Weeds generally appear when the grass is weak. Weeds can be from many factors such as under watering, mowing to short or even weed seeds blown from an unmanaged field nearby or weed seeds in your soil before installation.
Broad-leaf weeds such as dandelions, ground ivy, clovers, plantain, black medic, and thistles can be eliminated by treatment with post-emergent chemicals. Consult your local home and garden center. No matter the method of application, chemicals are most effective when weeds are growing vigorously. For difficult to control weeds, a second application may be necessary two or three weeks after the first. Always follow package directions carefully. Check with your local environmental bylaws to ensure it is permitted in your area.