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Dealing with Thatch


What is Thatch?

Thatch on a lawn refers to the intermingled organic layer of dead and living shoots, stems, and roots that develops between the zone of green vegetation and the soil surface. Hot humid weather contributes to the decomposition of the material which returns to the soil as nutrient rich organic matter. Thatch accumulation and the need to control it occur when thatch development is faster than the decomposition process.

When Thatch Becomes a Problem

A thatch layer of less than 1/2" is beneficial to the lawn but over time, it thickens and the lawn gets weaker and weaker. Excessive thatch provides favorable conditions for pests and the development of turf diseases. It can interfere with the movement of water and fertilizer into the root zone. This leads to over watering and ineffective fertilizer applications. Extreme thatch buildup can lead to the grass rooting into the thatch layer rather than the soil.

    Conditions that promote thatch build up include:
  • Heavy, wet soils; alkaline(high pH) soils and soil compaction
  • Pesticides that inhibit microorganism or earthworm activity
  • Over fertilizing, causes faster growth than the lawns ability to decompose it
  • Warm temperatures combined with creeping grasses like Bermuda, Zoysia, Bentgrass and Bluegrass

Contrary to popular belief, mulching your lawn clippings does not contribute to thatch development. Clippings are 85% water and break down rapidly. Returning them to the lawn actually adds nitrogen to the soil, reducing fertilizer needs.

Controlling Thatch

When a thatch layer starts getting thicker than 1/2", it may be necessary to remove some of it and/or improve the condition of the turf.

Dethatching a lawn can be as simple(and backbreaking) as vigorously raking the lawn or more complex, like vertical mowing. Vertical mowing dethatchers mow the turf vertically with spinning blades, cutting and pulling up thatch to the surface. A lawn care service can be used or machines can be rented at a garden center or tool rental outlet. Power rake dethatchers work by tearing into the thatch with a series of firm, spring loaded wires. Dethatching leaves the lawn a mess, so clean up the debris and use as compost. Since the grass is initially weakened by dethatching, to promote a quick recovery, it's a good idea to time it with a fertilizer application, overseeding(if necessary), and watering or rain.

Topdressing with humus high in organic matter or spraying organic 'teas' are good ways to add microbial activity to the soil and aid in decomposing thatch. It is a long process though and might not help a lawn with a severe thatch problem.

Core aeration is the act of making holes in the turf with hollow tines so as to remove a little 'core' of soil/thatch. Core aeration breaks up the surface allowing air and water to the soil while the cores fall apart and act as an organic topdressing, improving thatch breakdown.

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