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Herbicides – a Chemical Answer to Weed Control
by Dan Gill
by Dan Gill


Dan Gill earned B.S. and M.S. degrees in horticulture from Louisiana State University in Baton Rouge, and is an Associate Professor in Consumer Horticulture with the LSU AgCenter.

He is the spokesperson for the LSU AgCenter’s Get It Growing project, a statewide educational effort in home horticulture utilizing radio, Internet, TV and newsprint. Gardeners throughout Louisiana read his columns in local newspapers, watch his gardening segments on local TV stations and listen to him on local radio. In the New Orleans area, Dan appears weekly on the Channel 4 Morning News, writes a weekly gardening column for The Times-Picayune and hosts the Saturday morning WWWL Garden Show, a live call-in radio program.

Dan is co-author of the Louisiana Gardener’s Guide and author of Month-by-Month Gardening in Louisiana. His “South Louisiana Region Report” and “Only in Louisiana” columns appear monthly in the Louisiana Gardener Magazine.

October 26, 2008

In my previous column I discussed weed control in the landscape. I focused primarily on preventive and corrective methods of controlling weeds without using herbicides.

Using herbicides should be approached very carefully by the gardener. After all, you are introducing substances into your landscape that are designed specifically to kill plants. It is entirely possible that you could seriously damage your lawn, trees, shrubs and other landscape plants if you use them improperly.

That said, herbicides – if used properly – are exceptionally useful tools in our efforts to manage weeds in our landscapes.

You need to realize something else about herbicides. They are a tool you use in your continuing efforts at weed control. In the overwhelming majority of situations, you will need to apply herbicides more than once for effective control. I frequently talk to gardeners who have thrown in the towel in frustration because one or two applications of a herbicide did not totally eradicate the weed problem.

Make sure you are using the right herbicide, and then keep at it. Frequent monitoring and prompt, repeated, persistent effort is critical for weed control, whether you are using herbicides or physical controls.

When using herbicides you must first carefully assess the situation. The first step is determining where the weed is a problem – such as in the lawn, flowerbeds, shrub beds or vegetable garden or vines growing on structures or other plants. Where the weed is a problem will have a profound effect on the herbicides you should use in order not to 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 (Bermuda, torpedo), sedges (coco grass, nutsedge, kylinga), broadleaf weeds, annual weeds (warm- and cool-season), perennial weeds and vines.

Know the language

When discussing herbicide options, we first need to learn some terms.

Pre-emergence 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 pre-emergence herbicides to lawns to prevent problems with cool-season weeds that grow from October to April or May.

Post-emergence herbicides are applied to actively growing weeds to control a current weed problem.

Selective herbicides – will just kill the weeds and not the ornamental plants or turf when they’re applied over all the plants in an area. These are commonly used in lawns, but some products also are useful for weed management 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 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 a 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), what the weed is (bring some with you if you don’t know), what type of herbicide you want to use (systemic, selective, contact) and, if you intend to use a selective herbicide, the type of lawn grass you have or the ornamentals growing in the bed with the weed. With this information, you can choose the appropriate herbicide by looking carefully at the labels or talking to professionals (LSU AgCenter horticulture agents or nursery staff).

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

I have attempted to provide you with the basic knowledge needed to properly assess a situation and then use that information to select the proper type of herbicide. It would be far too complicated, in the format of this column, to look at the many different types of herbicides available and all the possible situations where weeds occur and then make specific recommendations. For help with identifying weeds and for herbicide recommendations, contact your local LSU AgCenter Extension office.

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