GARDENING: Perennial grassy weeds in lawns are noticeable

They show up as brown patches once grass starts to turn green.

Lawns are starting to green up and I have noticed people are beginning to mow the lawn. Good cultural practices in lawn care leads to a reduced use of herbicides. Thanks to reader Timothy Davis for pointing out that this was a bit confusing.

Last week I mentioned that you should hold off using pesticides on the weeds in May to allow dandelions to bloom for the bees. I should have said herbicides since I was referring to weeds. Let me explain.

I tend to use pesticides because it is the umbrella for all products used. Cide means to kill and of course, we know what pests are. However, many perceive pesticides are used only for bugs and insects only, but this is not the case.

Pesticides is the umbrella term – there is a variety of categories of pesticides under this umbrella. For instance, fungicide is for diseases, insecticide is for insects, rodenticide is for rodents, and so forth.

So yes Tim, in this case I should have referred to herbicides, as these are used to kill soft green tissue such as weeds. Thanks for the feedback. I always appreciate readers keeping me on track!

One herbicide that may be considered at this time is glyphosate for nimblewill and bentgrass. These grassy perennial weeds are still slightly brown in the green lawn. If you notice grassy areas that are or were brown when the lawn started to green up, it’s likely one of these.

In a thin lawn, this grassy weed tends to spread and take over. Keeping the lawn thick through proper mowing and fertilization helps to prevent spread.

Using a herbicide such as glyphosate is one way to eliminate nimblewill and bentgrass in the lawn. Glyphosate is a non-selective herbicide that kills anything green. Therefore, yes, it will also kill the good grass as well.

ExploreA Force for Good: New USAF museum exhibit highlights humanitarian missions

However, because nimblewill spreads, you need to spray out further than the brown patch to get any grass that is coming up from the rhizomes (underground spreading stems).

In addition, because of these rhizomes, you may not get all the nimblewill. Repeat treatments may be necessary.

You can also dig up these spots, ensuring that you get all the rhizome at the edge of the brown stuff. Or use a combination.

A very important note if you are going to use an herbicide – don’t spray until the nimblewill or bentgrass is green! If it’s brown, you waste time and money.

Once you are confident the weed is gone, you can reseed safely according to the label. Again, get the lawn thick to prevent and choke out any additional rhizomes.

Back to the spraying of pesticides, don’t spray when bees are actively pollinating dandelions and other blooming weeds in the lawn. Spray early in the morning before they come out.

Dandelions, as you may have noticed, still bloom even though you mow. They can grow short stems that are missed by the mower. Wait until they are at the seed stage or until fall if you want to eliminate dandelions.

And remember, it’s not NO Mow May but rather NO Spray May!

Pamela Corle-Bennett is the state master gardener volunteer coordinator and horticulture educator for Ohio State University Extension. Contact her by email at

About the Author