Torpedograss (Panicum repens) is one of the hardest weeds to eliminate from a St. Augustine lawn because both grasses belong to the same botanical family, and nearly every herbicide that harms torpedograss also damages or kills St. Augustine. There is no single product you can spray across your lawn that will selectively wipe out torpedograss while leaving St. Augustine unscathed. Getting rid of it requires a combination of physical removal, carefully targeted chemical treatments, aggressive cultural practices, and patience measured in growing seasons rather than weeks.
What Makes Torpedograss So Difficult
Torpedograss spreads through an underground network of thick, white rhizomes that can extend several feet deep in sandy soils. It also sends out aboveground stolons, giving it two independent routes of invasion. Even a small fragment of rhizome left behind after pulling or digging can regenerate into a new plant. That resilience is why torpedograss consistently ranks among the worst invasive grasses in warm-climate turf systems, and it is a particular menace in Florida, the Gulf Coast, and other regions where St. Augustine is the dominant lawn grass.
The real frustration, though, is chemical. Most grassy-weed herbicides work by targeting metabolic pathways shared by all grasses. They can distinguish between broadleaf weeds and grasses easily, but telling one warm-season grass from another is a much harder trick. Torpedograss and St. Augustine are close enough biologically that the handful of herbicides capable of suppressing torpedograss tend to injure St. Augustine as well. That leaves homeowners in a bind: you cannot simply broadcast a selective herbicide the way you would treat crabgrass in a cool-season lawn.
Pulling and Digging by Hand
For small patches, physical removal is the first line of defense and sometimes the only realistic option. The goal is to remove as much of the rhizome system as possible, because any piece left underground will sprout again. Use a garden fork or flat shovel to pry up the soil beneath the visible torpedograss, going at least six to eight inches deep. In sandy soils you can often trace the white rhizomes by hand and pull them out in long sections. In clay or compacted soil, this is harder, and you may need to remove a block of sod and soil together.
Hand removal works best when torpedograss has only colonized a small area, perhaps a few square feet. If you notice torpedo grass invading an edge of your yard, pulling it immediately and monitoring for regrowth over the next month is far more effective than waiting until it has spread across a large swath. Check back every two weeks and pull any new shoots as soon as they appear. Persistence matters more than technique: the rhizome stores enough energy to push up new growth several times, and each time you remove the top growth you deplete that energy reserve a little further.
Targeted Chemical Treatments
Because no selective herbicide can be safely broadcast over St. Augustine to kill torpedograss, chemical control comes down to precision. The most common approach is a spot application of glyphosate, a non-selective herbicide that kills any plant it contacts. You paint or dab glyphosate directly onto the torpedograss foliage, accepting that you will kill a small circle of St. Augustine around the weed as collateral damage. You then resod or plug that bare spot with fresh St. Augustine afterward.
Wipe-on applicators can make this kind of targeted work safer. Research on herbicide wiping devices has shown that applying herbicide by contact rather than spraying eliminates the risk of drift onto surrounding turf, which is the main danger with spot-spraying glyphosate near desirable grass.1New Zealand Plant Protection. Assessment of two herbicide wiping devices You can buy or improvise a wick applicator: a sponge or rope wick soaked in glyphosate solution, mounted on a handle, that you press directly against the torpedograss blades. Because the herbicide never becomes airborne, you reduce the chance of accidentally killing a wider area of your St. Augustine.
For this method to work, the torpedograss needs to be taller than the surrounding St. Augustine so you can wipe the torpedograss without the applicator touching the lawn. Letting the torpedograss grow up above mowing height before treating, or mowing the St. Augustine short and leaving the torpedograss alone for a week or two, creates the height differential you need.
What About Grass-Selective Herbicides?
Herbicides like sethoxydim and clethodim are sometimes discussed online as torpedograss treatments. These products are indeed effective at suppressing torpedograss in certain turf types. In research on centipedegrass, repeated applications of sethoxydim and clethodim reduced torpedograss cover by roughly 84 to 87 percent, though regrowth still occurred within weeks after the final treatment, and researchers concluded that multiple years of sequential applications might be needed.2Weed Technology. Selective Postemergence Herbicide Control of Torpedograss in Centipedegrass The problem for St. Augustine owners is that these herbicides are not labeled for use on St. Augustine and can cause serious injury to it. What works in centipedegrass does not automatically translate to your lawn.
Pinoxaden, another grass herbicide, has been studied on St. Augustine and produces a more complicated picture. Research found that St. Augustine cultivars respond differently to pinoxaden: Floratam and Palmetto tolerated it better, while Classic and Raleigh were more susceptible to damage. The same study found that combining pinoxaden with certain fungicides (chlorothalonil or propiconazole) had a safening effect, reducing injury regardless of cultivar.3Crop, Forage & Turfgrass Management. Response of St. Augustinegrass cultivars to pinoxaden alone and in combination with select fungicides This line of research is promising but not yet at the point where a homeowner can buy a product off the shelf and spray it with confidence. As of now, no grass-selective herbicide carries a label specifically approving its use for torpedograss control in St. Augustine lawns.
Quinclorac and Other Options
Quinclorac is another herbicide that gets mentioned in torpedograss discussions. It is labeled for use on St. Augustine and is primarily used for crabgrass and other weedy grasses. In torpedograss research, quinclorac has provided some suppression but generally performed worse than sethoxydim or clethodim at reducing torpedograss cover.2Weed Technology. Selective Postemergence Herbicide Control of Torpedograss in Centipedegrass It may slow torpedograss growth without eradicating it, and because it is labeled for St. Augustine, some lawn care professionals include it in a broader management plan. On its own, it is unlikely to solve a serious torpedograss infestation.
Cultural Practices That Tilt the Balance
Healthy, dense St. Augustine competes more effectively against torpedograss than thin or stressed turf. While cultural practices alone will not eradicate an established invasion, they are a critical part of any long-term strategy. Torpedograss exploits thin spots and weak turf, so everything you do to thicken your St. Augustine helps slow the invasion.
Mowing height is one of the simplest levers you have. St. Augustine performs best when mowed at 3.5 to 4 inches, depending on cultivar. At that height, the canopy shades the soil surface and makes it harder for torpedograss seedlings and stolons to establish. Scalping the lawn or mowing too low opens up light gaps that torpedograss exploits quickly.
Fertilization matters as well, though the relationship is not as simple as “more nitrogen equals less torpedograss.” A well-fed St. Augustine lawn grows more aggressively and fills bare spots faster, which helps outcompete torpedograss. But over-fertilizing, especially with nitrogen in summer, can also encourage the torpedograss if it is already present, because torpedograss responds vigorously to nitrogen too. The practical advice is to follow your local extension service’s recommended fertility schedule for your cultivar and avoid dumping extra fertilizer on areas where torpedograss is active, thinking it will help the St. Augustine outgrow the weed.
Irrigation plays a supporting role. Torpedograss tolerates drought better than many turfgrasses and thrives in wet conditions. Overwatering does not directly cause torpedograss, but chronically soggy soil or poor drainage creates conditions where torpedograss rhizomes spread more easily. Water deeply and infrequently rather than running sprinklers daily, and address any drainage issues in your yard.
The Full Renovation Approach
When torpedograss has spread through a large portion of your lawn, the patchwork approach of spot-killing and re-plugging becomes impractical. At some point, a full renovation may be more efficient. This means killing everything in the affected area with a non-selective herbicide like glyphosate, waiting for the torpedograss to regrow from surviving rhizomes, treating again, and potentially repeating a third time before resodding with fresh St. Augustine.
A single application of glyphosate will not kill the deep rhizome system of established torpedograss. Plan on at least two treatments spaced four to six weeks apart, allowing the rhizomes to push up new growth between applications. Some professionals recommend three rounds. In warm months, torpedograss regrows faster, so you will see new shoots sooner and can retreat more quickly. This cycle of kill, wait for regrowth, and kill again is tedious but is the most reliable way to exhaust the rhizome reserves.
After the final treatment, wait at least two weeks to confirm no new torpedograss is emerging before laying new sod. If you resod too soon and surviving rhizomes push through, you are back where you started but with the added expense of new turf. Some homeowners also lay a physical barrier, such as a root barrier membrane installed vertically along the property edge, to prevent torpedograss from reinvading from a neighbor’s yard or an adjacent ditch.
Solarization as an Alternative to Herbicides
If you prefer to avoid herbicides entirely, soil solarization can kill torpedograss in targeted areas during the hottest months. You mow the area as short as possible, water it deeply, and then cover it tightly with clear plastic sheeting, burying the edges to seal in heat. Over four to six weeks of full summer sun, soil temperatures under the plastic can exceed 140°F in the top few inches, killing rhizomes near the surface.
Solarization has real limitations with torpedograss. Deep rhizomes, especially in sandy soils where they can grow more than a foot down, may survive the heat. The method works best on small areas where you can ensure the plastic stays sealed and receives uninterrupted sunlight. Shady sections of your yard will not get hot enough. Still, combined with repeated hand-pulling of any regrowth after removing the plastic, solarization can reduce a torpedograss patch without introducing chemicals into your soil.
Where Torpedograss Comes From in the First Place
Understanding how torpedograss got into your lawn helps prevent it from coming back. The two most common entry points are contaminated sod and underground spread from adjacent properties. Torpedograss rhizomes are commonly found in sod fields, and if even a small fragment gets mixed into a pallet of St. Augustine sod, it will establish itself in your lawn within a season. When purchasing sod, buy from a reputable grower and inspect it for any grass that looks different from the St. Augustine, particularly anything with stiffer, more upright blades and a distinctly lighter green color.
The other common pathway is lateral spread. Torpedograss rhizomes can travel underground from a neighboring property, a drainage ditch, a canal bank, or a natural area bordering your lot. In parts of Florida and the Gulf Coast, torpedograss is so widespread in disturbed soils and waterways that preventing underground invasion entirely is unrealistic without physical root barriers. If you know torpedograss is present on adjacent land, monitoring the edges of your lawn regularly and treating new growth immediately gives you the best chance of containing it.
How to Tell Torpedograss from Similar Weeds
Before you invest significant time and money in treatment, make sure you are actually dealing with torpedograss and not a look-alike. Several grassy weeds invade St. Augustine lawns and can be confused with torpedograss at a glance.
- Bermudagrass: Also spreads by stolons and rhizomes, but bermudagrass has much finer leaf blades and a lower growth habit. It forms dense, wiry mats rather than the upright, coarser patches typical of torpedograss.
- Bahiagrass: Produces distinctive V-shaped seed heads on tall stalks. Its leaf blades are wider and coarser than torpedograss, and it does not have the aggressive underground rhizome network that makes torpedograss so difficult.
- Crabgrass: An annual that dies in winter. If the mystery grass disappears after the first frost, it is not torpedograss. Crabgrass also roots at nodes along its stems but does not have the thick, white rhizomes that torpedograss produces underground.
Torpedograss has a few distinctive features. Its rhizomes are thick, white, and sharply pointed at the tips (hence the name “torpedo”). The leaf blades are stiff with a prominent midrib, and the plant often has a blue-green tint compared to the darker green of St. Augustine. If you pull up a section and find those characteristic white, pointed rhizomes, you can be fairly confident in your identification. Your county extension office can also confirm the ID if you bring in a sample.
Realistic Expectations and Timelines
Torpedograss is not a weed you eliminate in a single weekend project. Even with the most aggressive chemical approach, you are looking at a minimum of two to three months to exhaust the rhizome system in a renovation scenario, plus several more months for new St. Augustine sod to fully establish. For the spot-treatment-and-replug approach, you may be managing and retreating individual patches over the course of a year or more.
Research consistently shows that torpedograss regrows within weeks of any single herbicide application, regardless of the product used.2Weed Technology. Selective Postemergence Herbicide Control of Torpedograss in Centipedegrass Each successive treatment weakens the rhizome reserves further, but expecting a one-and-done solution will leave you frustrated. Set your expectations around a multi-season management plan, and view each treatment cycle as incremental progress rather than a failed attempt.
The silver lining is that torpedograss is not impossible to control if you are consistent. Homeowners who combine physical removal, targeted chemical application, and strong cultural practices to keep their St. Augustine competitive can reduce torpedograss to a manageable nuisance within a year or two. The ones who lose the battle are usually those who try one approach, see regrowth, and give up rather than retreating on schedule. Torpedograss punishes inconsistency more than anything else.