Guides

How much mulch do I actually need?

Mulch is sold by the cubic yard in bulk and by the bag everywhere else, but you measure your beds in square feet. Here is the arithmetic that connects the two, the depth worth aiming for, and the point where a delivery beats loading bags into the car.

The formula

Everything follows from one line:

The 324 is not magic. A cubic yard is 27 cubic feet, and an inch is a twelfth of a foot, so 12 × 27 gives you the number that converts square feet and inches straight into cubic yards. A 200 square foot bed at 3 inches deep is (200 × 3) ÷ 324 = 1.85 cubic yards.

Working the other way, one cubic yard covers:

How deep

Three inches is the number most beds want. It is deep enough to block the light weed seeds need, and shallow enough that water and air still reach the soil. Two inches is fine around established plantings that already shade their own ground.

Going deeper is not better. Past about four inches, organic mulch starts holding water against the soil surface and keeping air out of the root zone, and fine-textured mulches can mat and shed rainwater instead of letting it through.

Keep mulch a few inches clear of trunks and stems. Piling it against bark — the cone shape sometimes called a mulch volcano — holds moisture against living tissue, invites rot, and gives rodents cover to chew the bark over winter. Mulch the ground, not the plant.

Organic mulch breaks down, which is the point: it is feeding the soil. Expect to top up roughly an inch a year rather than re-laying the full depth. Measure what is actually left before ordering — a bed that still has two inches on it needs one more, not three.

Bags or bulk

A standard bag holds 2 cubic feet, so it takes 13.5 bags to make a cubic yard. Some brands sell 3 cubic foot bags, which is 9 to the yard. Check the bag before you do the maths, because the two look identical on a pallet.

Bulk is cheaper per yard but carries a delivery fee, so the comparison is not per-unit, it is total. Bags win on small jobs because there is no delivery to amortise. Set the two totals equal and solve for the crossover:

With bags at $4 and bulk at $40 a yard delivered for $75, they break even a little over 5 cubic yards. With bags at $5 and bulk at $30, it drops to 2. So the honest answer is somewhere between two and five cubic yards, and which end you land on depends entirely on your local delivery fee. Run your own three numbers — it is one line of arithmetic and it is the only way to get it right for your supplier.

Two things that are easy to forget. A cubic yard of mulch runs roughly 400 to 800 pounds dry depending on the type -- pine bark at the light end, hardwood and cedar heavier -- and well past 1,000 pounds after a couple of days of rain. A half-ton pickup takes a dry yard comfortably and is at its limit with a wet one; two yards is asking a lot of the suspension either way. And bulk arrives as a pile on your driveway that has to move by wheelbarrow, which is real work — some people pay the bag premium happily for exactly that reason.

Ordering

• Add about 10% for irregular beds, spillage and settling. Running out with one bed left is worse than a leftover half yard.
• Bulk is usually sold in half-yard increments, so round up to the next half.
• Dyed mulches hold colour longer but the dye fades from the top down; a light top-up refreshes the look without a full re-lay.
• If you are laying mulch over bare soil for the first time, weed it properly first. Mulch suppresses seeds; it does not kill established roots.

Block out your beds in the planner

Draw the beds and the planner gives you the square footage and the cubic yards together, so you can try 2 inches against 3 and see what it does to the order before you buy anything.