Grassy weeds are among the most difficult to control in the landscape. Torpedograss is a scourge around the state, and common bermudagrass is a problem no matter where you go. Grassy weeds are a problem in both lawns and beds.
Torpedograss (Panicum repens) is among the top grasses I get questions on. Even if introduced into a small area, this weed can rapidly spread to become a major problem.
The name comes from the hard, sharp point on the rhizome that runs horizontally underground, like a torpedo going through the water. The rhizomes can travel a foot or more deep, and the hard points are able to punch through landscape fabric and weed barriers.
Native to Africa and/or Asia, it was introduced to the United States before 1876, primarily through seed used for forage crops. But what really did it was when the U. S. Department of Agriculture imported and distributed torpedograss seed in the early 1900s for planting in pasturelands. This was to provide forage for cattle. Ironically, it proved inferior for use as a forage crop.
Now it is found in the Gulf South from Florida to Texas and along tropical and subtropical coasts around the world.
Managing grassy weeds
In most situations, managing torpedograss and other grassy weeds can only be accomplished with diligent, repeated, frequent efforts. That means monitoring the situation frequently and promptly taking action anytime torpedograss is seen growing in an area.
Landscape fabric or weed barriers are generally not effective when it comes to running, perennial grassy weeds like torpedograss and bermudagrass.
Where you can just apply an herbicide to the foliage of the grassy weed, apply glyphosate at the highest label rate regularly as needed (Roundup, Killzall, Grass and Weed Killer, and other brands). This can be used near and around desirable ornamentals as long as you don’t get it on their foliage. Protect nearby plants by shielding them or cover them with plastic bags. Glyphosate is likely the best herbicide to kill grassy weeds, but be prepared to do follow- up treatments if new shoots appear.
Where you cannot just spray the foliage of the torpedograss, use a selective grass killer. These products can be sprayed on ornamental plants and torpedograss or other grassy weeds, and they just hurt or kill the grass, not the ornamentals. The herbicide fluazifop (Ferti-lome Over the Top, Ortho Grass B Gon, Fusilade, Ornamec and other brands) has a bit better activity on perennial grasses like torpedograss than the herbicide sethoxydim (Vantage, Hi-Yield Grass Killer, Poast). But both are useful. Make sure the ornamentals in the bed are listed on the label as tolerant. If they do not appear on the label, there is a chance they might be damaged.
Do this as needed, following label directions carefully. These herbicides will suppress torpedograss and kill most other grassy weeds.
In centipedegrass lawns, you can use the herbicide sethoxydim (Vantage or Poast) to suppress torpedograss and kill grassy weeds. It does not hurt centipedegrass if applied as directed. Repeated applications – at least three – through the summer will keep torpedograss suppressed – but not eradicated. If you ever stop spraying, it will come back.
In bermudagrass and zoysia lawns, several applications of the herbicide quinclorac, such as Drive (this is a commercial product) or Image Crabgrass Killer (homeowner version), will do a good job of actually killing the torpedograss with multiple applications. And they do a very good job on other grassy weeds.
No herbicides can selectively control torpedograss, bermudagrass and most other grassy weed in St. Augustinegrass. You can kill patches of grassy weeds that grow in summer with glyphosate (keep this off the desirable grass as much as possible). When the grassy weed is brown, remove it and patch the damage with a piece of sod. Doing this repeatedly over the years can maintain a lawn that primarily contains the desirable grass.
You also can use the “nuclear option.” Centipedegrass lawns or St. Augustine lawns severely infested with torpedograss or common Bermuda may need total renovation. This requires spraying the lawn area with a high concentration of glyphosate, with the goal of killing off everything and starting over with a new lawn. Sometimes it takes two applications to get torpedograss killed off.
If you decide to do this when torpedograss is the issue, consider installing zoysia or bermudagrass. Switching to zoysia or bermudagrass will allow the use of quinclorac, one of the more effective herbicides for managing torpedograss. But it is too damaging to be used on centipedegrass or St. Augustinegrass.
Renovation and switching to bermudagrass or zoysia are absolutely the last resort and definitely not the cheapest route to travel. But it may be the most effective way to manage severe torpedograss problems in a lawn.
Managing tough grassy weeds in the landscape takes persistent, repeated effort over the long term. There are no quick fixes or one-time applications that will properly deal with these weeds.
Dan Gill is a horticulturist with the LSU AgCenter and is known as a reliable source of helpful, useful advice on lawn and garden topics. He can be reached at DGill@agcenter.lsu.edu