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Rock mulch is a layer of crushed rock, pea gravel, or small stones that is spread over an area in landscape. The rock layer acts to control weeds while creating an aesthetically pleasing visual layer. Mulch layers are also useful for controlling moisture retention and temperature in the soil around plants, shrubs, and landscape trees. Rock mulch is considered an inorganic mulching material that has several advantages and disadvantages when compared with organic mulch materials.
When used around plants, rock mulch is best suited for areas around landscape shrubs and trees, in cactus or herb gardens, and in specialty gardens, such as Japanese or alpine-themed gardens. A layer of sturdy landscape cloth is spread over the soil around the plants before the rock mulch layer is applied. The landscape cloth helps to prevent weeds from growing up through the rocks. When using rock mulch around plants, heavy plastics generally should be avoided. Plastic prevents water from entering or escaping the soil, creating a very dry or very wet environment, both of which can harm or kill established plants.
In unplanted areas, rock mulch is used to cover the soil, prevent muddy conditions, and discourage weeds from growing. A thick layer of heavy black plastic spread over the area before the rocks are put in place also can help prevent weeds from growing up though the rocks. Walkways, driveways, and areas around large ornamental stones or sculptures are suitable places for a layer of heavy plastic and rocks.
One of the advantages of rock mulch is that it will not blow away in windy areas, as straw, wood chips or sawdust is likely to do. Another advantage it is that it does not break down over time, as is common with organic mulch materials. Though the initial preparation and installation is more labor intensive than organic mulch, it does not need to be replaced every year.
There are several disadvantages of landscaping with rock mulch, however. Rock mulches attract and hold heat after the sun goes down, increasing the evening temperatures around plants and outdoor living areas. Many types of trees and shrubs can suffer from the increased heat that is absorbed and reflected back from the mulch layer. Rock mulch is not generally suitable next to lawns, unless a border is put in place to prevent the rocks from getting scattered onto the grass and creating a hazard when mowing. Rock mulch also is not usually suitable around annual flowers, in vegetable gardens or around herbaceous perennials.
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