What It Is

• Grass grows more quickly than it breaks down, so there is a buildup of dead plant matter mixed in with the live matter at the root level of the plants. This is called thatch, and over time it will grow into a mat-like layer. Thatch will form more quickly in soils with high pH levels, soils that are wet and heavy, and soil that is highly compacted.
  
  

What It Does

• When thatch is 1/2 inch or less in thickness, it is actually beneficial to the lawn; it discourages weeds, holds in water and helps regulate the soil temperature. If the lawn and the soil are in good condition, thicker thatch shouldn't develop.
  
  

When It Should Be Dealt With

• If thatch is over 1/2 inch thick, the grass roots will begin to grown inside the thatch layer instead of in the soil, which makes it easier for the plants to be torn out. A thick thatch will block air from reaching the soil and provide a haven for pests, bacteria and fungus, which are hostile to the grass and very difficult to eradicate if they're under the thatch.
  
  

Preventing Problem Thatch

• It's much easier to prevent a thatch problem than to correct one. Don't overuse nitrogen fertilizer or over-water your lawn, because both practices will make the grass grow too fast. Cut the lawn regularly and remove no more than 1/3 of the grass blades' length so it will break down quickly. Aerate the lawn in the spring and again in the fall using a hollow-spoon aerator; this is a tool that removes small plugs from the lawn so that air can reach the soil. Leave the plugs on the surface of the lawn; as they break down they add organic matter to the soil.
  
  

De-thatching a Lawn

• If the lawn thatch is between 1/2 inch and 1 inch thick, use a de-thatching machine once a year (in the spring or the fall, when the grass is growing best). These machines look rather like lawnmowers, but they have vertical tines or blades that cut down into the thatch--the same principle as the aerator but on a grander scale. Make two passes over the lawn with the de-thatching machine, the second at right angles to the first. Applying a top-dressing--which is merely a 1/4-inch layer of compatible soil--after the de-thatching will add microorganisms that will help break down the thatch. If your thatch is thicker than 1 inch, it's best to remove the sod and plant a new lawn.