Mulching is one of the most beneficial ways to keep trees healthy. It makes growing situations more ‘friendly’ for trees in general. However, over-mulching can be one of the worst landscaping mistakes, causing significant damage to trees and other plants around them.

“All things in moderation, should be the mulching motto,” says Jim Skiera, executive director of the International Society of Arboriculture (ISA). “As beneficial as mulch is, too much can be harmful, in more ways than one.”

According to ISA, the generally recommended mulching depth is 51 to 102 mm (2 to 4 in.) When applied properly, mulch helps maintain soil moisture, control weeds, improve soil structure, inhibit certain plant diseases and give planting beds a uniform, well cared for look. It also protects plants and trees from weed whacker damage and lawnmower blight.

Too much mulch, whether in deep layers or piled high against tree trunks, can cuase major problems, including the following:

  •  Excess moisture in the rootzone, which causes palnt stress and root rot.
  • Insect and disease problems.
  • Micronutrient deficiency or toxicity.
  • Weed growth.
  • Smelly planting beds, caused by anaerobic (i.e. oxygen-free) conditions and ‘sour’ mulch.

Habitat creation for rodents that chew and girdle trees (i.e. remove a ring of bark, killing the tree). These problems do not negate the importance of mulching. Urban landscapes are typically harsh environments with poor soil conditions, little organic matter and significant fluctuations in temperature and moisture – all of which are ‘unfriendly’ growing situations for trees.

A suitable layer of organic mulch can mimic a more natural environment and improve overall plant health. When mulching, Skiera says it is important to remember the root system of a tree is not a mirror image of its top.

“The roots of most trees extend out of a significant distance from the trunk,” he says. “Also, most of the fine absorbing roots of trees are located within inches of the soil surface.”

He adds these shallow roots, which are essential for taking up water and minerals for trees, require oxygen to survive.

A thin layer of mulch, applied broadly and practically, can improve the soil structure, oxygen levels, temperature and moisture availability where these roots grow.