Spring is the focus of spraying a lot of weeds. Chickweed, in particular can be very persistent. It often take 2-3 applications of a herbicide to kill the weed. There is a lot of labor dedicated to spraying broadleaf weeds this time of year. Pre-emergent herbicide are used to control grassy weeds like crabgrass and goosegrass. However, broadleaf weeds like clover etc are sprayed post-emergently.