By Dan Gill

LSU AgCenter Horticulturist

(10/28/16) Using weed killers or herbicides should be approached very carefully by the gardener. After all, you are introducing into your landscape substances that are designed specifically to kill plants. It is entirely possible that you could seriously damage your lawn, trees, shrubs or other landscape plants if you use them improperly. That said, herbicides are exceptionally useful tools if used properly in our efforts to manage weeds in our landscapes.

You need to realize something else about herbicides. They are intended for continuing efforts at weed control. In the overwhelming majority of situations, herbicides must be applied more than once for effective control. Gardeners often have thrown in the towel in frustration because one or two applications of a herbicide did not totally eradicate a weed problem. Make sure you are using the right herbicide, and then keep at it. Frequent monitoring and prompt, repeated, persistent efforts are critical for weed management, whether you are using herbicides or physical controls.

When using herbicides, you must carefully assess the situation.

The first step is determining where the weed is a problem, such as in the lawn, flower beds, shrub beds, vegetable garden or vines growing on structures or other plants. Where the weed is a problem will have a profound effect on the herbicides to use and not harm desirable plants in the area.

Next, identify the weed, or at least the category it belongs to. Different herbicides and application methods will control different weeds. If you use the wrong herbicide or apply it incorrectly, you waste effort and money. Some examples of common weed categories include grasses, such as bermuda or torpedo; sedges, such as cocograss, nutsedge or kylinga; broadleaf weeds; annual weeds, warm-season and cool-season; perennial weeds and vines.

Know the language

When discussing herbicide options, we must first define some terms.

Preemergence herbicides are applied to weed-free areas to prevent annual weed problems by killing germinating weed seeds. For instance, now is a good time to apply preemergence herbicides to lawns to prevent problems with cool-season weeds that grow from October to April or May.

Postemergence herbicides are applied to actively growing weeds to control a current weed problem.

Selective herbicides will just kill the weeds and not the ornamentals or turf when applied over all the plants in an area. These are commonly used in lawns, but other products are useful for managing weeds in beds.

Nonselective herbicides will kill whatever you apply them to. These are useful for killing everything in an area prior to planting. Or they can be applied specifically just to the weed without getting any on desirable plants. This is called “spot treating.”

Contact herbicides only kill the part of the plant that they are applied to. They will not kill below-ground parts when sprayed on the leaves and are most effective on annual weeds.

Systemic herbicides are applied to the foliage where they are absorbed into the plant’s circulatory system. They kill all parts of the weed, even parts not directly sprayed, such as rhizomes or bulbs below ground, and are the best choice for perennial weeds.

So when you go to a nursery to purchase a herbicide, you need to know the situation (lawn or beds), the weed (bring some with you if you don’t know), the type of herbicide you want to use (systemic, selective, contact) and, if you intend to use a selective herbicide, the type of lawn grass or the ornamentals growing with the weed. With this information, you can choose the appropriate herbicide by looking carefully at the labels or talking to professionals, such as LSU AgCenter horticulture agents or nursery staff.

It is also critically important to completely read and understand the label of any herbicide. If you use it improperly, you can do great damage to landscape plants. I have talked to so many people that have severely damaged their lawns by improperly applying a herbicide to deal with a minor weed problem.

This is the basic knowledge needed to properly assess a situation and select the appropriate herbicide. For help with identifying weeds and herbicide recommendations, contact your local LSU AgCenter extension office or consult with professionals at your local garden center.

Always read the label to be sure the herbicide you choose matches your situation, weed, type of herbicide you want to use and the type of lawn grass or the ornamentals growing with the weed. Photo by Rick Bogren

Preemergence herbicides are applied to weed-free areas to prevent annual weed problems by killing germinating weed seeds. Photo by Rick Bogren