Lawns are scalped during lawn mowing when the thatch layer of the lawn is cut into, revealing the brown grass stems. While this is the only definition of lawn scalping, there are several causes of why it occurs, and most are preventable.
Occasionally we can mistakenly mow lawns too low, while the thatch layer can be of a normal thickness, the low mowing will cut into the thatch layer, revealing the brown stems and runners. Normal lawn care should resolve the problem in the growing season quite quickly and easily for Warm Season grasses which can repair themselves very effectively.
In most cases, mowing too low will not be the cause of lawn scalping, although it may be the first and easiest reason that comes to mind.
Uneven lawns will be regularly scalped as the wheels or rollers of the lawn mower follows the indentations of the lawn, thus lowering the blades when it occurs, and scalping occurs. This problem will continue until the problem is rectified and the lawn surface is evened out. See this article on how to level an uneven lawn.
This is the primary cause for lawns being scalped during lawn mowing. The thatch layer has continued to increase in thickness over several years, and the result is that normal lawn mowing now regularly cuts into the thatch layer, and scalping the lawn as a result.
Increasing lawn mowing heights may seem to fix the problem, but it will not. The thatch layer will only be hidden from view for a while longer, and with the longer green leaf the thatch will begin to increase in thickness at a far faster rate.
Whenever lawns get to this stage, they must be repaired and the thatch removed effectively.
This is usually done via three different methods:
Vertimowing is the most common method and usually done by a professional Vertimowing Contractor with the use of special Vertimowing machinery. Vertimowing can be done with slightly or heavily thatched lawns.
Low Mowing can be done by mowing Warm Season grasses at a very low height to remove all the thatch in one mowing, it's a long, difficult and dirty process and Vertimowing is always a preferred option.
Slowly Reducing Mowing Heights involves mowing more frequently, and slowly decreasing mowing heights over several months, the lawn will continue to be thatched, but will continue to look better after each mowing and as the process continues and completes itself.