Percentage of Soil to Mulch

Not Percentage but Depth

• The amount of mulch in a garden should not be calculated in relation to the amount of topsoil in the garden. Whether you have 2 inches or 6 feet of topsoil before reaching the subsoil layer, the amount of mulch is the same. What does make a difference mulch's depth is primarily based upon the type of mulch.

Light Mulch

• If a light mulch is used, such as hay or leaf litter, a layer 6 inches deep of mulch is appropriate. These light materials quickly compress when saturated with water or stepped on. If smaller quantities are used, weeds will still break through the barrier. It is hard to overmulch when using hay or leaf litter.

Dense Mulch

• Do not to overmulch with denser mulches. These include most commercially available mulches, such as pine straw, pine bark and cypress mulch. A 2-inch layer is fine for all of these mulches, regardless of the plants being mulching or time of year. Two inches of dense, carbon-rich material is the most that can be decomposed into organic matter in a year. For this reason, 2 inches is the best depth. There are other consequences of thinner or thicker mulch quantities.

Consequences

• Overmulching can have drastic consequences for your garden. While an extra-thick layer of mulch may help keep root systems warm and safe from frost in the deep winter, it can quickly cause a root rot epidemic in the spring. Mulch's ability to retain water in the soil is generally considered beneficial. If a mulch layer is too thick, no water escapes the soil, and the plants' roots quickly rot in oversaturated soil. If you mulch too thinly, the mulch simply won't do its job. Weeds break through the mulch layer, water rapidly evaporates from the soil and the plants' root systems won't be properly insulated.