Excess mulch reduces the soil's oxygen to the roots, stifling them and causing them to die. Roots in search of oxygen and water sometimes turn into excess mulch. During dry periods, the mulch dries out and the roots of the mulch die. So can too much mulch kill plants? Yes, too much mulch can kill plants, especially if it is piled up too close to the base of the plants.
Mulch that is too thick can stifle roots, overheat soil in hot, sunny weather, and promote illness due to excess moisture. Plant roots need oxygen to survive, and a very deep layer of mulch can limit supply. In addition, fungi can become a problem when the mulch layer is too thick, says Sam Schmitz, land supervisor and horticulturist at The Gardens at Ball. Fungal mats can form and, in fact, repel the water you are trying to conserve by applying mulch to begin with.
An inch of mulch is sufficient and cheaper. When wet mulch comes into contact with the coating, it creates a path for termites and other pests to use to reach your home. Baka says it's OK to use mulch against a concrete wall, but keep it at least 6 inches away from any type of wood or wood structure. While I was planting, I was also putting in some mulch to help my plants grow.
I started thinking about how other gardeners use mulch and thought I should publish my own ideas. As with all other mulch installations, be sure to add only what is missing to the depth of the mulch. Yes, too much mulch can kill plants, especially when the mulch is too close to the base of the plants. A layer of woody mulch also makes it more difficult to quickly cut weeds between the rows, Baka adds, noting that it must first be removed, the weeds removed, and then the mulch replaced.
The same moisture that is retained with the right amount of mulch grows exponentially when too much mulch is installed. Excessive use of the same type of mulch for a long period of time can cause changes in the pH of the mulch. If you already have 3 to 4 inches of mulch on the ground, it's not a good idea to keep adding more (see the guidelines above, depending on how fine the mulch is and how well the soil drains). Make sure to avoid the dreaded mulch volcano by installing mulch around trees (more info in the next paragraph).
Placing too much mulch can cause too much moisture to be retained, either in the soil or in the mulch. Also avoid accumulating mulch in the center of shrubs or shedding thick layers of mulch on perennials. Denver horticultural consultant Jim Borland writes a fall issue of American Nurseryman, says that while it's probably a good idea to remove or remove mulch from perennial herbaceous plants in spring to allow new growth to emerge, in areas likely to be affected by late frost spring, it would be just as good to leave the mulch in place. If you have beds where mulch has mixed with soil, rake to ground level and be sure to plant in the soil rather than with mulch.
Similar to preventing mulch from coming into contact with your home, leave some space between the mulch and the trunk of a tree. You'll also need to refresh the mulch by adding more as it dilutes, as the mulch will decompose, settle on the soil, and some may even disappear.