Thatch is one of the first characteristics to look for in Kentucky bluegrass that appears dead or dying. If the thatch layer is more than 1 inch thick, immediately peel back the sod and look for grubs and other insects. Then look at the root system for feeding injuries and discoloration. Root-infecting fungi that cause summer patch disease are often found in these discolored roots. Both grubs and summer patch are found in virtually all samples received for diagnosis in the summer when thatch is in excess.
What is thatch? If you have ever seen a soil core or cut a profile of sod, you may have noticed a layer that is above the soil surface and below the leaf tissue. It generally has a brownish appearance and the consistency of a Brillo pad. This network of fibrous material consists of undecomposed crowns, lateral stems called rhizomes, and leaf tissue. Other forms of debris may also deposit on the surface, contributing to the problem. In a balanced turfgrass ecosystem, this layer is kept in check by hungry fungi and bacteria. But once the system becomes out of balance, you're left with a thick blanket that can begin choking the life out of plants.
Thatch can cause assorted problems. Insects and fungal diseases have been mentioned. Thatch is very dense and hydrophobic, meaning that it has the ability to repel water. Imagine laying a plastic tarp across your lawn and punching small holes for the leaves to poke through. Now, try watering the turf on a hot day. Much of the water sits on the surface, and most evaporates into the atmosphere. Also, try applying fertilizer or pesticides to the turf. Again, it sits on the surface without reaching the roots where it is needed. Lastly, the organisms that decompose thatch begin to die along with the turfgrass because they lack proper gas exchange with the atmosphere.
The best way to control thatch is by using proper turfgrass-management techniques. Most importantly, don't overfertilize your lawn. Make sure your spreader settings are adjusted to give the correct coverage. If your grass grows faster than the microorganisms can decompose it, the thatch begins to build. Also make sure your grass gets about an inch of water per week during the summer. This is good not only for the grass but also for the microorganisms.
What if you already have more than an inch of thatch on your lawn? Without tilling up the whole yard, it may take several years to recover. Begin by cultivating your lawn in the fall, and possibly the spring, with a vertical rake, by core aerification, or by deep tining. These processes mechanically remove the thatch and allow gas exchange so the microbial flora can do their job of decomposition. You can also temporarily remove grass clippings. When thatch is under control, leaf clippings add little to the thatch because they are composed primarily of water. Leaf mulching is a great way to return nutrients and water to the plants while also reducing landscape waste in landfills. A thin coating of topsoil on the surface in the thickest areas speeds decomposition by attacking thatch from the top and bottom. Last of all, many of the newer Kentucky bluegrass varieties are beautiful and quick to establish but can also be somewhat aggressive. So keep a watchful eye out for thatch buildup with new sod and seed.