Spurge loves a thinning lawn that is mowed too short. These two pests are annuals, meaning they start from seed every spring or early summer, and die out each winter with the first freeze. They are most troublesome in the middle of the summer, but germinate and start growing in the spring. Goodspeed says a pre-emergent herbicide helps reduce these two most common weeds. These selective herbicides are applied to the ground before the weeds emerge only killing newly germinating weeds and allowing most established plants to continue growing.
Rhizomes have enough energy to push through the pre-emergent.
Established weeds such as dandelions, clover, and perennial grasses, are unaffected. Most pre-emergents need to be applied in April, and then again the first of June, Goodspeed says. Driveways, sidewalks, and other man-made structures and surfaces tend to warm the surrounding soil quicker than other areas of the lawn. I have seen extreme problems in some lawns where the weeds congregate around these areas. A pre-emergent may need to be applied sooner to these areas prone to early warming.