Bed size and depth to cubic yards and bag counts, with the point at which bulk delivery becomes the cheaper option.
Volume is area × depth, with depth converted to the same units. A cubic yard is 27 cubic feet, and that single figure drives everything: one cubic yard covers 108 sq ft at 3 inches deep, or 162 sq ft at 2 inches, or 81 sq ft at 4 inches.
A 25 × 20 ft bed is 500 sq ft. At 3 inches that is 125 cubic feet, or 4.63 cubic yards — which is 63 of the standard 2 cubic foot bags.
Bagged mulch is typically sold in 2 or 3 cubic foot bags, so a cubic yard is 13.5 or 9 bags respectively. Bulk delivery is almost always cheaper per unit, and the crossover is usually around 2 to 3 cubic yards — below that, delivery fees and minimum orders eat the saving; above it, the difference becomes substantial, and so does the number of bags you would otherwise be lifting.
Bulk also means a pile on your driveway that needs moving with a wheelbarrow, so factor in the labour honestly.
| Use | Depth |
|---|---|
| Around annuals and shallow-rooted plants | 1–2 in (25–50 mm) |
| General beds and borders | 2–3 in (50–75 mm) |
| Around trees and shrubs | 3–4 in (75–100 mm) |
| Paths and heavy weed suppression | 4 in (100 mm) |
Three inches is the general answer. Below two, weeds come straight through and the moisture benefit largely disappears. Above four, you start causing problems rather than solving them: thick mulch can become hydrophobic and shed water rather than letting it through, and it restricts oxygen to the roots.
The cone of mulch heaped around a tree trunk — a "mulch volcano" — is one of the most common and most damaging landscaping mistakes. Bark is not adapted to constant moisture. Held against the trunk it stays wet, softens, and invites rot, boring insects and rodents. It also encourages roots to grow up into the mulch and circle the trunk, eventually girdling the tree.
Keep mulch three to six inches clear of the trunk and spread it out to the drip line instead. A wide, flat ring does everything a volcano does not.
Suppressing weeds is the reason most people apply it, but the moisture retention matters more. Bare soil loses water rapidly to evaporation; a mulched bed can need substantially less watering through a dry summer. Mulch also moderates soil temperature, keeping roots cooler in summer and insulating them in winter, and organic types slowly break down and feed the soil as they go.
That decomposition is why organic mulch needs topping up — typically an inch a year. Measure what is already there before ordering, and top up to the target depth rather than adding a full new layer on top of the old one.
Shredded hardwood knits together and stays put on slopes. Bark chips last longer but wash away more easily. Pine straw is light, easy to spread and slightly acidifying. Compost feeds the soil best but breaks down fastest and suppresses weeds least. Rubber and stone do not decompose, which sounds like an advantage until you want to change the planting — they add nothing to the soil and stone can raise soil temperature considerably.
Avoid fresh wood chips directly against plants; as they decompose they temporarily draw nitrogen from the top layer of soil. Aged or composted chips do not have this problem.
108 square feet at 3 inches deep, which is the most common depth. At 2 inches it covers 162 sq ft and at 4 inches, 81 sq ft. The arithmetic is simply 27 cubic feet divided by the depth in feet. A cubic yard equals 13.5 bags at 2 cubic feet each, or 9 bags at 3 cubic feet.
Two to three inches for general beds, three to four around trees and shrubs, and one to two around annuals. Below two inches weeds push through and the moisture benefit largely disappears. Above four inches you start creating problems — thick mulch can turn hydrophobic and shed water instead of admitting it, and it restricts oxygen reaching the roots.
Usually not, if it is an organic mulch — it breaks down and improves the soil, which is part of the point. Measure what is left, and top up to your target depth rather than adding a full fresh layer over the top. If the existing layer has matted into a water-shedding crust, break it up with a rake first. Do remove it if there is visible fungal problem or it has built up well past four inches.
Because bark is not adapted to constant moisture. Piled against the trunk it holds damp, softens the bark, and invites rot, boring insects and rodents that shelter in it. It also encourages roots to grow upward into the mulch and circle the trunk, eventually girdling the tree. Keep it three to six inches clear of the trunk and spread it outward to the drip line instead.
Bulk is almost always cheaper per cubic yard, and the crossover point is typically around two to three cubic yards. Below that, delivery fees and minimum order quantities cancel the saving. Above it the difference grows quickly — and so does the number of bags you would be carrying, since a cubic yard is 13.5 two-cubic-foot bags. The trade-off is that bulk arrives as a pile you have to barrow into place.
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