Primetime Newspapers
By Calvin Finch, PhD, SAWS Conservation Director, and Horticulturist
Week of August 9, 2004

CONTROLLING WEEDS WITH CONTACT HERBICIDES 

            Is Bermuda grass taking over your garden? If there are no other plants around them, it is relatively easy to kill them with Roundup or Finale. Both products are contact herbicides that, when mixed and applied according to the label, are very effective at killing almost any weed.

            If the persistent Bermuda grass, however, is in amongst your flowers or groundcovers, Roundup and Finale cannot be used. They are non-specific contact herbicides. They will kill almost any plant if the spray is applied to the green part of the plant. Hand weeding is an option. Weeding Bermuda grass is good exercise, but in 90 degree F. plus temperatures it is certainly not much fun.

            There is a better option to control Bermuda grass that is invading your gardens. Consider one of the grass killing contact herbicides. Some brand names are Over the Top, Grass-Be-Gone, Poast, Vantage, and Fusilade. Each of them will kill grasses without hurting your broadleaf flowers, shrubs, or groundcovers.

Spray the product on the blades of the Bermuda grass; it reacts with the plant’s chemistry and is distributed through the plant to kill tops and root systems. The kill is not fast but, if the weed is actively growing, the kill is reliable if the label instructions are followed. Within one week you should see a slight color change on the blades of the grass and it will stop growing. In about three weeks the grass should be straw colored and the kill is complete.

Another pesky weed that loves the hot wet weather we have been experiencing is nutshedge, also called nutgrass. Nutshedge looks like a miniature lily. The sharp blades emerge from a central point. Instead of flowers the nutshedge bear raisin sized prickly nuts. The seeds have also been described as looking like miniature pinecones.

To kill nutshedge look for one of two products, Image or Manage. Again, follow the instructions closely. Image and Manage are expensive but a little bit goes a long way.

Roundup, Grass Be Gone, Image and the other contact herbicides described in this article are manufactured pesticides. It is difficult to find an effective organic contact herbicide. Vinegar is one organic herbicide that can be effective if you know its limitations. Vinegar burns the foliage of weeds to which it is applied. Unlike Roundup and the manufactured contact herbicides it does not usually kill the roots. The plant resprouts after the application. The exception is that vinegar will kill the whole plant of annual weeds like bedstraw, blue grass or henbit if it is applied when the weeds are young.

Two other manufactured contact herbicides to consider under certain conditions are MSMA and 24D.

MSMA is the short name for a salt that kills crabgrass and broadleaf weeds in zoysia, Bermuda or buffalo grass lawns. It does not work as well as Manage for nutshedge, but it was the herbicide of choice for the pest in the past. MSMA is not as useful in our area as other regions because it kills St. Augustine grass. MSMA is very effective in removing St. Augustine grass from other lawn grasses.

24D is the active ingredient of Weed Be Gone. Use it to kill broadleaf weeds like oxalis, clover, and dandelions in lawns. Again, read the label. It is not recommended for use in St. Augustine grass in high temperatures.

The contact herbicides are excellent tools for weed control. They usually work well. If you have tried a contact herbicide such as Roundup and it has not been effective, review the conditions of its use in terms of the usual reasons that they are less than 100% effective.

·        You selected the wrong contact herbicide for the target weed. Diagnosing the weed correctly is important. Contact your favorite nursery or the Texas Cooperative Extension for help in identifying weeds.

·        The contact weed killers do not work well on dry hardened off plants. They kill lush plants quickest. Sometimes it is worthwhile to water the target weeds three or four days before the herbicide is applied.

·        The spray is washed off the foliage before it dries. The herbicide will usually do its job if it sets on the plant for two hours before it rains.

·        The spray must be applied to the plant foliage. It does not work if you spray the soil.