Do you live in a warm climate? Rocks will absorb heat and can bake the roots. They can also block water so that the roses suffer from drought. It is also possible that the roses are suffering from something else - mole damage, herbicide drift, black spot or mildew. A better mulch for roses would be an organic mulch of wood chips, compost or shredded bark. One more thing - roses are heavy feeders. I use Osmocote on all my roses, especially roses in pots every June once the weather heats up. Then I also water the roses with Miracle Grow after the first flush of blooms to encourage more flowers. Early in spring a mulch of compost will help to make all the nutrients in your soil more available to the rose plants. As for the weeds, remove all weeds in early spring to keep them from going to seed and top off your compost with wood chips or bark chips to stop new weed seeds from blowing into the rose garden. Use the river rock to make a dry stream bed or to edge a garden bed instead of on top of the roses. Keep growing, Marianne Binetti