When Good Mulch Goes Bad

How to tell when your mulch might cause more problems than good for your trees and shrubs.

Proper mulching provides outstanding health benefits to trees and shrubs. It helps retain soil moisture, regulates soil temperatures, and improves soil conditions.

Despite these benefits, if mulch is installed improperly or left unattended, it can cause damage to your trees and shrubs. The problems described below are easily remedied but do require observation over time to ensure that your trees stay happy and healthy. Here is what to look for:

Mulch volcanoes happen when mulch is mounded around the base of a tree. Mulch placed too close to the trunk of the tree can damage the tree, making it more susceptible to attack from insects and disease. To prevent this damage, be sure no mulch is installed within 2-4” of the base of the trunk.

Hydrophobia, or water-repellant mulch, occurs when mulch is applied too deeply and allowed to sit too long without being turned over. Because this condition does not allow water through, it endangers newly planted trees and shrubs by depriving the root balls of water. During the summer, fresh woody mulches become hot from decomposition, which dries out the mulch. Fungi then colonize these dry, dusty mulches, making them thick and matted. This can cause water to run off to the sides.

To prevent hydrophobia, break up the crusted hydrophobic layers and refresh the mulch by turning it over. The best tool for this job is a potato hoe. Be sure not to dig too deep into the soil or use a pickax because that could potentially harm the roots.

Diseased or infested mulch Occasionally, mulch can harbor unwanted pests, mold, or fungi, some of which are harmless. Others, however, can cause minor problems, so it’s good to keep an eye out for them. Here are a couple of things you may encounter in your mulch and guidance on what to do:

  • Slime mold: this mold initially appears on top of mulch as brightly colored (orange, yellow or red) slimy masses that may be several inches to more than a foot across. The mold eventually dries and turns brown, sometimes leaving a dry, white powdery mass. Slime molds are a temporary nuisance and are harmless. Either let them be or break them up when you turn over your mulch.
  • Artillery fungus: harmless to your plants, but can stain your siding, cars, and other surfaces. The fruiting structures can shoot spore masses up to 20 feet. The spores look like specks of tar and can be very difficult to remove. To prevent this fungus from growing, refresh your mulch every 6-12 months. Also, consider using pure bark mulches or covering other types of mulch with a thin layer of pine straw. If you happen to see this fungus, no need to panic. Simply remove it before the spores land on nearby surfaces.

Sour mulch Sometimes mulch can start to smell like vinegar or sulfur, which is an indicator that it has become sour. The smell is created when a wood-derived mulch is piled high and the inside portion of the pile is deprived of oxygen. This causes anaerobic activity, which creates a build-up of acetic acid. If the mulch is spread on the landscape without treatment, the volatile acid will quickly cause plants to wilt and subsequently die.

To remedy sour mulch, spread it out thinly (only a few inches deep), soak it with water, and allow it to dry. After a few days of airing out, the smell should be gone and the mulch will be safe to spread around plants.

What is anaerobic activity?

Anaerobic activity is the process of microorganisms breaking down organic matter without the presence of oxygen. In mulch, the byproduct created by bacteria is called acetic acid which is toxic to plants.

While all of these problems are fairly common, they are also readily preventable and easily remedied. The benefits of mulch far outweigh these few and far between problems, but it’s still important to be mindful of them.

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