Mulch calculator

Depth is the whole decision

Mulch is bought by volume, and volume is area times depth, so depth is the only variable you actually control once the bed is dug. It is also the one people underestimate.

The point of mulch is to block light from reaching weed seeds. Under about two inches it simply does not do that — light penetrates, seeds germinate, and you have spent money on decoration. Three inches is the working figure for most beds. Four inches is worth it around established shrubs where you want moisture retention as well.

Going deeper than four inches stops helping and starts hurting: it holds water against stems and can starve roots of air. Never pile it against a trunk, however deep the rest of the bed.

Bags against bulk

The calculator gives both because the crossover point surprises people. Bagged mulch is convenient and easy to move, but expensive per unit volume. Bulk delivery is dramatically cheaper once you are past roughly half a cubic yard — which at three inches deep is a bed of only about 50 square feet.

Almost any real garden bed clears that threshold. The practical constraints on bulk are somewhere to tip it and the willingness to move it by barrow, not the price.

What to expect afterwards

Organic mulches — bark, wood chip, composted green waste — break down and feed the soil, which is the main reason to use them. They also settle, losing roughly a quarter to a third of their depth in the first year, so a three-inch layer is a two-inch layer by the following spring and wants topping up.

Fresh wood chip temporarily draws nitrogen from the soil surface as it decomposes. On a bed of established shrubs that is harmless. Around annuals and vegetables, use composted material instead.