Thatch is a soil surface layer of old leaves, clippings, stems, roots, and other organic material which has failed to decompose due to insufficient microorganism activity.

A layer limited to 1 to 1.5 centimeters (3/8 - 5/8") in thickness is generally considered beneficial because it promotes a more weed-free, drought tolerant lawn with better colour and more stress tolerance. However, when the accumulation becomes thicker than 2 centimeters (7/8"), water and fertilizer remains in the thatch layer instead of reaching the plant roots, and insects and diseases are protected from chemical applications.

The primary causes of thatch are poor soil drainage, compacted soils, over watering and too high a level of nitrogen application.

The thatch layer can be reduced by limiting the amount of organic material returned to the soil and by encouraging increased microorganism activity. Cut back nitrogen fertilizer applications to 30 percent of normal levels especially in the early spring. Reduce the amount and frequency of watering, and rake the lawn vigorously. To further promote microorganism activity, aerate the lawn to a depth of 7 to 10 centimetres (3-4") in May or September, then shatter the soil cores and leave them on the soil surface.

For very heavy thatch layers, the use of a dethatching machine may be necessary. Some lawn damage will result but, with proper care, lawns usually make a full recovery.

Next Tip: Soil Compaction