Hello, 

Thanks for reaching out! Here is a helpful UMN Extension article on crabgrass: https://extension.umn.edu/weeds/crabgrass 

After early July, treating crabgrass with herbicides is ineffective and not recommended. You could try removing small infestations by hand or raising your mowing height to reduce crabgrass growth. 

Otherwise, it is suggested to apply pre-emergent herbicides next spring before soil temperatures reach 55°F. More information can be found in the link under the "Herbicide Control" section. Always follow the pesticide label directions attached to the pesticide container you are using - remember, the label is the law.

I hope this helps!