Thatch is an organic layer of dead leaves, stem bases, roots, stolons and rhizomes in various stages of decay lying on top of the mineral soil, it also contains living roots etc. of the turf. Thatch is a natural development in all turf when earthworms are absent. It has a high lignin content and so resists microbial breakdown.

There are a number of factors which affect the rate at which thatch develops including; the species and even cultivars of turfgrass, fertiliser input, whether or not turfgrass clippings are removed, maintenance carried out, aeration of the rootzone and soil pH. Excessive thatch accumulates when the input of dead turf residues exceeds the rate of microbial breakdown, taking into account the amount removed by scarification.

Excessive thatch increases the risk of turf disease by providing a favourable environment for pests and disease causing pathogens to feed and multiply. During the summer months thatch tends to dry out and dry thatch is difficult to re-wet, which can lead to localised dry spots. In wet weather thatch acts like a sponge giving rise to excess water and poor aeration. It can reduce the effectiveness of insecticides and fertilisers by holding them up in its layers. Roots in a thatch layer are more susceptible to stress due to extremes of temperature and moisture content. Thatch can also increase the risk of scalping due to the sward being high above the soil.

A shallow layer of thatch of about 5mm may have a few benefits; the turf is more resilient and tolerant of wear and the cushioning effect may reduce the risk of soil compaction. However, action should be taken to prevent the build-up of thatch.

Maintaining a soil pH above 5.0 encourages bacterial activity thereby increasing the rate of decomposition. If the pH falls below 5.0 then the rate of decomposition will be decreased and the risk of a build-up of thatch increased. The excess use of fertiliser and over-watering both contribute to the build-up of thatch. Turfgrass clippings are often removed, but even when they are left, they are young and non-lignified so break down quite quickly compared with root tissue. Infrequent mowing which results in smothering large quantities of clippings onto the turf will cause problems.

The control of thatch requires a combination of several turf maintenance practices. The grass should be cut frequently at recommended heights and fertilisers and irrigation water should be applied in moderation to avoid excessive growth of the turf. These components of turf culture have to be complemented by the additional practices of scarification, aeration and topdressing. Scarification is needed to remove unwanted, dead turf residues, which then do not have to be decomposed within the turf. Aeration, which in the form of hollow tine spiking also removes some turf residues, it also helps ensure that the breakdown of residues is not restricted by anaerobic conditions. The application of a suitable topdressing worked into the rootzone surface will decrease its organic matter content to acceptable levels.

Michael J Higgins
Technical Manager