Torpedo Grass (Panicum repens) is a tenacious and aggressive invasive weed found in warm-season turfgrasses across the southern United States. Its presence in a St. Augustine lawn creates a difficult management problem, often called the grass-in-grass dilemma, because the weed is closely related to the desired turf. Effective control requires a precise, multi-step strategy combining cultural practices with highly targeted chemical applications. The goal is to eradicate the Torpedo Grass without harming the sensitive St. Augustine turf.
Identifying Torpedo Grass and Its Persistence
Accurate identification is the first step toward successful management. Torpedo Grass has stiff, rigid leaves with a sharp, pointed tip and a waxy, silver-green color, contrasting with St. Augustine grass. It tends to grow upright and rapidly, often standing taller than the surrounding turf.
The difficulty in controlling Torpedo Grass lies in its extensive network of rhizomes, which are woody, robust, and sharply pointed underground stems. These rhizomes give the weed its “torpedo” name, allowing it to penetrate compacted soil and spread aggressively. They can grow up to 20 feet long and are packed with carbohydrate reserves, enabling the plant to regenerate quickly after foliage is damaged or removed. This structure makes simply pulling the weed or applying a basic herbicide ineffective, as the rhizomes quickly sprout new shoots.
Manual and Cultural Control Strategies
Non-chemical methods can slow the spread of Torpedo Grass, but they rarely achieve complete eradication. Hand-pulling is viable only for small, isolated patches and requires meticulous removal of every rhizome fragment. If the entire rhizome is not extracted, the remaining portion quickly generates a new plant, spreading the infestation. Tilling the soil is strongly discouraged because it fragments the rhizomes and stimulates widespread new growth.
Cultural practices that promote a dense, healthy St. Augustine lawn help the turf outcompete the weed. St. Augustine should be maintained at its highest recommended mowing height (three to four inches) to create dense shade at the soil level. This shading limits sunlight reaching the weed’s new shoots, weakening their growth. Additionally, avoiding overwatering and ensuring deep, infrequent irrigation favors the established St. Augustine root system. This creates a less hospitable environment for Torpedo Grass, which prefers moist, poorly drained soils.
Solarization is an option for heavily infested areas that can be sacrificed, such as a small, isolated section of the lawn. This process involves covering the area with clear plastic sheeting during the hottest months to superheat the soil. While solarization can kill the rhizomes, it will also kill all the desirable St. Augustine grass underneath. Cultural methods alone cannot eliminate an established infestation, but they are a necessary component of a long-term management plan to minimize re-infestation.
Selective Herbicide Options for St. Augustine Grass
The most significant hurdle in chemical control is the lack of a selective herbicide that kills Torpedo Grass without damaging St. Augustine turf. Herbicides effective against Torpedo Grass, such as those containing Quinclorac, are not labeled for St. Augustine and will cause injury or death. Atrazine, a common herbicide used in St. Augustine lawns, offers temporary suppression but is not potent enough to kill established rhizomes.
For established infestations, the only consistently effective chemical approach is highly targeted spot-treatment using a non-selective herbicide, most commonly glyphosate. This method accepts that the St. Augustine turf in the treated area will be killed along with the weed. Application must be precise, often done by carefully painting or wiping the herbicide solution directly onto the Torpedo Grass foliage to minimize damage to the surrounding St. Augustine.
Multiple applications are necessary because the systemic herbicide must travel from the leaves down to the extensive rhizome network to deplete the plant’s carbohydrate reserves. The best timing for application is when the Torpedo Grass is actively growing, typically in late spring or summer, and the soil has adequate moisture. Avoid applying the product when the St. Augustine is under drought or heat stress, as this makes the turf more vulnerable to accidental contact. Homeowners should treat the area, wait two to three weeks for decline, and then apply a second, targeted treatment to any regrowth.
Post-Treatment Recovery and Prevention
Once the Torpedo Grass has been successfully treated and begins to die back, the focus shifts to the recovery of the St. Augustine lawn. The areas where the weed and surrounding turf were killed become bare spots vulnerable to re-invasion by weed seeds or remaining rhizome fragments. These bare patches should be filled promptly by plugging with healthy St. Augustine sod or installing new sod sections to quickly re-establish a dense canopy.
Long-term prevention relies on maintaining an environment where St. Augustine grass thrives and can outcompete any potential regrowth. This involves following a consistent fertilization schedule, which promotes vigorous growth and density. Aeration can help reduce soil compaction and improve St. Augustine root health, but it should be done carefully to avoid bringing dormant rhizome fragments to the surface. Continuous monitoring is necessary, especially in treated areas, to catch any new shoots immediately. Addressing new growth with immediate hand-pulling or a spot treatment prevents the weed from re-establishing its dense underground network.