How to Choose the Right Mulch for Your Garden Beds

The Short Answer

Choose mulch by what grows in the bed. Use wood chips or bark around trees, shrubs and other permanent plantings. Use straw, shredded leaves or grass clippings in vegetable and annual beds that get replanted every year. Use shredded bark or leaf mold in perennial borders, and compost where the aim is to improve poor soil. Spread most organic mulches 2 to 3 inches deep, and keep them about 3 inches back from trunks and woody stems.

The reasoning is about how long the material lasts against how often you dig. A coarse, woody mulch stays put for years, which is what a shrub border needs and what a vegetable row does not. A soft mulch that breaks down in a season suits a bed you turn over anyway.

The Main Mulch Types Compared

Mulch	Suits	Depth	Source for depth	Watch for
Wood chips	Trees, shrubs, paths	1 to 3 inches	University of Maryland	Do not mix into soil
Bark, shredded or fine	Ornamental beds, shrubs	2 to 3 inches at most	Clemson	Fine particles; stay within the limit
Pine bark nuggets	Shrub borders	Up to 4 inches	Clemson	Coarse; only this texture can go deeper
Straw	Vegetables, strawberries	About 4 to 6 inches	University of Wisconsin	Packs down; hay can carry weed seed
Leaves, shredded	Vegetables, perennials	2 inches at most	Clemson	Mats together if laid deeper
Grass clippings	Vegetable rows	1 to 2 inches	University of Maryland	Replace regularly
Compost	Any bed, soil building	About 1 inch	University of Maryland	Needs replacing each year

Wood Chips and Bark

Wood chips are the chipped branches and trunks that tree crews produce. Washington State University Extension’s guide to arborist chips reports that studies have found wood chips to be one of the best performers for moisture retention, temperature moderation, weed control and sustainability. That makes them a strong default for trees, shrubs and paths.

Bark mulch is sold shredded or as nuggets. Texture decides how deep it can go. Clemson’s fact sheet notes that many wood and bark mulches are made of fine particles and should not be more than 2 to 3 inches deep, while coarse pine bark nuggets let air move through and can be as deep as 4 inches.

The common worry with woody mulch is nitrogen. Washington State’s guide explains that there is a nitrogen deficiency at the boundary between the mulch and the soil, and that this probably helps by inhibiting weed seed germination. The same guide is firm that wood mulch should under no circumstances be worked into the soil as an amendment. Clemson adds that small wood chips decompose very rapidly and use soil nitrogen as they do, and notes that wood chips are usually completely decomposed within 2 years. The practical rule is to leave woody mulch on the surface and top it up as it thins.

Straw, Leaves and Grass Clippings

Straw is the standard mulch for vegetable gardens. It is light and easy to rake aside at planting. University of Wisconsin Extension advises applying straw and hay deeper than other organic mulches, about 4 to 6 inches, since they pack down. University of New Hampshire Extension goes further and lists 6 to 8 inches for grass hay and straw. New Hampshire also warns that leguminous hay can contain lots of weed seeds, which is the reason to ask for straw. Our guide to the best mulch for a vegetable garden covers how to check a bale.

Leaves cost nothing. Keep the layer thin: Clemson cautions that mulches of grass clippings or shredded leaves should never be deeper than 2 inches, because these materials mat together and restrict the water and air reaching the roots. University of Maryland Extension lists a 2 to 3 inch layer for leaves around trees and shrubs, so 2 inches satisfies both. Maryland also lists a 1 to 2 inch layer for grass clippings and notes they must be replaced on a regular basis.

Leaf mold is leaves that have already rotted down into a dark, crumbly material. The Royal Horticultural Society lists leaf mould among the best biodegradable mulches, alongside garden compost, wood chippings, processed conifer bark and well rotted manure, and notes that such mulches break down gradually to release nutrients.

When to Mulch Vegetable Beds

Timing matters more in the vegetable garden than anywhere else, because an organic mulch keeps soil cool. New Hampshire’s rule is that all natural mulches should be applied after the crop has begun to grow and after the soil has warmed. Wisconsin gives the same advice and puts dates on it for that state: about mid to late June on most soils, and mid May on sandy soils, which warm faster. In a warmer region the soil reaches that point earlier, so go by the soil and the crop and treat those dates as Wisconsin’s. The timing for ornamental and mixed beds is in how to mulch in late spring.

Depth around vegetables changes as the plants grow. Wisconsin recommends mulching transplants sparingly, about 1 inch, while they are young, then gradually increasing to 3 inches as they grow taller. Low crops such as lettuce and radishes get mulch on either side of the row at only 1 1/2 to 2 inches.

How Deep to Spread Each Mulch

More is not better. University of Maryland Extension’s instruction for trees and shrubs is to always keep mulch shallow, from one to three inches. Its material-by-material depths are 1 to 3 inches for bark and for wood chips, 2 to 3 inches for pine needles and for leaves, 1 to 2 inches for grass clippings, and about 1 inch for compost. The RHS gives a similar general figure for biodegradable mulches of at least 2 inches and ideally 3 inches.

One source sits outside that range. Washington State’s guide says coarse arborist wood chips maintained at 4 to 6 inches will control weeds. That figure applies to chunky, mixed chips around established trees and shrubs, not to bagged shredded bark, which Clemson caps at 2 to 3 inches.

Whatever the material, keep it off the plant. Maryland says to keep mulch about 3 inches away from the base of tree trunks and that mulch should never touch the trunk. Clemson gives 2 to 3 inches of clearance from the stems of woody plants. The RHS explains why: mulch in direct contact with the stems of trees or shrubs can cause the stem to soften, leaving it open to disease. Spread outward instead of upward. Maryland suggests mulching as far out as the spread of the branches where possible. A young fruit tree needs the same clear collar, as described in our guide to pruning a young apple tree.

How Much to Buy

Clemson gives the conversion: one cubic yard covers a 324 square foot area with an inch of mulch. From that, the formula is square feet times depth in inches, divided by 324.

A bed of 100 square feet mulched 3 inches deep works out to 300 divided by 324, or a little under 1 cubic yard. The same bed in straw at 4 inches needs about 1 1/4 cubic yards. Measure the bed and settle on the depth before ordering, since bulk mulch is sold by the cubic yard.

Mistakes to Avoid

Piling mulch against the trunk. A cone of mulch around a tree breaks every clearance rule above. Pull it back to leave about 3 inches of bare soil at the base.

Laying fine mulch too deep. Fine bark past 3 inches and shredded leaves or clippings past 2 inches go beyond Clemson’s limits for those materials.

Mulching vegetables too early. Organic mulch on cold spring soil keeps it cold. Wait until the soil has warmed and the crop is growing.

Digging wood chips in. Keep them on top. If you want something to work into the soil when you divide perennials or replant, use compost.