Mulch serves many important functions. Shredded wood chips and other coarse-textured, biodegradable mulches help to build and protect soil, buffer drought conditions, and support healthy tree growth.

Mulch:

  • Retains soil moisture
  • Moderates soil temperatures
  • Reduces competition from weeds and grass
  • Minimizes soil compaction
  • Prevents erosion
  • Returns nutrients to the soil as mulches break down slowly over time
  • Supports a healthy community of soil microorganisms
  • Visually defines planting areas and accenting trees and shrubs
  • Keeps mowers and string trimmers a safe distance away from tree’s trunks, preventing damage referred to as ‘mower blight’

Materials, such as shredded wood chips, rough compost and shredded leaves, produce the best long-term results. Bark mulches are also a fair option. These natural products are readily available; in fact, wood chips are often free from tree service companies eager to donate chipped material rather than pay tipping fees at the local landfill.

Mulch should be spread three to five inches deep around a tree in a three-foot radius from the trunk, although covering a larger area is recommended where possible. Less than three inches depth of mulch doesn’t effectively reduce the loss of soil moisture or discourage weeds, whereas more than five inches may prevent oxygen from reaching tree roots.

Proper mulching looks like a doughnut, not a volcano! Mulch should not touch the trunk or cover the root collar. Inspect the landscape annually and reapply new mulch as needed to maintain the optimal 3- to 5-inch depth.

Fine-textured mulches such as sawdust or grass clippings are not recommended. These materials tend to become anaerobic and smelly as they break down; they may self-compact into a thick mat that repels water, and frankly, they lack the “clean” visual appeal of coarser woody mulches.

Stone and gravel products are often used in dry climates where their colors and textures better complement native landscapes. These mulches are heavier and more costly to install, retain more heat around plants, contribute little soil nutrition, and still require regular maintenance to keep them contained within landscape beds. Stone mulches are best used in rock gardens, xeriscapes and some rain garden applications.

Also available are synthetic mulches made of recycled rubber such as tires; however, in the words of WSU horticulture guru Dr. Linda Chalker-Scott: “If the EPA defines discarded tires as pollution, then why isn’t recycled rubber mulch in the same category?”

So there you have it—good preventive healthcare for your trees includes good mulch—especially as we head into our typically hot, dry summer! Read more about mulch in this WSU fact sheet.