Excess Mulch Problems | University of Maryland Extension (2024)

Incorrect placement of mulch (mulch volcano). Photo: Fredric Miller Miller, Joliet Junior College, Bugwood.org

Updated: March 14, 2023

What is mulch used for?

Mulch is used around plants to conserve soil moisture, moderate soil temperatures, protect roots, reduce weeds, and help prevent trunk damage from lawnmowers and weed trimmers.
Many gardeners, as a routine garden task, add liberal amounts of mulch to trees and shrubs believing it is the “correct” thing to do.
Over time, mulch can accumulate to depths of five, six, or more inches causing plant problems.

Plant problems caused by too much mulch

Excessive mulch reduces soil oxygen for roots, suffocating them and causing them to die.
Roots in search of oxygen and water sometimes grow into excess mulch. During dry periods the mulch dries out and the roots in the mulch die.
Mulch piled up against the trunks of trees and shrubs keeps the bark moist underneath the mulch causing decay and a possible entry point for diseases and insects. Pull back the mulch about 3 inches so it does not touch the bark.
Light watering may only wet the mulch layer while the soil remains dry.
Too much mulch may delay the onset of dormancy in the fall by keeping root zone temperatures warm. In contrast, excessive mulch may prevent the breaking of dormancy in the spring due to cooler root zone temperatures.
Provides a hiding place for voles, a rodent that gnaws on bark and girdles trunks and the lower stems of shrubs.

Mulch tips

Keep organic (bark and wood products, compost, and leaves or grass clippings) and synthetic (plastic, stones, or landscape fabric) mulches 3 inches away from the base of the trunk to prevent damaging the bark by keeping the area too moist.
Organic mulches should not exceed three inches in depth. Although organic mulches break down over time they should only be replaced as needed to maintain their original depth.
Carefully monitor the watering of mulched plants to make sure the water percolates through the mulch layer into the soil.
As old mulch decomposes or before applying fresh mulch it can be gently worked into the planting bed to improve the soil or removed and worked into other garden areas.
Shallow-rooted plants like boxwood should not have more than one inch of mulch around their roots. Acid loving plants like azaleas can be mulched with pine bark or pine needles.
Additional mulch may be added occasionally to supplement and freshen-up the existing covering but do not let it exceed a depth of 3 inches.

Excess Mulch Problems | University of Maryland Extension (2)

Mulching Trees and Shrubs

Additional resources

Problems With Over-Mulching Trees and Shrubs | Rutgers University

Mulch and its Purpose:

Mulch is a crucial component in gardening practices, serving multiple functions to enhance plant health and overall garden well-being. The primary purposes of mulch include:

Conserving Soil Moisture: Mulch acts as a protective barrier, minimizing water evaporation from the soil and ensuring a more consistent moisture level.
Moderating Soil Temperatures: It helps regulate soil temperatures, preventing extreme fluctuations that can be harmful to plant roots.
Root Protection: Mulch provides a protective layer around plant roots, shielding them from potential damage caused by lawnmowers and weed trimmers.
See Also
Don't Make Your Trees Compete. Remove Grass & Weeds! - Friends of Trees
Weed Reduction: By blocking sunlight and suppressing weed growth, mulch serves as a natural weed deterrent, reducing competition for nutrients.
Preventing Trunk Damage: Properly applied mulch acts as a buffer, preventing damage to tree and shrub trunks from lawnmowers and trimmers.

Issues Caused by Incorrect Mulch Placement:

The article highlights several problems arising from the incorrect placement of mulch, commonly referred to as a "mulch volcano." These issues include:

Reduced Soil Oxygen: Excessive mulch can limit the exchange of oxygen to the soil, leading to suffocation of plant roots and subsequent root death.
Dry Mulch and Root Death: During dry periods, the excess mulch may dry out, causing the roots within it to die as they seek water and oxygen.
Bark Decay and Disease Entry: Piling mulch against tree trunks can keep the bark moist, leading to decay and creating an entry point for diseases and insects.
Temperature Impact on Dormancy: Too much mulch can delay the onset of dormancy in the fall by maintaining warm root zone temperatures. Conversely, it may prevent dormancy breaking in the spring due to cooler root zone temperatures.
Vole Infestation: Excessive mulch provides a hiding place for voles, rodents that can cause significant damage by gnawing on bark and girdling trunks and stems.

Mulch Tips:

To address these issues and ensure proper mulch application, the article provides several tips, including:

Trunk Clearance: Maintain a 3-inch clearance between the base of the trunk and the mulch to prevent excessive moisture around the bark.
Mulch Depth: Organic mulches should not exceed three inches in depth. Shallow-rooted plants like boxwood should have no more than one inch of mulch around their roots.
Monitoring Watering: Carefully monitor water penetration through the mulch layer to ensure it reaches the soil and the plant roots.
Mulch Replacement: Replace organic mulches only as needed to maintain their original depth. Decomposed mulch can be worked into the soil for improvement.
Plant-Specific Mulching: Consider the specific needs of plants; for instance, acid-loving plants like azaleas can be mulched with pine bark or pine needles.

Additional Resources:

The article references additional resources for further reading, providing valuable insights into the potential problems associated with over-mulching trees and shrubs. These resources include publications from Rutgers University and Penn State Extension, ensuring a comprehensive understanding of mulching practices.

In conclusion, proper mulching is a nuanced practice that requires attention to detail and an understanding of plant biology. By adhering to these guidelines, gardeners can harness the benefits of mulch while avoiding potential pitfalls associated with incorrect placement.