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ASK THE EXPERTS

When to use herbicides

POSTED: January 20, 2010 4:00 a.m.

In last week’s newspaper article, I wrote about how herbicides are classified as selective, nonselective, systemic or contact.  In this article, I will go a step further and write about how herbicides are classified as preemergence or postemergence, depending on the time the chemical is applied. 

  

It should be noted that although the majority of herbicides may be classified as preemergence or postemergence some, such as Princep, are notable exceptions. They are used as both preemergence and postemergence herbicides. Remember when using any pesticide to read, understand and follow the label found on the container.

  

Preemergence herbicides

  

Preemergence (PRE) herbicides are applied to the turfgrass site prior to weed seed germination and form a barrier at, or right below, the soil surface. Most preemergence 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 preemergence herbicides because their primary growing points escape treatment.

  

Postemergence herbicides

  

Postemergence (POST) herbicides are applied directly to emerged weeds. In contrast to preemergence herbicides, this group of herbicides provides little, if any, soil residual control of weeds. A complete chemical weed control program can be accomplished with postemergence herbicides, provided multiple applications are used throughout the year. However, due to the necessity of repeat applications and temporary turfgrass injury, most turfgrass managers use postemergence herbicides in conjunction with a preemergence weed control program.

  

Postemergence herbicides are useful to control perennial grasses and broadleaf weeds that are not controlled by preemergence herbicides. Certain postemergence herbicides also may be used on newly established turfgrasses.

  

When you have questions about weed control, call the Dawson County Extension at (706) 265-2442.

  

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

 

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