Thatch

By far the biggest problem in lawns is thatch. To understand what impact it has it is necessary to understand what it is and what causes it.

Thatch is not just dead grass. It is a fibrous layer of dead roots and stems that form on top of the soil. In normal healthy soil this layer is decomposed by the bacteria and fungi living in the soil, and is recycled back into the soil as humus, and the perfect balance is where the thatch is being degraded at the same rate as it is forming.

How does it form?

Most lawns start out as turf. In order for turf to have the necessary qualities it is usually made up of 3 or 4 grass species. Some of these will be classed as tufted varieties and some will be creeping. In other words, the tufted varieties have a root system that will grow straight down, and the creeping type will have a rooting system that will grow laterally. Mixing these types together ensures that when the turf is cut it does not fall apart. Additionally it is better to have more than one species of grass in a lawn to guard against disease, which may affect one but not another. The issue here is that some grass types are better suited to some soil types than others.

Let us look at some examples. Rye grass is a much maligned and misunderstood grass. It is the starting point in most turf. This is a tufted variety, and if the lawn was 100% rye then thatch would not be an issue. If you add a creeping fescue to it then you have a recipe for thatch. Its roots will grow laterally, either as rhizomes or stolons. As old stems die off, new ones are formed and it is the old and redundant ones that lead to thatch. These roots are, by nature waterproof, and so it follows that if the thatch layer is too thick it prevents water penetrating to the soil below. This then causes the roots to become shallow as they come back up looking for moisture, and in extreme cases the grass can die of drought stress.

The soil biology is the most important factor here. We know that on sandy loam soils the biology is less effective than on a better quality soil. So the decomposition of thatch is unbalanced and it is allowed to build up faster than it is broken down.

How can it be controlled?

Regular scarifying is the most important part 0f the programme where thatch is an issue. Let us not confuse this with lawn raking. Many lawn mowers come with attachments which really only then turn it in to a power rake.This can be useful for removing dead grass and moss, but will not remove thatch. We would use a verti cutter which has blades and gets deep into the thatch layer. Hollow tine aeration is also beneficial because it removes cores and therefore thatch.