By Dan Gill
LSU AgCenter Horticulturist
(08/05/16) If you maintain a landscape, you deal with weeds. At this point in our long, hot summer, I’m sure you have devoted considerable effort to keep your landscape from being taken over by various weeds in beds and lawns and vines growing up trees and on fences.
Non-herbicide weed control in beds is primarily done through mulching and barriers, such as landscape fabric. Proper mulching is the single most important technique we use in reducing weed problems in beds. Once weeds begin to grow, the non-herbicide control techniques are physical, such as hand weeding.
Gardeners should approach using herbicides very seriously. After all, you are introducing your landscape to chemicals 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 are useful tools in our efforts to manage weeds if used properly in our landscapes.
Once is not enough
There is something else you need to realize about herbicides. They are a tool you can 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. Make sure you are using the right herbicide, and then keep at it.
Before using herbicides
When using herbicides you must first carefully assess the situation. The first step is determining where the weed is a problem, such as the lawn, flower bed, shrub bed, 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 so as not to harm desirable plants in the area.
Next, identify the weed. Different herbicides will control different weeds. If you use the wrong herbicide, you waste effort and money. Some examples of common weed categories include grasses (Bermuda, Johnson), sedges (cocograss, nutsedge, kylinga), broadleaf weeds, annual weeds (warm- and cool-season), perennial weeds and vines.
Know the language
When discussing herbicide options, it’s important to learn some terms.
– Pre-emergence herbicides are applied to weed-free areas to prevent annual weed problems by killing germinating weed seeds.
– Post-emergence herbicides are applied to actively growing weeds to control a current weed problem.
– Selective herbicides will kill just the weed and not the ornamentals or turf when applied over all the plants in an area. These are commonly used in lawns, but some products also are useful in 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 just to the weeds to be controlled.
– Systemic herbicides are applied to the foliage and 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.
Choosing a herbicide
When you go to a nursery to purchase an herbicide, you need to know the situation (lawn, ornamental beds, vegetable beds or food crops), what the weed is (bring some with you if you don’t know) and what type of herbicide you want to use (pre-emergence, post-emergence, systemic, selective, contact). If you intend to use a selective herbicide, you also need to know the type of lawn grass you have or the ornamentals growing in the bed with the weed. In a bed of mixed ornamentals, bring a list of the plants with you to compare to herbicide labels. 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.
Do not ask, “What can I use to control weeds in my lawn?” You must include what kind of grass you have and the type of weeds to get a proper recommendation or to choose the right product by looking at the label.
It is also critically important that you completely read and understand the label of any herbicide you use. And make sure you read the entire label before you purchase a herbicide. 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.
Virginia buttonweed is a difficult weed to kill in a lawn and may require several herbicide applications to control. Photo by Dan Gill
Doveweed. Photo by Dan Gill
Crabgrass. Photo by Dan Gill