If the weeds are present in an established flower garden, there isn't any easy way to remove them selectively. Hoeing out the weeds may be easier on your knees and back than pulling them. The best thing to do, however, is to prevent the weeds from becoming established in the first place. For annual weeds, putting down a weed-preventing herbicide such as Preen (trifluralin) can help for certain flowers. (Make certain the ones you are growing are on the label.) A layer of mulch 2-4 inches thick also helps prevent weeds from sprouting.