Like many plants, grass has a belowground root system topped by a living, aboveground shoot that's green and growing. In between lies a layer organic matter called thatch. Some thatch is natural and good for a lawn, but too much can usher in problems.
Comprised of leaves, stems and roots – some living, some dead – thatch forms as a natural part of turf's growing process. It lies on top of the soil, a tightly woven layer beneath the visible grass blades. As long as it doesn't get thicker than ½ to ¾ of an inch, thatch actually contributes to lawn health in these ways:
- Mulching soil and slowing water loss
- Cushioning soil and decreasing compaction
- Insulating grass crowns from soil temperature swings
- Improving turf tolerance to foot and mower traffic
A problem arises when thatch develops into a thicker layer and forms a wedge between grass and soil. Too-thick thatch diminishes lawn health by:
- Forming an impervious layer that prevents water, fertilizer, and insect or disease controls from reaching soil
- Blocking sunlight from reaching lower grass blades
- Holding moisture against grass blades, which can foster disease
- Blocking soil so that grass roots grow into nutrient-lacking thatch, which forms a shallow-rooted lawn
- Creating an uneven lawn, which leads to uneven mowing and scalping
What Causes Thatch?
Although thatch forms naturally, some situations promote rapid thatch formation:
- Overfertilizing with too much nitrogen
- Always mowing too high
- Heavy clay soil
Certain grasses tend to form thicker thatch layers. Creeping turf types that spread by aboveground and underground stems often rapidly form thatch; this includes St. Augustinegrass, Bermudagrass and Zoysiagrass. Cool-season grasses, such as Bentgrass and aggressive Kentucky Bluegrass varieties, also can quickly form thatch, especially if soil is compacted and turf hasn't been properly fertilized for a few years.
If you use a mulching mower, mow often enough and leave grass clippings on the lawn – that does not lead to thatch formation. Those are healthy practices that can help prevent thatch formation. Typically grass clippings decompose readily. Thatch arises from grass stems and roots in the top inches of soil that die and don't fully decompose. For more on mowing your lawn properly, click here.
Does Your Lawn Need Dethatching?
Perform one or more of these tests to determine whether your lawn needs dethatching.
Feel The Lawn. A lawn that's spongy or bouncy underfoot, with a springy feel, often has a thick thatch layer.
Visually Inspect The Lawn. To determine how thick thatch is, examine the lawn closely. Is soil visible between turf crowns? If it isn't, you're likely looking at a thatch layer. Can you shove your finger through the visible thatch layer – or is it impenetrable? A thatch layer that's tough to wedge a finger through needs to be thinned.
Measure The Thatch. Another way to examine thatch is to excavate a lawn sample. Use a trowel or spade to remove a wedge-shaped layer of grass and soil about 3 inches thick, or just pry up a small section of turf. Look for the thatch layer lying directly on top of soil. Measure the thickness. A layer thicker than ½ inch signals it's time to dethatch.