Planting wild flowers on a rocky slope will work in some climates. The size of your river rock and rainfall will determine what will grow. Your best bet is to contact a local garden club or nursery about native wild flowers and alpine plants in your area. A mix of low growing shrubs, evergreen groundcovers and bulbs mixed with native wild flowers might be the most practical approach. Growing all wild flowers still takes a good amount of hand weeding to keep invasive plants under control. Keep growing, Marianne Binetti