identifying and eliminating two similar lawn "weeds" that take over lawn in July

Asked September 21, 2020, 10:16 PM EDT

I have two "weeds" that take over my lawn starting in July. Weed-1 has wider leaves and a slight curl. Weed-2 has finer, straight leaves.They both have spread into large patches in my lawn. Please help me identify them from attached pictures. What is the most effective way of preventing them from coming back next year? I have tried over-seeding and fertilizing in an attempt to crowd them out but have been unsuccessful over the last two years. I am not opposed to using a weed killer, or pre-emergent, if it would be effective. I have not performed a recent soil pH test if you think that may be a factor.

Howard County Maryland

1 Response

A soil test is recommended if you have not had one done in the last several years. Results will give pH, liming, and nutrient deficiencies. Here is our page on soil testing which includes a list of regional labs, our video, and FAQ's. https://extension.umd.edu/hgic/topics/soil-testing

The above weeds are difficult to control and require different methods.
Left and middle photos looks like Japanese stiltgrass. This a foreign invasive grass and is becoming widespread in forested areas and is now showing up in home landscapes and lawns. In natural areas, it is outcompeting native plants.
In the lawn it is best handled with a crabgrass pre-emergent applied before you would normally apply it for crabgrass in spring. Here is our webpage and tips for management in lawns. https://extension.umd.edu/hgic/topics/japanese-stiltgrass

Right photo - The weed looks like nimblewill. This is a native perennial warm season grass. It thrives in the shade in infertile soil but will move into sunnier areas.
Small infestations can be dug out or sprayed with a non-selective herbicide and then reseed. Another option is a selective herbicide with the active ingredient mesotrione (Tenacity). It is most likely too late this year as this product requires several applications and then reseeding. See our webpage for management in lawns including options for chemical controls.
https://extension.umd.edu/hgic/topics/nimblewill

Here is more on lawns and overseeding https://extension.umd.edu/hgic/topics/lawn-renovation-and-overseeding
and fertilizing https://extension.umd.edu/hgic/topics/lawn-fertilizing
Marian