Can Hardwood Mulch Burn Your Plants?

Burning the Roots

• All plants need nitrogen to survive and grow. If there is a nitrogen deficiency in the soil, the plant does poorly. However, too much nitrogen it may kill the plant. All fertilizers are based upon chemical salts, even natural ones. Salt draws moisture to itself. If excessive nitrogen salt in the soil drains the moisture from the plant roots, then the plant is "burned." Once this happens, the plant is usually damaged beyond recovery. Hardwoods are very poor in nitrogen and add minimal amounts to the soil.

Hardwood Mulch and Nitrogen

• Hardwood mulch eventually decomposes when on the top of the soil. The carbon in the wood interacts with available nitrogen in the soil and the trapped moisture under the wood, giving the soil microbes the perfect environment. This creates the opposite effect of having too much nitrogen in the soil to burn the plant roots, as the decomposing wood is using the nitrogen for itself. It is impossible for the hardwood mulch to burn the plants.

Gradual Decomposition

• Some gardeners fear that wood mulch draws too much nitrogen from the soil and leaves the plants with a deficiency. According to the Colorado State University, wood mulch on top of the soil only uses the nitrogen at the very top level, leaving the rest for plant roots. The university cautions against tilling wood mulch into the soil, as that creates a nitrogen deficiency in several years as the microbes decompose the entire shredded wood instead of just a small portion in contact with the soil. Since there is a nitrogen deficiency, then the wood mulch has the opposite effect of burning plants.

Use of Wood Mulch

• Replace wood mulch periodically since the gradual decomposition of the lower level eventually causes it to slowly sink into the ground. A 2- to 3-inch covering is best. Adding more mulch than necessary blocks the soil from getting the proper amount of water and oxygen. Wood mulch can be either hardwood or softer woods, which do not last as long as the harder woods. Soft woods also do not burn plants for the same reasons that hardwood do not.