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Growing Concerns: What are the worst lawn weeds?

May 5, 2019

“What is this weed in my yard and how do I get rid of it?”

This is a recurring question that homeowners ask every year. Each time I get the question, I recall a time early in my career here in Rochester when a lady asked me this over the phone. In a weak moment my Irish wit took over and I responded: “I don’t know, I can’t see it from here.”

After what seemed like a very long pause, the lady started laughing and said she should have known better because she had some horticulture training.

Since then, I have avoided the urge to make such a candid response to this question, but considering all the possibilities it would still be the most accurate answer without at least a good description or image of the weed. Herbicides and other weed control options have specific ways of working or “modes of action” that work on the biology of the weeds they are designed to control. If a herbicide is not effective on the specific weed, control efforts just won’t work.

When it comes to lawn weeds, not all of them are created equal. Annual weeds like crabgrass are easily controlled culturally by maintaining a vigorous lawn and raising mowing heights during the summer. They are also easily controlled with pre-emergent herbicides. If you do use pre-emergent herbicides they can be applied in early May.

Perennial broadleaf weeds vary in how easy they are to control, but all can be managed selectively with herbicides that are effective on them but safe on grass plants. Selecting the correct herbicide is still important to get good control of some of the tougher broadleaf weeds.

Some yards may require different products for different areas to manage the diversity of weeds that may be present. Most of the persistent perennial broadleaf weeds are cool-season plants that are best controlled in early spring or fall. Fall is also when lawn grasses produce new daughter plants through vegetative means allowing grass to quickly fill in voids left by dying weeds. These voids are often filled by other weeds after spring or summer weed-control efforts.

The worst weeds, however, are the cool-season grass plants such as quackgrass, because they have very similar biology as our cool-season lawn grasses. With few exceptions, these weeds need to be treated with herbicides that will also kill the lawn grass. Again, early fall (early September) is the best time because this is also the best time to re-seed areas that need to be treated.

One of the most common errors homeowners make is to misidentify quackgrass as crabgrass, in which case they often treat repeatedly and unsuccessfully with applications of pre-emergent crabgrass herbicides.

If you are dealing with troublesome lawn weeds, it may be necessary to contact a lawn care company that provides weed control services. They should be able to identify most lawn weeds and will have appropriate chemical products to control them.

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