Applying Mulch

Applying Mulch: Where, When & How

⟩Where to use mulch
⟩What to use
⟩When and how often to apply
⟩How much do I need
⟩How to apply
⟩Watch out for these

Where to Use Mulch

Mulches can be used in any location where soil is exposed:

• Flower or vegetable beds
• Areas around trees
• Walkways, driveways, and play and natural areas

Keep mulch at least six inches away from trees, so that you’re able to see the “flare” at the base of the trunk. Credit: UC ANR

Some additional considerations:

• Keep mulch 6 to 12 inches away from the base of trees and shrubs; allow the root flare (where the trunk meets the soil) to show.
• Mulch to the outermost edge of the tree's canopy, referred to as the “drip line.”
• Keep organic mulch, except compost, five feet or more away from the home and all structural foundations; consider a band of inorganic mulch against the foundation to deter termites and other pests.
• Non-combustible rock, gravel, concrete, and pavers should be used for areas less than five feet away from structures.

Deciding what to use

Pine needle mulch is useful around acid loving plants Credit: Wikimedia Commons

The selection of a mulch depends on its intended use. Consider:
• Appearance
• Size of the area
• Cost and availability
• Pros and cons of organic vs inorganic mulches
• Whether it will improve the soil
• How quickly it breaks down
• Whether it requires additional nitrogen
• Possible flammability

This chart includes information on the decomposition rate, permeability, and water retention of some common mulch materials.

For more information on the flammability of specific materials see our fire-smart mulch guidelines.

When and how often to apply

Green waste mulch is low to no cost, and good for the soil. Credit: Landscape for Life

Mulches used to enhance appearance and control weeds may be applied at any time. As the plants grow and fill in the bed areas, less and less mulch is needed. Mulch can be applied to landscape trees any time of the year, though best in the middle of spring, when soil temperatures have warmed.

Reapply or refresh organic mulches after they have begun to break down:

Grass clippings and leaves decompose very fast and need to be replenished frequently.

Pine needles and bark mulches typically last for around 2 to 4 years.

Inorganic mulches such as gravel and pebbles rarely need replenishing.

Determine how much you need

The amount of mulch to apply depends on the texture and density of the mulch material.

Fine mulch (less then half inch particle size)	Apply no more than 2 inches; thin layers are less likely to impede air and water.
Coarse mulch	Use 4-6 inches or more to control weeds in open spaces.
On poorly drained soils	Use lesser amounts
Grass clippings or shredded leaves	These should never be deeper than 2 inches; they tend to mat together, restricting the water and air supply to plant roots.

To calculate how many cubic feet of mulch is needed:

1. Determine the surface area to be covered in square feet, and the desired depth of coverage.
2. Mulch is often available in bulk, measured in cubic yards.
3. Calculate the volume required by:
- multiplying area (in square feet) X depth (in feet, not inches) and
- dividing by 27 (there are 27 cubic feet in a cubic yard).

How much will one cubic yard cover?

Area (square feet)	Depth (inches)
80	4
100	3
160	2
325	1

How to apply

Before applying mulch, remove weeds and water thoroughly. Most applications of mulch are done by hand to ensure good placement.

Watch out for these:

Nitrogen reducing	Organic mulches that are low in nitrogen, such as wood chips, can reduce soil nitrogen availability during the process of microbial decomposition.
Over-mulching around trees	Mulch extending up the trunk, smothering the root flare and root zone. Known as “volcano" mulching, it is hazardous to trees.
Mulch toxicity	“Sour” mulch can quickly damage plant tissue. Caused by poor handling or storing of mulch resulting in anaerobic conditions.
Matted mulch	When thick layers of dry mulch are applied or existing mulch dries, some nuisance fungi can grow, forming a mat of fine threadlike structures that repels water.
Shredded rubber	Toxic materials can leach from rubber as it degrades; also exhibits hazardous fire characteristics.
Dyed wood chips	Often produced from recycled wood that is ground into chips and dyed, the source wood may have been pressure-treated and contain chromate copper arsenate; shouldn’t be used around edibles.
Slime molds	Fungus-like organisms that typically appear suddenly during warm, damp weather. May be several inches to a foot or more in diameter and vary in color. They are unsightly but harmless.
Stinkhorn fungi	Can colonize hardwood bark mulch; the fruiting bodies or mushrooms often come up in the fall and exude a very unpleasant odor.

BACK TO BASICS

What is Mulch, Types of Mulch

Sheet Composting As Lawn Replacement Method