Thatch

Thatch is a complex mixture of dead grass roots, shoots and stems that forms between the soil and grass blades. While a small layer of thatch provides many benefits, problems arise when the layer of thatch becomes too thick. 

As grass is growing, new leaves, stems and roots replace those that die as part of the natural growth cycle. Dead roots and stems are decomposed by fungi and bacteria and returned to the soil. When the dead roots and stems are building up faster than they are being decomposed, the thatch layer increases. 

Some grass varieties are more prone to the accumulation of thatch. Rye grasses that have a rooting system that goes straight gown into the soil rarely have issues with thatch. Creeping fescues on the other hand have a lateral rooting system and may need regular dethatching. The majority of lawns are made up of several grass species. 

A thin layer of thatch, typically around 1cm, is beneficial to the lawn as it helps conserve moisture and keep an even temperature. When thatch becomes thicker, it acts as a barrier and limits the movement of water, nutrients and air to the grass roots. The moist layer becomes an ideal environment for lawn pests and diseases, such as brown patch and red thread which favour sheltered humid conditions. 

A lawn with too much thatch will feel soft and spongey underfoot. A soil sample will reveal a layer of thick, brown material sitting above the soil. Where thatch is more than 1cm thick it becomes harmful to the grass and should be reduced. 

Aeration will help break down a thick layer of thatch, especially where the soil and thatch have become compacted. Hollow tine aeration will remove small plugs from the lawn, allowing water, nutrients and air to reach the roots and encourage stronger growth. 

Scarifying will remove thatch from the lawn, and where thatch is an issue should be included as part of the annual maintenance programme. It is best carried out in spring or autumn. 

Following scarification, bare patches should be overseeded and a fertiliser applied to help the lawn recover.