6 min to read
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.
Managing torpedograss
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.
In beds
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 lawns
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.
Charlotte Peterson
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?
Michael
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.
Ken Wilson
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.
Send mail to:
Dan Gill, garden columnist
The Times-Picayune Living Section
3800 Howard Ave.
New Orleans 70125-1429
Send email to dgill@agcenter.lsu.edu.
Please include a phone number.
