Depends on the weed and the root. For example, if it's a horsetail root you might as well give up! If it's a dock, dandelion or a teasel, this will grow back from just a tiny bit of root left in the ground. The best thing to do is wait until there is a good piece of new leafy growth and spot apply glyphosate (Roundup) on the leaves. That way you don't have to spray all around and the weed will die back from the roots. Don't expect instant results as glyphosate takes a week or two to act.