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Herbicide Uses and Abuses

James R. Feucht Extension Landscape Plants Specialist Colorado State University Cooperative Extension

This is the time of year when weeds become a big problem in vegetable gardens, shrub borders and lawns.

A myriad of chemicals are available at garden centers to effectively reduce weed problems with less work than pulling by hand. Some weeds, in fact, are propagated when pulled by hand. Examples include field bindweed, the rubbery-leafed purslane, quackgrass and dandelion.

Before selecting an herbicide, you must know the weeds you have and whether they are annual or perennial. You also need to consider their proximity to desirable plants. For broad-leaved perennial weeds, such as dandelion, plantain and perennial chickweed, products containing 2,4-D can be very effective, especially in the lawn. Take care to avoid drift if you have trees or shrubs in or adjacent to the lawns. Spot treat problem weeds rather than broadcast applications to the entire lawn.

Some broad-leaved weed killers also contain a chemical called dicamba (banvel). This product is useful in managing the tough-to-control weeds such as bindweed, thistle and oxalis. Because it can leach into the root zones of trees and shrubs, this product should be used sparingly as a spot treatment, wetting only the foliage of the weeds.

In vegetable gardens, as well as shrub borders, avoid use of broad-leaved weed killers containing 2,4-D and/or dicamba. The fumes alone can injure your plants. It is best to clean, cultivate and hand pull existing weeds, then apply a pre-emergent herbicide (weed preventer) to control any weeds that would come up from seed. Several products on the market are labeled specifically for this purpose. They usually contain dachtal or surflan. Be sure to follow all label directions to avoid injury to your vegetables.

If you have weedy areas along fences, around buildings or in gravel or other mulched areas, spot treat with glyphosate (sold as Roundup or Kleenup). Avoid applications with a fine mist-type sprayer, as even small amounts can injure non-target plants. More than one application may be needed on bindweed and thistle. Glyphosate works best when weeds are actively growing with plenty of foliage.

Do not be tempted to control weeds in your landscape with "total vegetation control chemicals" such as Triox, Pramitol and Spike. By state law, effective Jan. 1, these and similar products can only be used by certified commercial applicators. These are soil sterilants and can migrate to tree roots, shrubs and other plants with long-lasting damage. Damage can result even if you follow the label carefully. These products also can harm your neighbors' trees if their roots extend into the treated area of your property.

When using herbicides, always read the label thoroughly and follow directions carefully. The misuse of an herbicide can lead to disappointment and to long-range problems in your garden.

For assistance in weed identification and appropriate controls, take weed samples to the Colorado State University Cooperative Extension office in your county.

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Date last revised: 01/05/2010