St. Augustine grass (Stenotaphrum secundatum) is a popular warm-season turfgrass in the southern United States and other subtropical regions. Homeowners favor this grass for its deep, blue-green color and its tolerance to heat and partial shade. Despite its widespread use, many people question if this plant is a weed, especially when it spreads aggressively. The confusion stems from the fact that a plant’s designation as a weed is based on context, not biological classification.
Defining Turfgrass Versus Weed
Botanically, St. Augustine grass is a true grass, belonging to the Poaceae family. This is the same scientific group as wheat, corn, and other turf species. In its intended area, it is a desirable turfgrass that forms a thick, carpet-like mat. The definition of a “weed” is functional, describing any plant growing where it is unwanted.
St. Augustine grass becomes a weed when it invades areas outside the lawn boundary. This includes vegetable gardens, flowerbeds, or adjacent lawns planted with different grass species like Zoysia or Bermuda grass. When it creeps into planted beds or over sidewalks, it acts as a functional weed. It competes with desirable landscape plants for resources like sunlight and water.
Characteristics That Lead to Aggression
The reputation St. Augustine grass has for aggression is linked to its unique method of horizontal growth. This species spreads exclusively via above-ground runners known as stolons. These thick, whitish stems creep along the soil surface and feature nodes that produce both roots and new grass blades. This allows the plant to rapidly colonize new territory.
This growth pattern contrasts with grasses that spread underground using rhizomes, such as Bermuda grass. Because the stolons are entirely above-ground, they quickly grow across bare soil or into the edges of landscape borders. During the peak growing season, these runners can extend several inches per week. This enables the grass to quickly fill in thin areas and overwhelm less vigorous species.
Strategies for Controlling Unwanted Growth
Managing St. Augustine grass acting as a weed requires focused, non-broadcast methods. It is difficult to spray an herbicide without harming the desirable lawn nearby. The most common and effective strategy for containing the spread is mechanical removal. Regular edging along sidewalks, driveways, and garden beds severs the creeping stolons before they can establish roots.
In flowerbeds, the primary recommendation is to hand-pull the invading stolons as soon as they are observed. Although labor-intensive, this prevents the runners from rooting and forming a dense mat. Physical barriers, such as deep-set plastic or metal edging installed six inches into the soil, can significantly slow its advance. For non-turf areas like patios or gravel walkways, targeted application of a non-selective herbicide containing glyphosate can be used.
This must be applied carefully to the leaves of the unwanted grass, avoiding drift onto any desirable plants. When St. Augustine grass invades a lawn of a different turf type, selective herbicides cannot be used. These products are formulated to kill grassy weeds and would damage the neighboring grass.