St. Augustine grass is a popular warm-season turf known for its coarse texture and broad leaf blades. This turf spreads via above-ground runners called stolons, resulting in a dense growth habit. Proper mowing technique is a significant factor in maintaining a healthy lawn, as improper cutting frequently causes thinning and decline. Understanding the ideal cutting height promotes a thick, resilient lawn.
The Optimal Cutting Height for St. Augustine Grass
The optimal cutting height for St. Augustine grass generally falls within the range of 3.5 to 4.5 inches. Setting the mower deck toward the higher end of this range ensures the blades maintain sufficient surface area to capture sunlight, fueling photosynthesis.
A taller cut creates a dense canopy that provides natural protection to the soil and the delicate stolons beneath. This shading effect helps to keep the soil cooler, reducing heat stress and water evaporation during hot periods. The increased shade also acts as a natural weed suppressor by blocking the sunlight that weed seeds need to germinate and grow.
Maintaining a greater blade length encourages the grass to develop a deeper, more extensive root system. When the grass is consistently cut too short, it directs energy toward replacing lost leaf tissue instead of growing roots. This results in a shallow root base less tolerant of drought and heat. This optimal height range should be the standard setting for the majority of the active growing season.
Seasonal Adjustments to Mowing Height
While a static height range is a good starting point, the optimal cut must be adjusted dynamically for environmental changes. During periods of extreme heat, drought, or disease pressure, raising the cutting height to the maximum setting (4.5 inches) provides maximum protection. The extra leaf tissue insulates the soil and reduces stress, helping the turf survive challenging conditions.
Shaded sections of the lawn also benefit from a consistently higher cut, typically at the 4-inch mark or slightly above. Taller blades can maximize the limited sunlight available in these areas, ensuring adequate photosynthesis continues despite the reduced light intensity. In contrast, a slight reduction in height may be appropriate in the late fall as the grass prepares for dormancy or a cooler season.
Lowering the height to about 3 inches just before the grass slows its growth can help remove excess leaf material and prevent the build-up of a thick layer of thatch. This practice can reduce the risk of winter-active fungal diseases like brown patch, which thrive in dense, moist conditions beneath a heavy canopy. However, this reduction should be done gradually over several cuts, not all at once.
Mowing Frequency and the One-Third Rule
The frequency of mowing is directly tied to maintaining a consistent, healthy height through adherence to the “One-Third Rule.” This principle states that you should never remove more than one-third of the grass blade’s total height in a single mowing session. Violating this rule severely stresses the plant, leading to brown tips and an increased susceptibility to pests and disease.
To illustrate, if your target height is 4 inches, you must mow the lawn before it exceeds 6 inches in height. For St. Augustine grass during the peak growing season of late spring and summer, this often requires weekly mowing to prevent excessive removal. If growth is particularly vigorous due to favorable weather and fertilization, it may even necessitate mowing every five days.
If the grass has grown significantly taller than the recommended maximum (perhaps due to a missed week), it is necessary to bring the height down gradually. Cutting only one-third of the existing blade length, then waiting a few days to cut another one-third, prevents the shock of removing too much green tissue. This staged approach ensures the grass remains healthy while returning it to the desired length.
Common Mowing Errors to Avoid
The mechanical aspects of mowing are as important as the height setting, and several common errors compromise turf health. A frequent mistake is operating a mower with dull blades, which tear and shred the grass instead of providing a clean cut. This tearing leaves frayed edges that lose moisture rapidly and create an entry point for fungal pathogens, resulting in a stressed appearance.
Another serious error is scalping, which involves cutting too short and unevenly exposing the brown stems and stolons. Scalping damages the living growth points and leaves sensitive runners exposed to intense sunlight, leading to significant dieback and large brown patches. This is particularly damaging on uneven terrain where the mower deck dips suddenly, causing the blades to cut far lower than intended.
Finally, managing grass clippings improperly can negate the benefits of a correct cutting height. Leaving the clippings on the lawn (mulching) is generally beneficial as they decompose quickly and return nutrients and organic matter to the soil. However, if the grass is excessively long and clippings are heavy or clumpy, they must be removed to prevent smothering the turf or spreading disease.