Mulch Volcanoes: Why They’re Bad for Trees

If you’ve taken a walk through many suburban neighborhoods or commercial landscapes, chances are you’ve come across what appear to be young trees erupting from mounds of mulch. These mounds, resembling miniature volcanoes, have given rise to the term “mulch volcanoes.” Although they may seem harmless or even aesthetically pleasing to some, mulch volcanoes can pose a significant threat to tree health.

What Are Mulch Volcanoes?

Mulch volcanoes refer to the excessive piling of mulch around the base of trees. Instead of spreading out evenly across the ground, the mulch is heaped around the tree trunk, creating a steep mound. This phenomenon is particularly prevalent among young trees in landscaped areas.

While mulch, when used correctly, can provide numerous benefits to trees and soil, over-mulching can lead to several problems.

The Problems With Mulch Volcanoes

1. Root Rot and Fungal Diseases: Excessive mulch around the trunk creates a constantly moist environment. This can lead to the bark becoming soggy, making the tree susceptible to fungal diseases and rot.
2. Insect Infestation: The wet and sheltered environment created by mulch volcanoes provides an ideal habitat for pests, such as beetles and termites, which can harm the tree.
3. Girdling Roots: Trees surrounded by deep mulch can sometimes grow secondary roots into the mulch. These roots can encircle the trunk and constrict the tree, a phenomenon known as girdling. Girdled trees can suffer from reduced nutrient and water uptake.
4. Thermal Stress: Mulch has insulating properties. While this is beneficial in moderation, excessive mulch can cause the underlying soil to retain too much heat or cold, stressing the tree’s roots.
5. Deprivation of Oxygen: Roots require oxygen for respiration. A thick layer of mulch can suffocate the roots by blocking the exchange of gases between the soil and the atmosphere.

Proper Mulching Techniques

To harness the benefits of mulch without causing harm, it’s essential to employ proper mulching techniques:

1. 2-4 Inch Rule: Ideally, mulch should be spread in a layer of 2 to 4 inches deep. This provides adequate insulation without causing suffocation.
2. Donut, Not Volcano: Instead of piling mulch against the tree trunk, spread it in a donut shape, leaving a gap around the base of the tree.
3. Choice of Mulch: Organic mulches, such as shredded bark or leaves, are often preferred as they decompose over time, enriching the soil. Avoid using fine mulches like sawdust, which can compact easily and hinder air movement.
4. Annual Maintenance: Check and replenish mulch annually. Also, ensure that old, decomposed mulch isn’t building up to harmful levels.

The Benefits of Proper Mulching

When done correctly, mulching can:

1. Retain Soil Moisture: Mulch reduces the evaporation rate from the soil, helping to maintain moisture for tree roots.
2. Control Weeds: A layer of mulch can prevent weeds, which might compete with the tree for nutrients and water.
3. Regulate Soil Temperature: Mulch acts as an insulator, protecting tree roots from extreme temperature fluctuations.
4. Improve Soil Quality: Organic mulches decompose over time, enriching the soil with essential nutrients.
5. Prevent Soil Erosion: Mulch can protect the soil from being washed away during heavy rains.

Conclusion

Mulch volcanoes, while commonplace, are an avoidable landscaping mistake that can threaten the health and longevity of trees. By understanding the dangers posed by over-mulching and adopting correct mulching techniques, homeowners and landscapers can ensure that trees thrive and landscapes remain beautiful for years to come.