mowing; excessive use of pesticides; improper soil pH; soil compaction; or heavy clay soils can all cause thatch.

A little bit of thatch — less than 1/2 inch — isn’t bad. The thin layer cushions the turf, reduces soil compaction, and helps conserve moisture. A thick layer of thatch between grass blades and soil blocks the movement of air, water, and nutrients to grass roots, resulting in either a shallow-rooted lawn or a spongy, bumpy lawn. Thatchy lawns are also more susceptible to insects and disease. If your lawn feels spongy when you walk on it, check for thatch by removing a small wedge of the lawn (go deep enough to get some soil) with a knife or shovel. You can see the thatch — the spongy layer of brownish, peat-moss-like stuff below the grass blades and above the soil — when you examine the turf. If your lawn’s thatch is more than a 1/2-inch thick, you need to dethatch or aerate.

Dethatching involves actually cutting through the thatch with knife-like blades and then removing the debris. You can use a thatching rake, which has knifelike blades rather than normal tines to basically comb out the debris, but it’s hard work and really only practical for small lawns. The more practical and effective method is to rent a gas-powered machine called a dethatcher, vertical mower, or power rake. Available for rent, a dethatcher easily cuts through the thatch with rotating blades or stiff wire tines, but the machines can be fairly heavy and a bit difficult to maneuver. For thick grasses, use a vertical mower with steel blades. Use the wire-tine type of dethatcher on Kentucky bluegrass or fescue lawns.

A dethatcher works best when the lawn is lightly moist — not too wet or too dry. Here’s how to do it:

1. Mow the lawn a little lower than normal right before you dethatch.
2. Make at least two passes — with the second pass at a 90-degree angle to the first — over the lawn with the dethatcher.
3. Rake up all the debris.
   If you haven’t used any pesticides on the lawn and it’s not a weedy grass, such as Bermuda grass, you can compost the debris or use it for mulch.
4. Water and fertilize the lawn.

Dethatching is stressful on a lawn. The lawn ends up looking pretty ratty, but if you dethatch at the right time, the lawn recovers quickly and fills in. For a quicker fill-in, some people prefer to reseed the lawn right after dethatching. If you don’t want to reseed but worry that weed seedlings may take over before the grass recovers, apply a preemergent herbicide (it prevents weed seeds from germinating) after dethatching.

Aerating is the process of punching small holes all over your lawn. The most effective type of aerating is with a gas-powered machine called a core aerator that pulls out small cores of grass and soil.  ....read more