Controlling broadleaf weeds in cool season lawns

Controlling broadleaf weeds in cool season lawns

While a good pre-emergent program will keep most annual weeds out of your lawn, perennial broadleaf weeds like dandelions, white clover and buttonweed will be unaffected and will need to be removed using a selective post-emergent herbicide.

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Post-emergent herbicides come in a variety of forms.  Weed and feed type products are the worst choice, because:

  • The timing for fertilization and weed control is never the same.
  • It must be applied to the entire lawn whether there’s weeds or not to avoid differences in color caused by some areas being fertilized and some not
  • The granules have to stick to the weed making them ineffective.

For these reasons we will skip over weed and feed  products and talk about standalone liquid and granular herbicides. Most contain the same active ingredients:

  • 3-way herbicides contain 2,4D, mecoprop and dicamba. These herbicides will control a very large number of broadleaf weeds like dandelions and clover. They are cheap and very effective, making them an excellent first choice when dealing with broadleaf weeds. Examples are Ortho Weed-B-Gon and Trimec.

When spraying broadleaf herbicides, the following rules should be observed:

  • Always read the label and follow all instructions exactly to avoid damage to your lawn.
  • If you only have a few weeds in your lawn simply spot spray rather than applying to the entire lawn.
  • Apply just enough to wet the leaf.
  • Do not apply to lawns which are stressed, damage may occur.
  • Do not apply to new turfgrass seedlings until after the grass has been mowed at least three times.

Herbicides are most effective when weed are actively growing – a dormant weed will not absorb the herbicide. A dose of fertilizer followed by a few days of watering can help bring the weed out of dormancy, at which point it can be sprayed. Adjuvants such as an non-ionic surfactant  greatly increase the effectiveness of your herbicide application by breaking water surface tension and helping the spray droplets stick to the leaf instead of beading up and rolling off. Even under ideal conditions, repeat applications every 10 days may be needed for some stubborn weeds like ground ivy.

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