Not sure where you live but for rocky hillsides in zones 5 to zones 7 there are low growing phlox, euphorbias and creeping thymes that may survive. Look for rock garden plants many with a parentage that is from native alpine wildflowers. Candytuft or Iberis is one example. Euphorbias that can reseed along with the Corsican hellebore that has a green flower may work. Some Mediterranean herbs survive rocky hillsides so the thymes, rosemary and salvias are other possibilities.  I suggest you try some different plants this spring, see how they survive the summer than add more of what does best. Blooming "weeds" can also be called wild flowers so try to notice what grows naturally in your area that also has a bloom. You will need to dig hole, remove the rocks then add pockets of soil to get the new plants established. Keep growing, Marianne Binetti