St. Augustine grass, a warm-season turfgrass, is a popular choice for lawns across the southern United States. Its health and appearance depend heavily on proper maintenance, with the correct mowing height being the single most important factor. Cutting this grass too short is a common mistake that can lead to severe stress and long-term damage. Understanding the specific height requirements for St. Augustine is the first step toward maintaining a dense, resilient lawn.
The Recommended Mowing Height Range
The absolute shortest St. Augustine grass should be cut is 2.5 inches, but this height places the plant under considerable stress. The standard, ideal height range for a healthy St. Augustine lawn is between 2.5 and 4 inches. Experts advise maintaining the grass at the upper end of this range, typically 3 to 4 inches, to promote optimal health. A height below 2.5 inches should be avoided entirely, as it severely compromises the plant’s ability to thrive, especially during periods of high heat or drought.
The higher range is recommended because it allows the plant to develop a deeper, more robust root system and provides better insulation for the soil. Setting the mower to its highest or second-highest setting ensures the cut is consistently within this beneficial range. While some cultivars may tolerate a slightly lower cut, aiming for the 3.5 to 4-inch mark is the most reliable strategy for a resilient lawn.
Physiological Reasons for Higher Mowing
The length of the leaf blade, or shoot, has a direct relationship with the depth and density of the root system. Taller grass blades allow for a greater surface area exposed to sunlight, maximizing the rate of photosynthesis. This increased energy production fuels the development of deep, extensive roots, which are necessary to access water and nutrients during dry conditions.
When the grass is cut too low, the plant diverts stored energy from its roots to rapidly regrow its leaves, a process known as compensatory growth. This constant resource reallocation results in a shallow, vulnerable root system that is less tolerant of environmental stresses like heat, drought, and foot traffic. St. Augustine spreads via above-ground runners called stolons, which are easily damaged or exposed to harsh sunlight when the protective leaf canopy is removed by low mowing. Protecting the stolons is essential for the grass’s lateral spread and recovery.
The One-Third Rule and Preventing Scalping
The frequency of mowing is governed by the “One-Third Rule.” This principle dictates that you should never remove more than one-third of the grass blade’s total height in a single mowing session. For instance, if you maintain a desired height of 3 inches, you should mow before the grass exceeds 4.5 inches tall. Adhering to this rule minimizes the shock placed on the grass plant, allowing it to recover quickly.
The most damaging consequence of cutting too low or removing too much tissue is “scalping,” which exposes the underlying, yellowish stolons and the soil beneath. Scalping not only leaves the lawn with an unsightly, brown appearance but also stresses the grass, making it highly susceptible to weed invasion, disease, and desiccation from the sun. Following the one-third rule ensures that mowing frequency, which may mean weekly cuts during peak growing season, is correctly balanced with the chosen height.
Situational Adjustments to Cutting Height
The ideal mowing height is not static and must be adjusted to accommodate specific environmental conditions. In areas of moderate to dense shade, the cutting height should be raised to 4 inches or higher. Taller blades provide a larger surface area to capture the limited available sunlight, ensuring the plant can still generate enough energy through photosynthesis. This adjustment is important because St. Augustine struggles to thrive under low light.
During periods of high heat, drought, or other environmental stress, raising the cutting height acts as a protective measure. A taller grass canopy provides natural insulation for the soil, which helps to keep the roots cooler and significantly reduces the evaporation of soil moisture. A dense, taller canopy also serves as a form of natural weed control, as the shading effect blocks the sunlight necessary for many weed seeds to germinate and establish themselves within the lawn.