Mulch is used in the landscape to control weeds, conserve moisture and protect the root system of plants from extreme temperatures. Many different materials are considered mulch, from black plastic to gravel and shredded tires. While they all serve the same purpose, organic mulches, such as shredded bark or chipped wood, are the friendliest to the environment and tend to look most natural. As they decay, they enrich the soil and provide nutrients to the plants. However, unsightly annual weeds often grow through organic mulches. You can control these weeds by applying a preemergent for weed control. Buy a preemergent herbicide that can be applied around perennial plants. It will say on the package if it is suitable for placing around perennials. Some preemergent herbicides can damage perennials, including trees, so it is important to look on the label for warnings. Remember, preemergent means it keeps all seeds from germinating for a certain period of time, usually three to four months, and that includes desirable flower and plant seeds.
Rake the layer of existing mulch out of the flower bed. Pile the mulch up or rake it over to one side, treat one side of the flower bed, then move the mulch to the other side. While you can apply preemergent over the top of mulch, this procedure is not recommended for most preemergents because you cannot get even coverage.
Spread the prememergent over the soil surface in the flower bed according to directions. It is important to not over apply or you can kill the perennial plants and trees.
Spray water over the treated area to wash the preemergent into the top layer of soil. Do not spray water to the point of runoff because you want the preemergent to stay in place.
Replace mulch or add new mulch as needed around the plants and trees. A 2- to 3-inch layer of mulch is usually effective against the reemergence of weeds. You may need to reapply the preemergent for complete control, depending on the product.