Post Emergent Weed Control





Post emergent weed control is
one of two categories
of chemical herbicides that kill weeds.

The other is pre-emergent.


Click to view the big overview
outline of herbicides.

Selective herbicide


These are herbicides that are applied to weeds growing above soil.


The Action process falls under two categories - Systemic and Contact
They are applied directly on the weed breaking down it's tissue.  There are both Selective and Non Selective Herbicide.

This page details Selective Herbicides.


Systemic


Systemic herbicides work by the chemicals first being sprayed on the weed. The weed absorbs chemicals and is transported throughout the tissue, roots and all.

These chemicals attack only the weed that they are placed on and not threatening any surrounding vegetation.

The process is not instant. Depending on the chemical used, it can take hours up to a week for the weed to break down. It can be effective on both annual and established perennial weeds.  This systemic process uses selective herbicides. 



     a) Selective Herbicides

           (usually non-residual)

  • Selective herbicides kill specific types of weeds. This works by sourcing the different hormones found in each weed group type.

    When the chemicals are applied they block the weed's hormones. This will then control, or inhibit normal growth, ultimately killing the weed.

    Because these chemicals are limited to specific weeds they are also Non-Residual. Non-Residual means that the chemicals don't percolate beyond the weed keeping surrounding soil healthy rather than sterilizing it. Residual are under the non-selective herbicide page.

The weed group types that selective herbicides target
are Dicots, Monocots and Sedges.

      1) Dicots (dicotyledonous)

      Dicots and broadleaf plants emerge from two major groups of angiosperms. An angiosperm is a seed producing plant. 'Di' means two, forms two leaves when emerging from the seed. These are more often termed Broadleaf Weeds.

      A dicot tends to be oval shaped and flat. Leaf veins branch out and have one long central tap-root that travels deep into the ground.

      Examples of Broadleaf weeds are:

        Annuals
        • Common Chickweed
        • Knotweed


        Perennials



      2) Monocots (monocotyledon)

      Monocots are weeds that emerge from the seed having one leaf, 'mono' meaning one. They are produced one at a time and can be lawn grasses and grassy weeds (Poaceae family/formerly known as Gramineae). Monocots are both annual and perennial weeds.

      A monocot has short fibrous roots that are closer to the soil's surface. Leaves usually are long and narrow having a blade-like appearance with veins that run parallel.

      Examples of these weeds are:
        (Monocot weeds are often mistaken for grasses)




      3) Sedges

      Sedges are different weedy grasses than Monocots.

      Sedge grass stems feel round or triangular and have three rows of leaves growing out from them. Their leaves are solid and jointless and the flowers at the top are spike-like in appearance.

      Sedges also like to grow in soils that are rich with moisture.

      Examples of Sedge weeds include:

        • Beak Rush
        • Kyllinga
        • Purple Nutsedge
        • Texas Nutsedge
        • Star of Bethlehem
        • Yellow Nutsedge
        • Wild Onion

      If you are unsure of which type of grass you are dealing with here's one big tip... a disappointing tip, but still a tip:

      Herbicides used for the Monocot grasses will
      NOT be effective on Sedges.


      Depending on the sedge grass, they are not always considered a weed. Many times they look beautiful in your garden used as an ornamental grass.




Even with the use of selective herbicides it should be noted that this method of post emergent weed control has drawbacks...

Many times these targeted weeds can grow back larger and more challenging. It is recommended that you rotate different chemicals rather than using the same one over and over again.  Please note, however, soils lose important microbes while weeds are becoming more resistant to these chemicals.

Also, it has been proven that post emergent weed control is reducing pollinator numbers because of the chemicals (especially glyphosate) used in these over the counter products. 






Related Articles:

         Soil Testing Methods -
               Soil also matters when it comes to control - both
               pre-emergent and Post emergent weed control.





Back from Post Emergent Weed Control to How to Kill Weeds


Site Map:  Butterfly Website


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