Kochia is a common annual weed that is often found in waste areas and in bare patches in lawns. Killing young Kochia plants is not difficult; any broadleaf herbicide should do the trick. The only way to keep it out is to help the lawn grasses fill in bare spots. The Kochia isn’t actually taking over your lawn as much as it is filling in the bare ground that the grass is not covering. Proper watering, mowing, and fertilizing, possibly combined with some overseeding, should make your lawn grass out compete most annual weeds and many of the perennial weeds.