Mulch 101: Types, Depth, and How Much You Need

What mulch does and why it matters

Mulch is any material spread over bare soil. It does several jobs at once.

Iowa State University Extension lists the main ones. Mulch controls annual weeds, holds soil moisture, and evens out soil temperature. It also cuts erosion and keeps rain from splashing soil onto fruit and leaves.

There are two families. Organic mulch was once alive: bark, wood chips, straw, leaves. It breaks down and adds organic matter to the soil, per University of Minnesota Extension. Inorganic mulch is rock, gravel, or rubber. It lasts for years but does nothing for the soil.

The choice follows the job. Beds you plant do best with organic mulch. Paths and no-plant zones can use rock.

How to mulch a bed, step by step

Pick the depth, work out the volume, then spread. The whole job takes three measurements and one formula.

1. Pick a depth. Most beds want 2 to 4 inches. Iowa State puts coarse mulch, like wood chips, at the high end and fine mulch, like grass clippings, at the low end. The calculator's presets follow the job:

• 2 inches: a top-up over mulch that is already there
• 3 inches: a new bed, for weed control and moisture
• 4 inches: heavy weed control in open ground, kept off stems and trunks

2. Work out the volume. Measure the bed in square feet. Then use one shortcut:

Cubic yards = square feet × depth (in inches) ÷ 324.

The 324 folds two conversions into one. Inches to feet is ÷12, and cubic feet to cubic yards is ÷27. So 12 × 27 = 324.

A 20 ft × 10 ft bed is 200 sq ft. At 3 inches, 200 × 3 ÷ 324 = 1.85 cubic yards. That is 50 cubic feet, or 25 bags of 2 cu ft mulch.

A round bed works the same way. A 10 ft circle is about 78.5 sq ft (π × 5²). At 3 inches that is 0.73 cubic yards, or 10 bags.

3. Spread it. Pull weeds first, since mulch stops new seeds, not plants already growing. Water dry soil before you spread. Then rake the mulch level and pull it back from every stem and trunk.

Add about 5 to 10 percent to your order. Beds are rarely flat, and a spare bag is easier than a second trip.

How much mulch to buy for your bed size

Here is the formula run for ten bed sizes at the three calculator depths. Bag counts assume standard 2 cu ft bags, rounded up.

Notice how fast bags pile up. By 200 sq ft at 3 inches you are loading 25 bags, which is where bulk starts to make sense.

For more worked shapes, see how much mulch do I need. For any other size, the Mulch Calculator does it in one step — or jump to the bags version or the cubic yards version if you already know which unit you want.

Bags per cubic yard

A cubic yard is 27 cubic feet. Divide by the bag size and round up.

• 1 cu ft bags (28 L): 27 per yard
• 1.5 cu ft bags (42 L): 18 per yard
• 2 cu ft bags (57 L): 13.5 per yard, so buy 14
• 3 cu ft bags (85 L): 9 per yard

The bags of mulch in a yard guide covers odd bag sizes and pallets.

Choosing a mulch type

Match the mulch to the spot, not just how it looks in the bag. The table below pulls depth, lifespan, and best use for 13 mulches from Iowa State and Colorado State Extension.

Where a source gives no special depth, the row uses the general 2 to 4 inch range.

Shredded leaves are the free option if you've got fall trees nearby. Leaves as mulch covers shredding them with a mower, how much they shrink, and how many bags a bed needs.

Sources: Iowa State University Extension and Colorado State University Extension.

A few notes behind the table:

• Wood chips: Iowa State warns that chips made from old lumber or pallets break down fast and may contain chemicals. Some are dyed to hide where they came from.
• Grass clippings: let them dry first, or they mat and smell. Skip clippings from lawns treated with broadleaf weed killer until after two or three mowings.
• Cocoa shells: they can make dogs sick if eaten. Iowa State says dog owners should watch their pets closely.
• Gravel: Colorado State says stones must be under about 1/2 inch to act as mulch. Bigger rock does not hold moisture or block weeds the same way.

Mulch in the vegetable garden

Vegetable beds get their own pick. Iowa State says straw, dried grass clippings, and shredded leaves are ideal there, because they break down within a season.

Mulch pays off in a food garden in ways you can see. Iowa State notes it can cut the severity of some diseases, such as blight on tomatoes. It also keeps fruit clean of rain-splashed soil, which means less rot and spoilage.

Fast mulches suit beds you dig each year. Colorado State says leaves and clippings can simply be turned under in fall, then laid fresh next season. Save slow wood chips and bark for paths and perennial plantings you don't dig.

How far one cubic yard goes

Depth changes coverage more than people expect. Going from 2 inches to 4 inches halves how far each yard spreads.

This is the same ÷324 math in reverse. Divide 324 by the depth in inches to get square feet per yard.

Bulk vs bagged: where the cost flips

Bagged mulch is simple. There is no delivery minimum, and you carry home just what you need.

Bulk usually wins once you pass about 1 cubic yard. That is 13 to 14 bags of 2 cu ft. Below that, bagged is simpler and you skip the delivery fee.

Storing a bulk pile? Keep it modest. Iowa State says huge, soggy piles can go "sour." The mulch ferments and makes acids that can burn or kill annuals, perennials, and low shrubs within hours to days. Sour mulch smells like vinegar or silage. Good mulch smells like fresh bark or compost. Keep piles no more than 10 feet high, well drained, and turned.

When to lay mulch, and how often to top it up

Spring and fall both work. Iowa State says to wait in spring until the ground warms and perennials come up. Mulch laid too early keeps the soil cold and slows plants.

In fall, wait until plants go dormant. Mulch laid too early can delay dormancy and raise the risk of cold damage. For tender crops still producing when the nights turn cold, a DIY cold frame does more than mulch alone, since it traps daytime heat instead of just insulating the roots.

Organic mulch shrinks as it breaks down. Before you add more, check the old layer. If it is already 3 inches deep, a light top-up is enough. You don't need to strip old mulch every year, just keep the total in the 2 to 4 inch range.

Rock needs less topping up, but it is not weed-free forever. Iowa State notes that leaves and soil slowly collect on top, and weeds root in that layer.

Mulch problems and how to avoid them

Most mulch problems come from using it wrong. Iowa State says the benefits far outweigh the risks. This table sums up what to watch for.

On termites, Iowa State is clear on one point. There is no evidence that wood chip mulch draws in termites that aren't already nearby. The gap at the foundation is about keeping their path in plain sight.

The one takeaway

Pick organic mulch for beds you plant, lay it 2 to 4 inches deep, and keep it off every trunk and stem. To order, multiply square feet by depth in inches and divide by 324.

Ready to skip the math? Open the Mulch Calculator. Enter your bed size and depth, and it returns cubic yards, bags, and coverage. Mulching a new raised bed? Fill it first with the raised bed soil guide, and feed it with how much compost do I need.