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Understanding herbicides

POSTED: August 24, 2011 4:00 a.m.

Buying herbicides can be a challenge.

There are so many different types of herbicides that are used on different plants, how does a homeowner decide which one is best?

Remember to always read the label. By knowing certain chemical terms the label will be easier to understand.

Herbicides may be classified according to chemistry, method of application, timing of application, persistence, selectivity and mode-of-action.

1. Selective. A selective herbicide controls or suppresses certain plant species without seriously affecting the growth of another plant species. Selectivity may be due to differential absorption, translocation, morphological and/or physiological differences between turfgrasses and weeds. The majority of herbicides used in turfgrasses are selective in nature.

2. Nonselective. Nonselective herbicides control plants regardless of species. These are primarily used to control all plants as in the renovation or establishment of a new turf area, "spot treatments," or a trimming material along sidewalks. Glyphosate (Roundup) is an example of nonselective herbicide.

3. Systemic. Systemic herbicides are extensively translocated (moved) in the plant's vascular system. The vascular system translocates the nutrients, water and other materials necessary for normal growth and development. In contrast to the quick kill observed with contact herbicides, systemic herbicides require several days or even a few weeks to be fully translocated throughout the plant's vascular system, and therefore, require a longer period of time before kill. Systemic herbicides are also classified as selective or nonselective.

4. Contact. Contact herbicides only affect the portion of green plant tissue contacted by the herbicide spray. These herbicides are not translocated in the vascular system of plants or only are to a limited extent.

Therefore, underground plant parts such as rhizomes or tubers are not killed. Usually repeat applications are needed with contact herbicides to kill regrowth from these underground plant parts. Adequate spray volumes and thorough coverage of the weed foliage are necessary for effective control.

These herbicides kill plants quickly, often within a few hours of application. Contact herbicides may be classified as selective or nonselective.

Herbicides may also be classified as pre-emergence or post-emergence, depending on the time the chemical is applied. It should be noted that although the majority of herbicides may be classified as pre-emergence or post-emergence some, such as Princep, are notable exceptions. They are used as both pre-emergence and post-emergence herbicides.

Remember, when using any pesticide to read, understand and follow the label found on the container.

Pre-emergence herbicides

Pre-emergence herbicides are applied to the turf grass site prior to weed seed germination and form a barrier at, or right below, the soil surface. Most pre-emergence herbicides prevent cell division during weed-seed germination as the emerging seedling comes into contact with the herbicide. Weeds that already have emerged (visible) at the time of application are not controlled consistently by pre-emergence herbicides because their primary growing points escape treatment.

Post-emergence herbicides

Post-emergence herbicides are applied directly to emerged weeds.

In contrast to pre-emergence herbicides, this group of herbicides provides little, if any, soil residual control of weeds.

A complete chemical weed control program can be accomplished with post-emergence herbicides, provided multiple applications are used throughout the year.

However, due to the necessity of repeat applications and temporary turf grass injury, most turf grass managers use post-emergence herbicides in conjunction with a pre-emergence weed control program.

Post-emergence herbicides are useful to control perennial grasses and broadleaf weeds that are not controlled by pre-emergence herbicides.

Clark Beusse is the Dawson County extension agent. For more information, call (706) 265-2442.

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