A mulch of large rocks will be better for blocking weeds but not as good for nourishing the plants. After a few years a rock mulch will also collect leaves and debris and weeds can grow. Use a rock mulch around heat loving plants and away from falling leaves. A rock mulch also works near driveways or where you have water runoff as the water can pass through the rocks. A mulch of compost or bark will be better under rhododendrons, clematis and other shrubs that like cool roots. You can also plant groundcovers such as ajuga in a bark mulch to create a living weed block. A mulch of small rocks or gravel will become a breeding ground for small weeds after just a year or two. Larger rocks at last he size of a walnut are better for blocking weeds. Keep growing, Marianne Binetti