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How deep should mulch be?

2 to 4 inches for beds and trees, 3 to 4 inches on paths, 1 to 2 inches of fine mulch in vegetable plots, and bare space around trunks. How much each depth takes per 100 square feet.

The short answer

Spread mulch 2 to 4 inches deep on beds and around trees, as the University of Minnesota and Penn State Extension both give, and keep it off tree trunks. Penn State adds: no more than 4 inches, and less where the soil drains poorly.

Depth by use

Mulch depth by where it goes
WhereDepthSource
Beds and borders2 to 4 inUniversity of Minnesota Extension
Around trees2 to 4 in, no more; less on poorly drained soilPenn State Extension
Paths and high-traffic areas3 to 4 in of coarse mulch such as wood chipsNC State Extension
Inside vegetable plots1 to 2 in of finer mulch such as leaves or strawNC State Extension
Top of a raised bedLeave 1 to 2 in of space below the rim for itIowa State University Extension

Keep it off trunks and stems

The University of Minnesota Extension says to leave a 6-inch mulch-free ring around tree trunks to avoid root rot and insects. Penn State’s rule of thumb is to pull mulch 3 to 5 inches away from young trees and 8 to 10 inches from mature trees. The mulch calculator has a tree ring shape that leaves that bare circle out.

How much to order

One cubic yard covers 324 square feet 1 inch deep, or 108 square feet at 3 inches, the University of Florida IFAS Extension says. Per 100 square feet, that is 0.62 cubic yards at 2 inches, 0.93 at 3 inches and 1.23 at 4 inches.

Check what is already on the ground and order only the new layer: if 1 inch remains and you want 3, order 2. The mulch calculator starts the extra at zero, so the amount it gives is what the area and depth take. None of the extension services we cite gives a percentage to add for mulch. The table lists the allowances a public body, extension services and a maker give for other jobs.

Extra to order, where a public body or maker gives a figure
JobExtra to orderWho gives it
Mulch on beds and bordersNo figure given; the calculator starts at zeroNone of our sources
Compacted gravel path or baseAbout 11%: multiply the compacted volume by about 1.11Maine Department of Environmental Protection
SodAbout 5% to about 10%Mississippi State University Extension; Purdue Extension
Raised-bed soil, for settling10%Frame It All, in its own soil calculator

Ask your supplier for its figure for the product you buy: how much a material compacts or settles depends on the material.

References