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Landscapes' Last Defense Against Weeds
You've tried removing weeds by hand and preventing them with mulches and landscape fabrics. Your last line of defense is chemical control.

If you're not opposed to using herbicides, a large array of them are available. In fact, so many are on the garden center shelves that many people get confused.

Much of the confusion comes from the fact that many herbicides are no longer patented, so any company that wants to can sell them. Some of the many brands you might recognize are Ortho, Spectracide, Bonide, Dragon, Hi-Yield and Acme. As if that weren't enough, most herbicides, but not all, come in both granular and sprayable forms.

Think 'Active Ingredients'

If you keep track of the name and percentage of active ingredients, you can shop around among brands to get the best buy.

There's no difference in the active ingredients except the percentage and possibly the formulation. The diquat in a Ortho product is the same diquat in a Spectracide brand.

That doesn't mean all products are the same. Some sprays have different surfactants or wetting agents. And different materials in granular forms can affect the herbicide release rate. These factors can affect a brand's ability to control weeds.

More Herbicide Terms

Other herbicide terms to keep clear are selective, nonselective, pre-emergent and postemergent.

If a herbicide is selective, it will control some plant species and not others. Ornamec, for example, controls only grasses. A nonselective herbicide controls any vegetation that it is applied to. A good example is Roundup, which can kill any plant it comes in contact with.

Pre-emergent herbicides control weeds that originate from seed and haven't yet germinated. They rarely work on perennial weeds that germinate from bulbs, corms, rhizomes, stolons or other vegetative structures.

Their active ingredients include dichlobenil, prodiamine, isoxaben, pendimethalin, metolachlor, trifluralin, simazine, oxadiazon and oryzalin. Some control only a few tough weeds. Most control various ranges of annual grasses and broadleaf weeds.

Postemergent herbicides' active ingredients include bentazon, triclopyr, clethodim, glufosinate, halosulfuron, fluazifop, diquat, glyphosate, sethoxydim or potassium salts of fatty acids.

These products generally control actively growing grasses or other emerged annual weeds, including tough weeds like purple and yellow nut sedge and some woody plants.

A few products can be both pre- and postemergent.

Mode-of-Action

Mode-of-action is a term homeowners probably won't see but should be aware of. A herbicide's MOA, or the specific way it works, is usually understood, but now always.

For example, glyphosate (Roundup) makes plants unable to make key proteins and enzymes, so they starve to death. But we don't fully understand how dichlobenil (Casoron or Norosac) works.

There are many restrictions on the uses of both pre- and postemergent herbicides. Some can be applied over the top of actively growing ornamentals, for instance, and some can't.

Follow Directions

For this reason, it's wise to read the directions provided with the herbicide. Precisely following the label can keep you from damaging landscape plants or wasting costly herbicides.

If you're unsure about appropriate uses or have questions about herbicides, contact your county Extension Service agent.

Here are some herbicides homeowners commonly use. They don't require a licence when used on your own property. Their active ingredients can be sold alone or in combination with other herbicides.

Pre-emergent Herbicides

  • Casoron and Norosac (dichlobenil) control many tough weeds, such as Florida betony and horsetails, in many established woody ornamentals. Use them only in the late fall, winter or early spring. They can provide postemergent control of some weedy plants.
  • Factor, Barricade and others (prodiamine) control many annual grasses and small seeded broadleaf weeds.
  • Gallery and others (isoxaben) control many annual broadleaf weeds. Isoxaben provides poor control of annual grasses but provide excellent pre-emergent control when mixed with other herbicides like Surflan.
  • Pendulum (pendimethalin) controls many annual grasses and small seeded broadleaf weeds.
  • Pennant (metolachlor) controls many annual grasses and small-seeded broadleaf weeds. Pennant provides some pre-emergent activity on yellow nut sedge.
  • Preen and Treflan 5G (trifluralin), have a weed-control spectrum like that of Surflan. If possible, apply them before planting, under mulch, and incorporate into top 1 to 3 inches of soil.
  • Princep (simazine) controls many annual grasses and small-seeded broadleaf weeds.
  • Ronstar and others (oxadiazon) controls many annual grasses and small-seeded broadleaf weeds.
  • Surflan (oryzalin) controls many annual grasses and small-seeded broadleaf weeds. Excellent tank-mixed with Roundup.
Postemergent Herbicides

  • Basagran T/O (bentazon) controls yellow nut sedge and certain broadleaf weeds. It can be used over the top of many ground covers such as English ivy, liriope and pachysandra.
  • Brush-B-Gon (triclopyr) controls woody plants. Paint it onto freshly cut stumps to prevent resprouting.
  • Envoy (clethodim) controls a broad spectrum of actively growing grasses. It doesn't control broadleaf weeds or sedges. It can be used over the top of broadleaf ornamentals.
  • Finale (glufosinate) controls most emerged annual weeds. It doesn't control perennial weeds well. Don't allow the spray to contact foliage of actively growing, desirable plants.
  • Manage (halosulfuron) is excellent at controlling yellow and purple nut sedges. It must be applied directly to the sedges without contacting desirable ornamentals.
  • Ornamec and Grass-B-Gon (fluazifop) control a broad spectrum of actively growing grasses. They don't control broadleaf weeds or sedges. They can be used over the top of broadleaf ornamentals.
  • Reward and many others (diquat) control most emerged annual weeds. They don't control perennial weeds well. They provide visual control of weeds quickly. Don't allow the spray to contact the foliage of actively growing, desirable plants.
  • Roundup and others (glyphosate), in many formulations, control most actively growing weeds. It may take 10 to 14 days, but they provide excellent control of perennial weeds (but no pre-emergent control). Don't allow the spray to contact the foliage of actively growing, desirable plants.
  • Scythe and others (potassium salt 00A1 s of fatty acids) provides contact control of actively growing weeds. Don't allow the spray to contact the foliage of actively growing, desirable plants. 25DC
  • Vantage (sethoxydim) controls many actively growing grasses. It doesn't control broadleaf weeds or sedges. It can be used over the top of broadleaf ornamentals.
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