Can You Put Too Much Mulch Around a Tree?

Can You Put Too Much Mulch Around a Tree?

Most people know about the advantages of mulching around trees, but few understand the drawbacks when not done properly. Plants with shallow root systems, such as shrubs, benefit from light mulching, however, putting a thick layer and too much mulch around tree trunks detracts from the aeration of the soil, smothering the plant. In this guide, we highlight why it is important to properly mulch trees and the drawbacks of putting too much mulch around trees.

Problems With Putting Too Much Mulch Around Trees

Lack of Oxygen

The biggest problem with over mulching around plants is the lack of oxygen delivered to the roots. Repeatedly mulching around trees in areas with high rainfall will waterlog the soil, retain moisture, and cause root rot.

Suffocation of the roots is the most common cause of tree and shrub death from over-mulching. When oxygen levels in the soil drop below 10%, it affects normal root growth. If the roots die, the tree dies. Water and nutrients are vital to the health of your tree.

Applying a two to four-inch layer around the tree to the outermost area of the tree’s canopy, known as the drip line, is ideal.

Inner Bark Death

Death of the phloem (inner bark) in trees results from applying mulch directly against the tree trunk and above the root flare of shrubs. The tissues in these areas differ from the roots and can’t survive when the environment is constantly moist.

Trees and shrubs breathe through the Lenticel pores in the root flare. Over-mulching in these areas results in a diminishment of the gaseous exchange during irrigation or heavy rainfalls. This effect results in stress to the tree or shrub and the death of the phloem.


Piling wet mulch up against trees results in heat accumulation in this area. This effect results in rapid decomposition of the bark. Temperatures under the mulch can reach 120° to 140° F, similar to composting.

The elevated temperature can kill the phloem or prevent the tree from undergoing the hardening period during the fall. This is when the trees and shrubs prepare for the winter. If the bark and root flare don’t harden before the arrival of cold weather, the phloem is at risk of dying. This effect results in the roots not receiving enough photosynthetic sugars, and the health of the tree starts to decline.

Soil Acidity

Adding excess piles of mulch changes the pH level of the soil. Mulch usually has a pH of 3.5 to 4.5, causing the soil to become more acidic as seasons pass. Organic mulches break down and improve soil.

Lack of Nitrogen

Uncomposed fresh mulch can cause a nitrogen deficiency in young trees, shrubs, and plants. The bacteria responsible for mulch decomposing requires ample nitrogen to start the decomposition process.

When the mulch lacks enough nitrogen to spark decomposition, it draws it from the soil. Plants need nitrogen to grow; you’ll feed the mulch instead of your flowerbed, shrubs, or trees. You’ll notice this problem when the foliage starts to turn yellow.

Pests and Disease

Not properly mulching around trees and shrubs invites pests and rodents, and provides ideal conditions for the development of fungal pathogens like mold.

Most homeowners won’t realize there’s a problem until the following season when the plants or trees look unhealthy. If the pests eat away 50% of the bark around the base of the tree, it results in damage and death to the phloem.

How to Properly Mulch Around a Tree

To properly mulch around a tree:

  • Choose quality mulch like organic mulch made from wood chips or bark.
  • Measure the tree’s diameter and extend the mulch to the drip line.
  • Clear weeds and create a flat surface.
  • Spread mulch in a ring, starting a few inches from the trunk.
  • Maintain a consistent depth of two to four inches.
  • Keep mulch away from the trunk to prevent moisture buildup.
  • Refresh mulch periodically and water after application.
  • Avoid piling mulch against the trunk to prevent rot or disease.

By following these steps, you can properly mulch around your tree to enhance its health and appearance while also promoting a healthy growing environment.

Omaha Tree Services

At Omaha Tree, our mission is to provide tree care services & mulch products that improve the HEALTH, BEAUTY, & SAFETY of our customers’ trees. We recycle all of our wood waste material from the trees we trim and remove in the greater Omaha metro area. The wood waste material is brought back to our 8-acre site at 3606 McKinley Street, where it is processed into a beautiful & consistent mulch product that is local, fresh, and clean. Contact a certified arborist today.