Weed War: When and How to Use Chemical Herbicides

First, let's get familiar with the terminology. Herbicides are used as pre-emergents, by inhibiting plant seeds from germinating, or as postemergents, meaning they work on actively growing plants.

Some are nonselective and affect any plants they contact, while others are selective and will control only specific plants.

Contact herbicides affect only the plant tissue on which they are applied, while systemic herbicides are absorbed into the entire plant and root system.

Chemical controls may be natural or synthetic. Ideally, all herbicides should be applied by a licensed pesticide applicator.

Natural Herbicides

The active ingredients in natural herbicides come from plants or minerals. These products are subject to government regulations for personal and environmental safety. Read all product labels thoroughly and follow them with care.

Corn gluten meal. The protein part of a corn kernel is a selective, pre-emergent herbicide most commonly used to control annual weeds — such as oxalis, purslane and spurge — in lawns. It’s also about 10 percent nitrogen, so it helps promote healthy turf. Corn gluten meal is most effective when applied twice a year. Apply it before the seed germinates and forms a root. A dry period following germination is also necessary. Learn more about timing corn gluten meal applications here.

Vinegar. It's a nonselective, postemergent and contact herbicide for annual weeds. Apply horticultural vinegar solutions, which have less than 20 percent acetic acid, as a spray to the weeds’ foliage. The acid acts as a contact desiccant (“burning” the foliage but not the roots) and is most effective when applied to annual weeds in the heat of summer.

Soap. Horticultural soaps, derived from fatty acids, are nonselective, postemergent, contact herbicides. Sprayed on the weeds' leaves, the product smothers the foliage, inhibiting the plants' growth. Horticultural soaps are most effective on young, actively growing, annual weeds.

Iron. The newest kid on the block uses a 1.5 percent solution of FeHDTA (an iron chelate) as its active ingredient. A selective, systemic, postemergent herbicide, the applied iron dose is toxic to several common broad-leaf weeds but does not have a detrimental effect on turf grasses.

Synthetic Herbicides

Synthetic herbicides have man-made components subject to government regulations for personal and environmental safety. Remember: Read and follow product labels carefully.

Glysophate and glufosinate ammonium. These nonselective, systemic herbicides may be effective on noxious perennial weeds such as field bindweed, myrtle spuge and quackgrass. Research shows that a very specific application regime — the time of year, the stage in the plants’ life cycle, and the method of application — is crucial to the most efficient and effective use of this herbicide.

2,4-D and Tryclopyr. Both of these compounds are selective, systemic, postemergent herbicides useful for controlling many annual and perennial broadleaf weeds such as puncturevine, kochia, Canada thistle and orange hawkweed. They're effective on weeds in lawns and around conifers.

Your local Cooperative Extension Office will have more research on how to identify and control the weeds in your region.

More:
5 Ways to Naturally Win the Weed War
Tackle Weeds the Natural Way
5 Weed-Smothering Ground Covers