Stubborn weed in lawn
I have a slowly extending zone in my lawn with a very resistant weed. It doesn't seem to respond at all to a broadleaf weed killer that includes post-emergent crabgrass coverage. It partially responds to Tenacity, with some initial whitening, but then it recovers, even after repeated applications a couple of weeks apart. This weed emerges in the spring and persists through the fall. I'm attaching 3 photos, and I'll appreciate your assistance!
This looks like Japanese stiltgrass, an invasive grass. Japanese stiltgrass is a foreign invasive plant that is wreaking ecological havoc all over the Eastern United States. Research is ongoing. We were swamped with questions about Japanese stiltgrass last year, probably because weather conditions were optimal for it and it is increasing exponentially all the time.
Pull it out of your beds before it goes to seed in late summer/early fall. (Keep an eye on the tips of the blades, where you will see the seedheads emerge.) If you can't pull it all, spray or cut down the plants before the seeds mature.
In the lawn, use a crabgrass pre-emergent herbicide in early spring (early to mid-March). Since it germinates before crabgrass the pre-emergent should be applied a couple of weeks before you would apply it to control crabgrass. Here is our webpage (be sure to read the links, too)
Seeds of Japanese stiltgrass stay viable in the soil for up to 5 years, so one big key is to keep seeds out of the soil. If you have a mower that can bag clippings, use it when the stiltgrass is going to seed late August to early September. The rest of the year, we don't recommend bagging clippings. Also, be sure that the mower does not shoot clippings into beds or natural areas.
Keep up with lawn care best practices to achieve a healthy, dense turf (soil test, fertilize in the fall, mow high, etc.). https://extension.umd.edu/hgic/topics/best-practices-lawn-care
You have to decide how much you want to do to keep battling this grass. Here is information about managing Japanese stiltgrass in a lawn. https://extension.umd.edu/hgic/topics/japanese-stiltgrass
In sum, there are no easy solutions to this plant that has become highly invasive and widespread. In some case, people are just accepting it in their lawn because it is just too much/too difficult to control effectively. If you have deer in the area, they are effective dispersers of the seeds (carried on their fur).
Here is additional information from Rutgers University. https://njaes.rutgers.edu/fs1237/