Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
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Mark Czarnota, Ph.D., Ornamental Weed Control Specialist
Department of Horticulture, 1109 Experiment Street
Griffin, GA 30223
If you have been unsuccessful at removing weeds by hand, or preventing their growth by using physical barriers such as mulches and landscape fabrics, your last line of defense is chemical weed control. If you are not opposed to using synthetically produced herbicides to control unwanted plants, you will find a large array of herbicides available.
Consumers get confused about the large number of herbicides available to them at most garden centers, home improvement stores, and department stores. Much of this confusion comes from the fact that many herbicides are "off patent", which means they can be sold by any company that wants to sell them. This results in the production and sale of the same herbicide being sold under many different brand names. Brands such as Ortho®, Spectracide®, Bonide®, Dragon®, Hi-Yield®, and Acme® are commonly found in garden centers. The list of manufacturers is continually changing with the success and failure of businesses. Another point of confusion is that most herbicides are sold in both a granular and sprayable formulations. In general, granular formulations are easier to apply and have less of a chance of being incorrectly applied. However, due to cost or formulation constraints, some herbicides are available only in one type of formulation. A good example of this is Casoron, which is available only in a granular formulation.
Write down the name and percentage of the active ingredients in the product, the compare this information with other brands in order to get your best buy. There is no difference in the active ingredients! The diquat in an Ortho brand is exactly the same as the diquat in a Spectracide brand. Difference can occur in the percent of diquat in the formulation and the formulation itself. Some manufacturers add different surfactants or wetting agents to their sprayable concentrates. Granular formulations can be made with different materials, which can affect the rate at which herbicide is released. All of these factors can affect a herbicide's ability to control weeds.
Terms you should know when discussing herbicides are preemergent, postemergent, selective and non-selective. A preemergent herbicide is one that generally controls ungerminated seeds. Postemergent herbicides generally control emerged plants. A few herbicides, such as Casoron, have both pre and postemergent activity. Herbicides may be selective or non-selective. If a herbicide is selective, it will have activity on some species of plants and not others. A good example is Ornamec®. Ornamec is a grass herbicide that only controls grass-type weeds. A non-selective herbicide is one that controls any vegetation to which it is applied. A good example is Roundup®. Roundup controls any vegetation it contacts.
Another term you should be aware of is mode-of-action. All herbicides (like medicine) work by a specific chemical mechanism or mode-of-action. A herbicide's mechanism is usually understood, but the mode-of-action of some herbicides is unknown or poorly understood. For example, Roundup inhibits the production of three amino acids. Plants treated with Roundup cannot manufacture these amino acids. Lacking these three amino acids, plants treated with Round-up cannot manufacture proteins and key enzymes. This causes the plants to simply starve to death.
The following are lists of preemergent and post-emergent herbicides that consumers commonly encounter. These herbicides do not require you to have an applicator's licence when using these products on your own property. The active ingredients in these products can be sold alone or in combination with other herbicides.
Preemergent herbicides generally control weeds by preventing the germination of seed. Perennial plants that germinate from bulbs, corms, rhizomes, stolens or other vegetative structures are generally not controlled by a preemergent herbicide.
| Preemergent Herbicides | ||
| Trade Name | Active Ingredient | Weeds Controlled |
| Caseron | dichlobenil | Spectrum of weed control is limited, but controls many difficult weeds such as Florida betony and Horsetails in many established woody ornamentals. Volatile and should only be used in the late fall, winter or early spring. Does provide postemergent activity to some weedy plant species. |
| Gallery | isoxaben | Controls many annual broadleaf weeds. Poor on annual grasses. Excellent preemergent control when mixed with other herbicides like surflan. |
| Pennant | metolachlor | Controls many annual grasses and small-seeded broadleaf weeds. Some preemergent activity on yellow nutsedge germinating from tubers. |
| Surflan | oryzalin | Controls many annual grasses and small-seeded broadleaf weeds. Excellent tank mixed with Roundup. |
| Ronstar | oxadiazon | Controls many annual grasses and small-seeded broadleaf weeds. |
| Pendulum | pendimethalin | Controls many annual grasses and small-seeded broadleaf weeds. |
| Factor, Barricade and others | prodiamine | Controls many annual grasses and small-seeded broadleaf weeds. |
| Princep | simazine | Controls many annual grasses and small-seeded broadleaf weeds. |
| Preen and Treflan | trifluralin | Weed control spectrum similar to that of Surflan. Applied prior to planting, under mulch, and incorporated into top 1 to 3 inches of soil (if possible). |
Generally, postemergent herbicides only control plants that are actively growing. Most postemergent herbicides, like Roundup, will not provide any preemergent activity and need to be applied to green tissue of a plant in order to be affective.
| Postemergent Herbicides | ||
| Trade Name | Active Ingredient | Weeds Controlled |
| Basagran T/O | bentazon | Controls yellow nutsedge and certain broadleaf weeds. Can be used over-the-top of many ground covers such as English ivy, Liriope and pachysandra. |
| Envoy | clethodim | Controls a broad spectrum of actively growing grasses. Does not control broadleaf weeds or sedges. Can be used over-the-top of broadleaf ornamentals. |
| Reward and many others | diquat | Provides control of most emerged weeds. Does not control perennial weeds well. Provides visual control of weeds very quickly. Do not allow spray to contact foliage of actively growing, desirable plants. |
| Ornamec, Grass-B-Gon | fluazifop | Controls a broad spectrum of actively growing grasses. Does not control broadleaf weeds or sedges. Can be used over-the-top of broadleaf ornamentals. |
| Finale | glufosinate | Provides control of most emerged weeds. Does not control perennial weeds well. Do not allow spray to contact foliage of actively growing, desirable plants. |
| Roundup and others | glyphosate | Many formulations available. Provides control of most actively growing weeds. Control may take 10 to 14 days, but provides excellent control of perennial weeds. Provides no preemergent weed control. Do not allow spray to contact foliage of actively growing, desirable plants. |
| Manage | halosulfuron | Excellent at controlling yellow and purple nutsedge. Must be applied directly to sedges without contacting desirable ornamentals. |
| Scythe and others | potassium salts of fatty acids | Provides contact control of actively growing weeds. Do not allow spray to contact foliage of actively growing, desirable plants. |
| Vantage | sethoxydim | Controls many actively growing grasses. Does not control broadleaf weeds or sedges. Can be used over-the-top of broadleaf ornamentals. |
| Brush-B-Gone | triclopyr | Used to control woody plant material. Can be painted onto freshly cut stumps to prevent resprouting. |
There are many restrictions on the uses of both preemergent and postemergent herbicides. Some can be applied over the top of actively growing ornamentals, and some cannot. For this reason, it is extremely important to read the directions (or labels) that are provided with the herbicide. This will prevent unnecessary plant damage, or wasteful uses of herbicides. If you have any questions about herbicides or are unsure about an appropriate use of a herbicide, don't hesitate to contact your local County Extension Agent.
Trade and brand names are used only for information. The Cooperative Extension Service, University of Georgia College of Agricultural & Environmental Sciences does not guarantee nor warrant published standards on any product mentioned; neither does the use of a trade or brand name imply approval of any product to the exclusion of others which may also be suitable.
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