Scarifying: What is its purpose and what must be taken into account?

The word “thatch” doesn’t necessarily refer to a roof or a thick mop of hair, but like the latter, it does need to be kept under control. Here, the method used is scarifying. Lawn thatch consists primarily of dead, but also of some living shoots and also partly of roots. It can be found between the soil and the visible turf and can measure from a few millimetres to as much as 2 cm in thickness. However, not all lawns automatically contain thatch.

This depends largely on the varieties of grasses that have been sown or are present. But few laypeople can distinguish between the various grass types, despite this being fairly simple and in fact rather interesting.If a lawn consists mainly of ”Lolium perenne“ (perennial or English ryegrass), there is unlikely to be any lawn thatch and in the case of short cut lawns, the soil can nearly always be seen through the grass blades. “Lolium” dominant lawns consequently hardly ever need to be scarified. Lawns predominantly comprising “Poa pratensis” (common meadow-grass) and/or a high proportion of “Festuca” varieties (red fescue, sheep fescue) are considerably more prone to the formation of thatch. Worst of all in this respect are “Agrostis” dominated lawns (with a high proportion of creeping bentgrass). These grass types are typically used for golfing greens and are rarely contained in seed mixtures for domestic gardens. In terms of a propensity towards lawn thatch, there are significant differences between the individual grass varieties and even some “Lolium” varieties, for example, can display slight thatch formation. Unfortunately, those grasses which are particularly well suited for domestic gardens, i.e. the very dense-growing “Poa” lawns, are highly prone to thatch.

So what is the problem with lawn thatch? This thatch forms a kind of “sponge”, which absorbs the water during brief rain showers or light watering, preventing it from penetrating into the root-bearing soil. Most of the water evaporates from the thatch and is therefore of little use to the lawn. Particularly in domestic gardens, it is significant that in many cases watering is carried out too frequently, but in insufficient amounts (please refer to my article on “correct” watering dated 25/6/2014 in this section). Unbeknown to the layperson, the lawn will react significantly to “incorrect” watering over the long term. The grass roots grow in search of the dampest areas, i.e. immediately beneath the layer of thatch and even into it. The lawn therefore forms an extremely flat system of roots and then responds very sensitively to even a brief lack of water. It becomes “addicted”, so to speak, to constant watering. Moreover, once the lawn is only superficially rooted into the soil, its “shearing resistance” is significantly impaired so that it becomes increasingly sensitive to tread loads.

What can be done against lawn thatch? Scarifying! Sounds simple, but doing it “right” is not quite so easy. This starts with the choice of machine, continues with the correct time to scarify and the appropriate depth settings for doing so. The hand-held tools available from retailers are of little use: using these, you will only manage a couple of square metres. Likewise, most of the “low-cost” machines are unsuitable for really “de-thatching” the lawn – they are usually insufficiently robust to really get the thatch out of the lawn. Scarifying should be carried out during dry weather and when growth is strongest. May, for example, is ideal because scarifying naturally also causes significant injury the living lawn structure, which then needs to regenerate as quickly as possible. The lawn must always be mown prior to scarifying.

Setting the correct depth is the most difficult part of scarifying. The tools (blades, spring tines etc.) should be set to a depth at which they just reach the soil in order to remove as much thatch as possible. However, if you have a heavily thatched lawn with a flat network of roots, or even an “Agrostis” dominant lawn, then you will completely destroy the lawn with such a setting. Large chunks will be ripped out, simply because the roots are extremely superficial.

The only advice here is therefore: try out different depth settings on small portions of the lawn which are not in direct view. For the above-mentioned reasons, many shy away from a deep setting so that hardly any thatch is removed, resulting in a great deal of wasted effort. Consequently, a little courage, but also the appropriate care and sensitivity are required. The material removed from the lawn must of course be collected and is well-suited for composting.

I hope you enjoy your next scarifying operation!

Prof. Karl E. Schönthaler
VIKING garden expert

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