Given the benefits of properly mulching your trees you may be surprised to hear that mulching can cause problems. The problems can be divided into two categories: composition and application.
Typically mulch is composed hardwood chips, carefully shredded into uniform pieces. Mulch that is not properly shredded contains large chips and many twigs that make it difficult to spread and unsightly. This may this may be suitable for less formal areas of your landscape, or in your wood lot. Chips from fine textured trees like willows will have numerous fine twigs. Sharp blades in the chipper will usually solve the problem of stringy chips.
If the mulch is composed of pine, spruce or other soft woods, as it breaks down it will make the soil more acidic. This can be a problem if your trees and shrubs prefer a more basic soil. This can be of benefit if you have plants that thrive in acidic soils. Mulches made from rocks and gravel are ground covers and don’t provide the same benefits as wood chip mulch. Limestone crushed into mulch sized pieces will make the soil become very basic and damage your plants.
Some trees produce chemicals that eliminate the competition with natural chemical herbicides. Sugar maple, hackberry, cotton wood and black walnut are a few of the worst offenders. Allowing the mulch to sit in a pile for a season reduces the concentrations of these chemicals. You should avoid putting these materials directly into your vegetable garden. Tomatoes and some herbs are particularly sensitive to these natural herbicides. If your mulch is composed of sawdust or newspaper it will steal nitrogen from your trees as it decomposes. Composted mulch is best.
Beware the Mulch Volcano! Many people make the mistake of piling mulch too high against the base of the tree. Mulch that is applied too deeply can cause rot on the bases of trees. Mulch should not be in contact with the bark at the base of the tree. If this happens, the root flare area will rot or sprout adventitious roots both which injure the tree. Mulch should be 2 to 4 inches away from the bark of the tree to allow for air movement. The mulch should then be an even 3 to 4 inches out to the drip line. If you are putting plastic or weed barrier under your chips you will be excluding oxygen from the roots of the tree and suffocating the roots.
Commercially made mats of compressed mulch look very nice. These mats often maintain there shape and don’t decompose or allow the tree to grow. This can result in bark damage. Mulch that has been used to soak up road debris may have excess salt. If you have an area where salt splash or spray is a problem change your mulch after each winter to prevent the salt from leaching into the soil. Refresh your mulch as needed and don’t allow it to burry the plants. Mulching is easy and effective if you know the basics and understand the benefits it can provide. When in doubt ask an ISA Certified Arborist, they will have the latest information on mulching.