Answer: Since you didn't fill in your Locale, it's difficult to provide a useful answer of what to do when. However, thatch is a layer of straw-like dead grass stems, roots, and stolons that builds up on top of the soil faster than it decays. A thin layer (one-half inch) of thatch is beneficial, acting as mulch to maintain soil moisture and consistent soil temperature, reduce compaction, protect the grass crown and even act as a "cushion" to protect the turf from excessive wear. When thatch gets too thick it prevents water and fertilizer from penetrating to the roots. Thatch can also provide a nice environment for pests and diseases and make level mowing a challenge. To determine how thick thatch is, use a hand trowel or knife to dig out a two-inch deep wedge of turf from several areas. Measure the straw-like matter between the green grass and the soil. Dethatching is best done when grass is actively growing so it can recover quickly. You can use a thatch rake, or for large areas, dethatching blades can be attached to some rotary mowers. Dethatching machines can be rented. Basically, vertical blades slice through thatch, lifting it to the surface. It's important to make sure blades are set so they don't reach through the soil and damage the grass crowns (growing points).
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