- Torpedograss Weeds: Tips On Torpedograss Control
- Torpedograss Identification
- How to Get Rid of Torpedograss in Beds
- Eliminating Torpedograss in the Lawn
- How to Kill Torpedo Grass
- Texas Invasive Species Institute
- Native Origin
- Current Location
- Torpedograss – one of the most difficult weeds to control
- Torpedograss invading St. Augustine grass
Torpedograss Weeds: Tips On Torpedograss Control
Torpedograss (Panicum repens) is native to Asia and Africa and was introduced to North America as a forage crop. Now torpedograss weeds are among the most common and annoying pest plants here. It is a persistent plant that pierces soil with pointed rhizomes that grow a foot or more into the earth. Eliminating torpedograss in the lawn is a tricky business, requiring tenacity and usually multiple chemical applications. The weed is nearly indestructible and has been known to come out through weed barrier fabric.
The methods on how to get rid of torpedograss do not encompass selective herbicides or mechanical measures. This is bad news for those of us who prefer not to use chemicals on our landscape. You could just leave the stuff alone but it would first take over your lawn and then move to the garden beds.
Torpedograss weeds spread by their numerous seed but also from even tiny fragments of rhizome. This makes for a formidable foe and indicates the necessity of herbicide use as the primary torpedograss control.
The first step in any weed control is to correctly identify it. Torpedograss is a perennial that can grow up to 2.5 feet in height. It produces stiff stems with thick, rigid, flat or folded leaf blades. Stems are smooth but the leaves and sheaths are hairy. The color is grayish green. The inflorescence is a vertical loose panicle, 3 to 9 inches long.
This annoying plant can flower all year long. The rhizomes are a key to torpedograss identification. They stab down into soil with pointed tips that spear soil and grow deeply. Any part of the rhizome that remains in soil will resprout and produce new plants.
How to Get Rid of Torpedograss in Beds
Torpedograss control is nothing to jest about due to its difficulty and general unpredictability. As mentioned, weed barriers have little effect on the plant and hand pulling can leave behind rhizomes, causing more problems later.
There have been some studies showing burning as being effective but this is only in conjunction with herbicide use. In garden beds, use glyphosate applied directly to the weed. Do not get any of this non-selective chemical on your ornamental plants.
You may have to repeat again to ensure complete torpedograss control. You can also try a selective herbicide like fluazifop or sethoxydim. Repeated applications are again recommended. Both the latter chemicals will suppress torpedograss but likely not kill it.
Eliminating Torpedograss in the Lawn
The type of chemical you use in grass infestations will depend upon the species of grass growing in your lawn. Not all herbicides are safe on all types of sod. Kill patches of torpedograss in the lawn with glyphosate. It will take out a bit of the turf but you can remove the dead vegetation and reseed.
A kinder, gentler method in Bermuda grass or zoysia grass is to use a formula with quinclorac. In centipede turf, use sethoxydim. This will kill the torpedograss but not damage the lawn. Many other lawns have no recommended selective herbicide.
jhurst – posted 20 May 2001 14:25
Any controls for torpedo grass in tifway 419 bermuda? Right now I’m having to spray with RoundUp and resod.
wdrake – posted 20 May 2001 17:50
Best action would be to move!However, UF’s Lawn Handbook says to use Asulam which is sold as Asulox®
seed – posted 22 May 2001 02:08
jhurst, the selective control for torpedograss in bermudagrass is Drive 75DF by BASF. Asulam (Axulox) will have little or no effect.
Drive 75DF can only be applied at 2 pounds per acre per year, and that’s not enough to to remove torpedograss in one year. Therefore, it should be applied in small increments, e.g., three applications of 2/3 pounds per acre, which causes a slow kill.
It may take three or years of serial treatments to get rid of torpedograss.
kellyis1 – posted 19 April 2002 13:30
Is there a selective control for torpedo grass that can be used in centipede lawns?
sam – posted 20 April 2002 09:14
quote:Originally posted by kellyis1:Is there a selective control for torpedo grass that can be used in centipede lawns?
You can use Vantage to control torpedo grass in centipede. I,ve used it with good success. You will need to reapply to keep torpedo grass under control.
lil_pupcyclist – posted 18 July 2005 10:46
I have tried that and it came back the next year.
quote:Originally posted by jhurst:Any controls for torpedo grass in tifway 419 bermuda? Right now I’m having to spray with RoundUp and resod.
Empire1 – posted 21 July 2005 19:19
I have just purchased the Drive 75 for torpedo grass in zoysia, central Fl. I am going to do three treatments every two weeks at a reduced rate. I need to find some methylated seed oil to mix with it.I found lesco spreader sticker but I don’t wan’t to buy a gallon. Any ideas where to find small quantities of a adjuvant??I am only spot treating with a sprayer.
lawngrower – posted 16 August 2005 17:28
what can i use to kill torpedo grass in my centipede lawn without killing my centipede?
bot – posted 11 April 2009 01:50
The only way to get rid of torpedo grass in flower beds is to carefully dig very deep around the grass without injurying it. Then flood the grass and the area around it with a grass and weed killer and leave it exposed for a week or two before replacing the soil. It is a very tough weed and grows underground in extensive networks.
PensacolaPat – posted 27 May 2009 22:07
I have some areas with Torpedo Grass in my St Augustine lawn and also in a flower bed. Ican kill it with MSMA but it kills the desirables as well.
Last year after pulling and digging a lot, I sprayed a circle with cooking oil. In the heat of the summer it killed the rhizomes. I am planning on trying the cooking oil with a jersey glove this year just without the pulling.
BigOilMan – posted 23 July 2011 15:01
Pensacola Pat, I realize this is an old thread but how did the cooking oil treatment work out. I’ve got torpedo grass taking over my backyard (centipede) and the flower beds. Just bought some Vantage. We’ll see how it does.
- Will Quinclorac 1.5L work to kill torpedograss without killing the wanted Centipede grass?
- Will Celsius WG Herbicide kill torpedo grass?
- Is QuinKill Max Crabgrass and Weed Killer safe for zoysia grass?
- Will Bonide Grass Beater Over-The Top Grass Killer Concentrate kill torpedograss in my ornamental beds?
- How often can I safely apply Hi-Yield Grass Killer Post-Emergent Herbicide to my Centipede lawn to control torpedo grass?
- Will Dismiss NXT Herbicide kill torpedograss?
- Can I use Solitare WSL to eliminate torpedograss from my centipede grass?
- Will Dismiss NXT control torpedograss in St. Augustine?
- Can I use Image Kills Nutsedge Concentrate to kill Torpedo grass?
- What grassy weeds will QuinKill Max Crabgrass and Weed Killer control?
- Will Pastora kill torpedo grass in a Bermuda grass pasture?
- Will Opti Amine 2-4-d Herbicide kill Spatterdock and Tussock Torpedograss?
- Can Fusilade II be used in a centipede lawn to control Bermuda grass and torpedo grass and not harm centipede?
- Can Grass Out Max be used on a centipede lawn to control bermuda and torpedograss without harming the centipede?
- Can Dismiss NXT Herbicide be used to kill Torpedograss in St.Augustine?
- Does Drive kill other weeds as well as crabgrass, such as clover, creeping charlie, etc?
- Will Quali-Pro MSM Turf Herbicide kill Torpedograss without killing a Centipede lawn? What product would take of the torpedograss without killing the lawn?
- Does Garlon 4 Ultra Herbicide kill Torpedo grass (Florida)?
- Will Solitare kill Torpedo grass in Centipede lawns?
- How effective is QuinKill Max Crabgrass and Weed Killer on Torpedo grass and how often can I apply? Does heat and humidity affect the result?
How do I get rid of torpedo grass in my lawn? -Sherry
They don’t call torpedo grass (Panicum repens) “creeping panic” for nothing, since it’s very difficult to control once it finds its way into your yard! Most weed killers barely slow torpedo grass down, and pulling or digging only makes it grow faster. Unfortunately, there’s no good way to kill torpedo grass without killing your lawn grass, too.
I wish I had an easy solution for you, but when you want to get rid of torpedo grass, you’ve got to pull out the big guns. Here are some ideas to try:
- Glyphosphate: (such as Roundup) is considered the most effective herbicide against torpedo grass, though it kills lawn grass and other plants, too.
- Imazapyr: (such as Ortho Ground Clear) is also effective on torpedo grass, along with all other plants, but has a residual effect in the soil that can harm trees and shrubs.
- Keep at It: Any herbicide treatment will likely need to be applied more than once.
- Target Weeds: Use a plastic bottle to minimize overspray onto your lawn.
- Protect Waterways: Torpedo grass often grows near and in water. Never use herbicides if the spray can come into contact with waterways.
Torpedo grass with Bermuda
Other Control Measures
- Solarization: If you are clearing a larger area of lawn, you can solarize it by covering with clear plastic for a month or two during the summer. The sun will bake everything underneath, leaving you with (hopefully) a clean slate in the fall.
- Burning: Researchers at the South Florida Water Management Division have successfully managed torpedo grass by burning it during the winter, then spraying it with herbicide as soon as it starts to sprout again.
- Pulling and Digging: Manually removing torpedo grass isn’t very effective, since the bits of broken plant spread and sprout anew!
Torpedo grass is a marshy plant that tends to invade areas with poor drainage or that have recently been disturbed (such as by a controlled burn, tilling, or grading). You can help discourage infestations of torpedo grass by:
- Improving drainage and soil quality in your yard to keep the existing lawn grass healthy.
- Replacing disturbed soil with sod, lawn grasses, or naturalized beds before weeds have time to invade.
- Torpedograss (University of Florida)
- Torpedo Grass (USDA Plants Profile)
- How to Control Weeds in Your Lawn (article)
- Lawn Weed Control (video)
How to Kill Torpedo Grass
grass image by green308 from Fotolia.com
Torpedo grass (also known as bullet grass) is a grass variety found throughout coastal, sandy-soil areas of the United States and other regions worldwide. Torpedo grass can grow up to a height of 3 feet, and because it grows rapidly, it is not a desirable option for landscaping your home lawn or garden. Eliminating torpedo grass from your lawn is simple and requires but a few weeks time and some basic landscaping skills and inexpensive materials.
Cut torpedo grass with a lawn mower if patches are larger than about 6 square feet.
Chop up the soil with a shovel or hoe and remove as much of the torpedo grass and underlying root structure as possible.
Spread old newspaper or a plastic dropcloth at least 1 mil thick over remaining torpedo grass patches in your lawn or garden. Use bricks or other heavy, weather-resistant objects to hold the newspaper or plastic drop cloth in place. The heat of the sun will intensify under the covering while blocking sunlight and air in order to kill any budding torpedo grass.
Leave the covering in place for at least two weeks in the summer months and three to four weeks in the spring and fall.
Remove the plastic drop cloth or newspaper and spot check the area to determine whether new torpedo grass growth exists. Hand pull any remaining torpedo grass and continue to spot check the areas on a weekly basis.
If necessary, use a specially formulated herbicide, such as Drive 75 by BASF, to eradicate persistent torpedo grass. The formula comes in a plastic spray bottle with a trigger handle for applying to targeted areas on the lawn. This should be a last resort; herbicides can be dangerous to pets and wildlife.
Reseed the treated areas with a different grass species to discourage torpedo grass from coming back.
Texas Invasive Species Institute
Native Origin: Africa and Eurasia
Habitat: Torpedo grass can be found on marshy shores or disturbed areas such as canals and poorly drained soil. Torpedo grass has no tolerance for cold weather and will die off following a frost thus limiting the possible range to the southern United States.
U.S. Present: AL, CA, FL, HI, LA, MS, NC, SC, TX
Other common names for torpedo grass: Bullet grass, coastal bermuda grass, couch panicum, creeping panic, dogtooth grass, panic rampant, quack grass, and wainaku grass.
Preventative: Control can be accomplished by preventing the spread and fragmentation of rhizomes. This can be very difficult because if even a tiny fragment of rhizome is left in an area, it will reestablish itself. Control of infestations near waterways will prevent long-range spread via water and this should be a priority. If mowing or tillage is used, care must be taken to prevent transport of rhizome or stolon fragments.
Cultural: Weeds such as torpedo grass generally invade open or disturbed areas following a burn, clearing mowing, etc., so these areas are particularly vulnerable to invasion. Therefore, a healthy ecosystem with high species diversity will help to deter infestation.
Mechanical & Biological: There are limited agents being studied for biological control of torpedo grass, although Dr. Charudattan at the University of Florida has been evaluating a species of fungus. Torpedo grass is very palatable for cows and goats, and grazing may be integrated in an overall management scheme.
Google Search: Panicum repens
Google Images: Panicum repens
NatureServe Explorer: Panicum repens
USDA Plants: Panicum repens
Invasive Plant Atlas of the United States: Panicum repens
Bugwood Network Images: Panicum repens
Center for Aquatic and Invasive Plants. 2009. Panicum repens: Torpedo Grass. University of Florida, IFAS.
Hitchcock, A.S. 1950. Manual of the grasses of the United States. USDA Miscellaneous Publication No 200. Agricultural Research Administration, Washington, D.C. Pp. 697.
Torpedograss – one of the most difficult weeds to control
For Release On Or After 07/19/13
By Dan Gill
LSU AgCenter Horticulturist
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.
Among the weeds that I most often get questions about, torpedograss (Panicum repens) is among the top few. Torpedograss is a scourge in area lawns, flowerbeds and landscapes. Even if introduced into a small area, this weed can rapidly spread to become a major problem.
View full sizeKenneth Harrison / The Times-Picayune
The name comes from the hard, sharp point to 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.
Some legends have grown up around this weed. I often hear it said that the Corps of Engineers brought torpedograss in to stabilize the levees, and from there it spread into landscapes. This is actually not the case. Torpedograss was established in the southeastern United States long ago.
Native to Africa and Asia, it was introduced to the United States before 1876, primarily through seed used for forage crops. But torpedograss really became established in the early 1900s when the U.S. Department of Agriculture imported and distributed seed for planting in pasturelands 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.
So, how did this weedy grass find its way into the New Orleans area? The primary way is believed to have been the use of fill soil containing bits of torpedograss rhizomes. Torpedograss grows rampantly and lushly in the fertile, alluvial soil deposited by the Mississippi River in the Bonnet Carre spillway. This area has long been a source of spillway sand fill.
When I was extension horticulturist in Orleans Parish back in the early 1980s, there were no noticeable torpedograss problems. I did not hear about this weed until the mid- to late 1980s. The initial reports I remember were from the Chateau Estates area, where tons of spillway sand had been used for fill. Because of this, generally avoid using spillway sand when filling in your landscape.
Torpedograss is not difficult to eradicate: It is nearly impossible to eradicate.
So, dealing with torpedograss is often not so much a matter of how to get rid of it, but how to keep it from taking over. And that 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 not effective; I’ve seen torpedograss come up through thin asphalt.
Where you can apply an herbicide only to the torpedograss foliage, 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. Glyphosate is likely the best herbicide to kill torpedograss, but it will still need to be applied repeatedly as needed as new shoots appear.
Where you cannot limit spraying to the foliage of the torpedograss, use a selective grass killer. These products can be sprayed on the ornamentals and the torpedograss (or other grassy weeds), and it just hurts or kills the grass, not the ornamentals. Fluazifop (Ferti-lome Over the Top, Ortho Grass B Gon, Fusilade and other brands) has a little better activity on perennial grasses like torpedograss than 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 but not kill it.
In centipede lawns, you can use the herbicide sethoxydim (Vantage or Poast) to suppress torpedograss. It does not hurt centipedegrass if applied as directed. Repeated applications (at least three) through the summer will keep it suppressed — not eradicated. If you ever stop spraying, it will come back.
In bermudagrass and zoysiagrass lawns, several applications of 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.
There are no herbicides to selectively control torpedograss in other types of lawn grasses, such as St. Augustine grass. You can kill patches of torpedograss with glyphosate (keep this off the desirable grass as much as possible). When the torpedograss is brown, remove it and resod the spot with your lawn grass.
Doing this repeatedly over the years can maintain a lawn primarily of the desirable grass.
There is also the “nuclear option.” Centipedegrass or St. Augustinegrass lawns severely infested with torpedograss 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 the torpedograss killed off.
If you decide to do this, consider installing zoysiagrass or bermudagrass. Switching to zoysiagrass or bermudagrass will allow the use of quinclorac, a herbicide that is one of the more effective ones for managing torpedograss, but which is too damaging to be used on centipede or St. Augustine.
Renovation and switching to bermudagrass or zoysiagrass is absolutely the last resort and definitely not the cheapest route to travel, but it may be the most effective way to manage severe problems with torpedograss in the lawn.
Managing torpedograss in the landscape takes persistent, repeated, frequent effort over the long term. Even then you generally only control it, not eradicate it. Unfortunately, there is no quick, efficient, easy or permanent way to deal with this weed.
DAN GILL’S MAILBOX
View full sizeDrought stress in Knock Out roses may be the result of fungal root rot.
We have an established 3-year-old Pink Knock Out rose bush planted in our flowerbed. It has always done well, but we noticed the leaves have begun to curl downward and there appears to be a silvery powdery mildew on it. Please look at the attached photos explain what is happening and what can we do. My daughter who lives on the north shore has lost five Knock Out rose bushes due to the same problem.
Looking at the photos, I couldn’t see the mildew. But, the curling down of the leaves appears to be drought stress. That is, it appears the leaves are not getting enough water. So, they are curling under or wilting. There are two reasons this will happen:
The first is that the soil in the bed is too dry. This is not a major issue and is easily corrected by watering properly. However, given recent rains and the fact that I’m sure you are watering your bushes, this scenario is unlikely.
The second is that the root system has stopped functioning properly. The rose bush can only obtain the water it needs from the soil through its roots. If something goes wrong with the roots and they stop absorbing water properly, the upper part of the plant will wilt just as if the soil is too dry. Fungal root rots are often responsible for the problems in these situations. Unfortunately, root rot is life-threatening. If the roots are sick and cannot absorb water, the plant will often die of thirst no matter what you do.
This situation is most common when excessive rainfall and/or excessive irrigation happens. Hot weather plus the soil staying too moist, too long are perfect conditions for root rot to occur (in landscape plants in general). Knock Out roses are extremely tough and low-maintenance and require little or no irrigation once they are established. The problems we are seeing with Knock Out rose bushes dying are generally associated with poorly drained beds, excessive rainfall and/or over-irrigation, leading to root rot.
All you can do at this point is to make sure that you do not compound the problem by watering the roses. Water, in this situation, is like throwing gas on a fire. Hopefully, the roots are still in decent enough shape to recover.
When my wife waters her African violets, it stirs up a bunch of tiny ant-like insects. Can you provide any info on this?
These are likely springtails, primitive insects that live in the potting soil. These tiny bugs do not attack or damage African violets, and one option is to ignore them. Allowing the plants to become thoroughly dry before watering (African violets are remarkably drought-tolerant) will help reduce the population. These insects must have a moist environment. If you feel it necessary to control the springtails, you could move the plants outside and drench the soil with an insecticide (Malathion has commonly been used; permethrin should also work well). Allow the plants to drain thoroughly and then move them back inside.
I am planning to fertilize my lawn using Scotts turf builder 20-0-08. I would like to know if it is OK to use on my St. Augustine lawn at this time of year.
There are no temperature limitations for fertilizing lawns. Midsummer is an excellent time to make another application of fertilizer to your lawn. Follow label directions of the product you use carefully.
Dan Gill is extension horticulturist with the LSU Ag Center.
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Torpedograss invading St. Augustine grass
Unfortunately we do not have any herbicides that will kill the torpedograss and can be used in St. Augustinegrass lawns. There are two herbicides which do a fair job on torpedograss, Monument and Drive, but both of these herbicides are not labeled for use in St. Augustinegrass and if used would cause damage to the St. Augustinegrass. About all you can do is treat with Roundup very carefully and then replace any of the St. Augustinegrass that is killed or damaged by the application of the Roundup.
Here is an excerpt from the LSU site.
Torpedograss – Torpedograss is a perennial, rhizomatous grass that is considered one of the most invasive grasses in the world. Although the plant does produce seed, the seeds are not viable. The weedy grass solely reproduces vegetatively by robust rhizomes. The spread of torpedograss in Louisiana is mainly attributed to the movement of soils infested with torpedograss from the Bonnet Carré Spillway. The spillway is just west of New Orleans and is the main source of torpedograss for southeast Louisiana, especially within the New Orleans metro area. For years, there were no reports of infestations in north Louisiana; however, torpedograss infestations have been confirmed at several locations in northern areas of the state.
Control: Complete control of torpedograss may not be possible. Grass-killing herbicides normally prescribed for flowerbeds like sethoxydim and fluazifop are just not very effective on torpedograss, although fluazifop is a little better than sethoxydim. Glyphosate is the best herbicide on the weed, but high rates are necessary for control. Directed sprays of glyphosate are not always safe in landscape plantings due to the potential for drift. It may be safer to wipe the weed with a glyphosate/water solution that is at least 10 percent glyphosate when it is in a landscape bed. Using a chemical-resistant glove inside a cotton glove to wipe the solution on the torpedograss foliage is an effective method of application. Repeated applications are always necessary for torpedograss.
Thanks for the question.