Thatch is accumulated organic matter that is persistent (does not decompose). The persistent organic layer (thatch) forms a distinct layer on top of the soil. You would think that thatch would be appealing – it has all the right qualities – organic and persistent, but there in is the problem. A distinct layer of thatch has physical and biological problems. Physically, the thatch interferes with soil moisture – it is hard to wet and it dries out quickly. The thatch forms a barrier to water moving up and down in the soil profile. Biologically thatch is nearly dead – no biological activity – which is why it persist (no biological activity – no decomposition). Also the thatch is a magnet for chemicals and nutrients interfering with the actions of both.
St Augustine Canopy Details
The photo shows a healthy St Augustine lawn cut-away. The lawn has been mowed at 4 – 6” consistently over the years and has formed a very typical St Augustine turf canopy. Starting at the top is a canopy height of 5” (as shown by the yellow ruler) - measured from the soil to the leaf tip – a very good canopy height. Between the leaves and the soil surface is the stem mat of about 2”. It is made an open weave of thick stiff stems. These stems are protected from the sun and elements by the leaf canopy above the stems. Roots grow from stems to a depth of about 6”.
Thatch and St Augustine Turf
St Augustine has a unique culture of thick above ground stems, large open space between stems and a tall protective canopy above the stems. It is this mat of stems and tall canopy that is often – and wrongly – called thatch. This stem mat and tall canopy is an important part of the anatomy of the St Augustine turf – it is not thatch.
Mat + Tall Canopy = Composting (Not Thatch)
When St Augustine is mowed at 4” or higher it creates a perfect condition for composting organic matter. Leaves, clippings, etc. fall into the tall canopy and are composted into plant nutrients. Organic matter never builds up in St Augustine turf - no accumulation of organic matter – no thatch.
Mowing height is critical for composting – and it protects stems and roots from the hot Florida sun. The stem mat is about 2” thick above the soil in a healthy lawn. If you are mowing 2”, you are mowing stems – it is that simple. If you mow at 3” you have a leaf blade of 1” which will expose the stems to the brutal Florida sun. Mowing heights of about 4” or above will provide a thick protective canopy of erect stiff leaves providing the optimum light reception, insulation of the stems from the harsh sun and perfect conditions for composting. That is a win/win/win – high photosynthesis, max protection for the stems and composting.
Rarely will St Augustine form a harmful thatch layer. The spongy cushiony feel of St Augustine is natural and very necessary for the survival and health of a St Augustine lawn. However close mowing – below 4” will destroy the leaf canopy, expose the stems and reduce the vigor of your lawn. For best results mow your lawn at 4” or higher and spend your time and efforts improving your irrigation system – don’t worry about thatch.