Thatch is a layer of undecomposed stems, leaves and roots between the grass blades and the soil surface. It can be beneficial, but when it builds up too much, it keeps air, water and fertilizer from reaching the soil. Grass becomes less tolerant of drought and heat, more susceptible to disease and insects, and more prone to scalping when mowed.
Dethatching or core aeration is typically performed every year or two, and the optimum timing depends on grass type.
Aeration is the most useful treatment for moderate thatch. It consists of perforating the soil—and any thatch above it—with small holes. Breaking through the soil with an aeration device is easier and less drastic than dethatching, and it causes less stress to the grass and less debris to be cleaned up. A power aerator can usually be rented locally, or you can pay a lawn care or landscaping service to do it for you.
Dethatching—whether with a thatching rake, vertical mower or dethatching tines on your rotary mower—cuts through the thatch, tears it loose and brings it to the surface. Verticutting is the preferred treatment for St. Augustinegrass lawns in the South.
Bluegrass lawns can be dethatched in spring or fall, though fall is preferred. In the spring, just wait until the soil dries out enough to be workable. In the fall, do it early enough that the lawn has a month or more to recover before growth slows down or stops for the year. Bentgrass lawns, especially on the West Coast, can also be done in spring or fall—but spring is preferable. Bermudagrass lawns should be done in late spring, when the grass is thriving; fall is not recommended, but summer is okay.