How Many Bags of Mulch Do You Need? Depth, Area and Round Beds Around Trees
Mulch does three useful jobs: it keeps moisture in the soil, it smothers weed seedlings, and as an organic mulch breaks down it feeds the soil underneath. It only does them at the right depth. Too thin and weeds push through; too thick and water struggles to reach the roots. Getting the depth right starts with buying the right amount.
The GardenStuffs Soil Volume Calculator handles mulch the same way it handles soil and compost. Choose a rectangular area or a circular one, enter the length and width (or the diameter) in feet and the depth in inches, pick Mulch as the material, and it returns cubic feet, cubic yards and the number of 1, 1.5 and 2 cubic foot bags, rounded up to whole bags.
How deep to spread it
For shredded bark, wood chips and similar coarse mulches, 2 to 3 inches is the range most garden guidance settles on for beds and borders. Fine mulches such as shredded leaves or compost can go thinner, because they pack down. Going deeper than about 4 inches rarely helps: thick layers of fine material can mat and shed water, and they keep the soil underneath wet and short of air.
Worked example: a 10 x 20 foot border
A border 10 feet deep and 20 feet long is 200 square feet. The depth you choose changes the order a great deal:
| Depth | Cubic feet | Cubic yards | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|
| 2 inches | 33.33 | 1.235 | 23 | 17 |
| 3 inches | 50 | 1.852 | 34 | 25 |
| 4 inches | 66.67 | 2.469 | 45 | 34 |
Each extra inch on this border adds about 17 cubic feet, eight or nine more of the 2 cubic foot bags. If last year's mulch is still there but has thinned, you may only need to top it up. An inch over the same border is 16.67 cubic feet, or 9 bags of 2 cubic feet, which is often all an established bed needs in spring.
Mulching a vegetable bed
Vegetable beds take mulch too, usually after the soil has warmed in late spring, because a cold mulch laid early keeps cold soil cold. A 4 x 8 foot raised bed at 2 inches needs 5.33 cubic feet, or 3 bags of 2 cubic feet. Use a material that breaks down within the season, such as weed-free straw, shredded leaves or compost, so it can be turned in or raked off when you replant. Keep it an inch or two back from the stems of young plants, and pull it back from rows of small seeds until the seedlings are up, or slugs will shelter under it right where you need it least.
Bags or bulk?
Bulk mulch is sold by the cubic yard. A cubic yard is 27 cubic feet, so it equals 13.5 bags of 2 cubic feet or 9 bags of 3 cubic feet. To compare prices, multiply the bag price by 13.5 (for 2 cubic foot bags) and set it against the delivered price of a yard, including any delivery charge.
For a large job the difference adds up quickly. A 25 by 40 foot area, 1,000 square feet, mulched 3 inches deep needs 250 cubic feet. That is 9.259 cubic yards, or 125 bags of 2 cubic feet. Few people want to carry 125 bags, and at that volume bulk is usually the practical choice. For a single small bed, bags are cleaner and easier to store.
Round beds around trees
Switch the shape to circular for a mulch ring around a tree. A ring 6 feet across at 3 inches deep is 7.07 cubic feet, or 4 bags of 2 cubic feet. An 8-foot ring at the same depth is 12.57 cubic feet, or 7 bags.
The calculator fills the whole circle, but the area right around the trunk should get no mulch at all. Leave a bare gap of several inches between the mulch and the trunk. On the 8-foot ring, leaving a 1-foot circle bare in the middle brings the volume down to 12.37 cubic feet, so the bag count does not change; the gap matters for the tree, not for the order.
Keep mulch off trunks and stems
Mulch piled up against a tree trunk, the "volcano" shape often seen in parking lots, keeps the bark constantly wet. That invites rot and disease, gives rodents cover to chew the bark, and can encourage roots to circle the trunk. The rule is a flat ring, wider rather than deeper, with the trunk flare visible at the base. The same applies to shrubs and to the stems of perennials and vegetables: pull mulch back an inch or two from the stems.
Choosing a mulch
- Shredded bark and wood chips last a season or more and suit borders, paths and tree rings.
- Straw is common in vegetable beds; use straw sold as weed-free, not hay, which carries seeds.
- Shredded leaves and compost break down quickly and feed the soil as they go; see how much compost to add to a garden bed.
- Cocoa-shell mulch smells of chocolate and contains theobromine, the compound that makes chocolate toxic to dogs. Dogs are attracted to it and can be poisoned by eating it. Do not use it anywhere a dog can reach.
- Dyed mulches are colored wood chips. Check what the product is made from if it will go near food crops.
- Fresh wood chips from tree work are fine on paths and around trees. Worked into the soil of a vegetable bed, they can tie up nitrogen for a while, so keep them on the surface.
What the calculator does not cover
- Settling. Fluffy mulch settles. A layer spread at 3 inches will look thinner after a few weeks of rain.
- Bag labels. Mulch bags are usually sold as 2 or 3 cubic feet. The calculator counts 1, 1.5 and 2 cubic foot bags; for 3 cubic foot bags, divide the cubic feet by 3 and round up.
- Weight and moisture. Wet mulch is heavy, and the calculator does not estimate weight.
- Planting areas. The volume covers the whole area. Plants, stepping stones and the gap around trunks reduce what you need slightly, which works in your favor.
Spreading it safely
- Wear gloves; bark mulch splinters and bulk mulch can hide sharp debris. Keep cuts covered and your tetanus vaccination up to date.
- Lift bags with your knees bent and your back straight, or slit them in the wheelbarrow and tip them out.
- Bulk piles of wood mulch can heat up as they decompose. Spread a delivery within a few days rather than leaving a tall pile against a fence or building.
- Keep mulch out of gutters and storm drains, and sweep it off paths.
Water the soil well before you mulch, then spread the mulch on moist soil, and you will lock that moisture in. To see how much water the bed still needs afterward, try the Watering Schedule Planner or read how soil type and rainfall change your watering.