How deep mulch should be, by plant and by material

The depth table, with who says so

Where and what	Depth	Source
Trees and shrubs, wood chips or shredded bark, drained soil	3 to 4 in	Iowa State
Trees and shrubs on heavy or poorly drained soil	2 to 3 in	Iowa State, Bloomingdale forestry
Beds in general, coarse organic mulch	2 to 4 in	Illinois, Mulch and Soil Council, Iowa State
Composted or fine-textured mulch	2 to 3 in	Illinois, Oregon State
Bark, chunk grade	2 to 3 in	Washington State
Sawdust, shavings, other very fine mulch	1 in at most	NRCS Washington
Compost as a mulch	1 to 3 in	Washington State
Cocoa shells	1 to 2 in	Iowa State
Arborist wood chips on perennial beds	4 to 6 in	Washington State, South Dakota State
Straw or hay, vegetable garden	4 to 6 in, up to 8 in spring	South Dakota State, Washington State
Pine needles	4 to 6 in	South Dakota State
Shredded leaves	2 to 3 in	South Dakota State
Grass clippings	1 to 2 in dried in thin layers, up to 4	NRCS Washington, South Dakota State
Stone, gravel, lava rock, tumbled glass	2 to 3 in	Utah State
Cinders or gravel on a conservation site	3 to 4 in	NRCS Washington
Playground wood chips, 10 ft fall height	9 in uncompressed, 9 in compressed	CPSC handbook table

The band is narrow because the reasons are the same everywhere. You want enough depth to shade out weed seedlings and hold moisture. You do not want so much that roots grow up into the mulch or that fine particles mat into a crust.

Trees and shrubs: depth and the bare ring

Iowa State's answer is the most specific. On well-drained soil it is 3 to 4 inches of chips or shredded bark, and on heavy soil 2 to 3. Keep the mulch at least 6 inches away from the trunk. Illinois says 2 to 4 inches and at least 4 inches from the trunk. Maryland keeps it shallow at 1 to 3 inches and 3 inches back. Colorado State goes to 4 or 5 inches over the backfill of a new tree and 6 inches from the trunk. The disagreement is in the ring, not the depth. The Illinois and Bloomingdale pages say the same thing about the mound against the bark that landscapers call a volcano: do not build it. Spread the ring as wide as the branch spread where the lawn allows; a tree's feeder roots run well beyond the trunk.

Fine and coarse are different materials

Coarse mulch, chips and chunk bark, lets water and air through, so it can go 3 to 4 inches deep. On a perennial bed where weed control matters more than looks, it can go 4 to 6. Fine mulch, shredded bark, compost and especially sawdust, packs. Oregon State puts fine bark at no more than 2 to 3 inches. The conservation specification puts sawdust and shavings at 1 inch at most, and Washington State gives compost 1 to 3. A layer of fine material that has matted sheds rain instead of passing it; raking it loose each spring does more than adding to it.

Vegetable gardens

Straw and hay are light and settle fast, which is why the vegetable-garden figures are the largest in the table. South Dakota State's guide gives 4 to 6 inches. Washington State's guide gives up to 8 inches in a spring garden around strawberries and small fruit. Shredded leaves go 2 to 3 inches and pine needles 4 to 6. Dried grass clippings go 1 to 2 inches at a time so they do not slime. These depths are after settling, so the loose pile when it goes down is deeper.

Stone and other inorganic mulch

Rock does not decompose and does not need depth for weed suppression the way bark does. It needs enough to hide the ground and stay put. Utah State's water-wise guide gives 2 to 3 inches for gravel, crushed stone, lava rock and tumbled glass against 3 to 4 for organic mulch. The conservation specification allows 3 to 4 inches of cinder or gravel where erosion is the concern. Rock is heavy, about 2,500 lb per cubic yard for crushed stone. The inch you leave off a 2 to 3 inch layer saves real tonnage. That is why the rock calculator asks for the depth first.

Playgrounds are a safety depth, not a garden depth

Under play equipment the depth is set by the fall it has to absorb, and the federal handbook's test table is the reference. In the table, 6 inches of wood chips protects a fall from 7 feet, 9 inches from 10 feet and 12 inches from 11 feet. The same table says 9 inches of chips compressed by use still protects 10 feet. Loose fill packs and scatters, so playground chips are installed deeper than the table figure and kept topped up. The table quoted is from the 1997 edition hosted by the Government Publishing Office, so take the installed depth from the current edition or from the equipment maker.

What the depth costs in mulch

Every inch of depth over 500 square feet is about 1.5 cubic yards. So the choice between 2 and 4 inches is the difference between 3.1 and 6.2 yards, or 42 and 84 bags of 2 cubic feet. Utah State's rule of thumb, about 6 cubic yards per 1,000 square feet for a 2 inch layer, matches the arithmetic. Top-ups after the first year are thinner than the first layer: rake, measure what is left, and add only the difference to the target. The mulch calculator takes the depth from this table for each bed and turns it into bags or a bulk order.