Invest in a weed preventer that works, like the GrazonNext 2 gal. HL Specialty Herbicide Concentrate. This weed killer is designed to be a broadleaf weed prevention product that will kill more than 100 different types of weeds, including the more difficult-to-kill perennials. With a post-emergent design, this herbicide concentrate covers up to 5 to 10 acres with a longevity of 2 to 4 weeks for long-lasting use.