Thatch in St Augustine Lawns

The first time I walked across a healthy St Augustine lawn, I thought to myself “Thatch!” The lawn looked great, it was the middle of the summer (prime time for heat and water stress) and I recommended verti – mowing (sometimes called dethatching). Now 9 years later and a  lot more experience with St Augustine turf, I do not recommend verti-mowing (dethatching). Thatch is a minor problem and the verti-mowing causes more damage to the lawn the thatch will cause.

About Thatch

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.

Thatch and St Augustine

St Augustine has a unique culture. It has a lot of above ground stems and large open space between this stems. This stems, creeping horizontally along the soil surface, form a 2” open weave but stiff firm mat on the soil that is easily damaged by the sun. The leaves grow erect from the stems and protect it and the roots grow downward into the soil. It is this mat of stems that is often – and wrongly – called thatch. The mat is important part of the anatomy of the St Augustine canopy – it is not thatch.

Mowing Height

Mowing height is critical to the stems health and survival (no stems – no thick lawn). 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 and insulation of the stems from the harsh sun. That is a win/win – high photosynthesis and max protection for the stems.

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”. Most of the soil activity is in the top 6”. Photo 3 shows the 3 layers of St Augustine lawn – the middle layer, the stem layer is not thatch. The stem layer is between the protective leaf canopy and the soil. Also note that removal of the leaf canopy will expose the stems to the sun – by design, St Augustine requires this leaf canopy above this stem layer to thrive.

Conclusion

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.