Mulch in bags and yards, at the depth plants want

Two inches over perennials and three over shrubs, and four is the ceiling for anything. The band is narrow because mulch has two jobs that pull against each other: it has to be thick enough that light never reaches a weed seed, and thin enough that a summer shower gets through it to the roots. Below two inches weeds come through within weeks. Past four, shredded bark mats into a thatch that sheds rain sideways, keeps the soil cold and airless, and encourages the plant to send feeder roots up into a layer that dries out completely in the first hot week.

Four inches over the roots and nothing at all against the bark — three to six inches of bare ground on every side of the trunk. The mulched area wants to be as wide as you can make it, out toward the drip line, because that is where the feeder roots are and every square foot of ring is a square foot not being mown, compacted or hit with a string trimmer. The clear circle in the middle is not a detail. This calculator subtracts it from the area so the ring and the gap are ordered as one shape.

It keeps the bark permanently wet, and bark that stays wet is no longer a barrier. A trunk is built to shed water and dry off; buried in damp mulch it softens, cankers get a foothold, and voles and mice tunnel through the pile and girdle the tree under cover. The second failure is slower and just as fatal: roots grow into the mulch cone and circle the trunk, and years later those girdling roots strangle the tree from underneath. Both are avoidable by pulling the pile back until the root flare — the place the trunk widens into the ground — is visible.

No, in most beds you top it up, which is why this page asks how much is already down. Mulch that has broken down into the soil is doing exactly what it should, and removing it removes the organic matter you paid for last year. Two situations do call for stripping: a bed that already measures four inches, where more would take it past the useful depth, and a layer that has matted into a water-shedding crust, which is usually fine shredded bark laid too thick. In the second case, breaking the crust with a rake often solves the problem without hauling anything away.

At around fourteen bags, which is the point where you are handling a cubic yard two cubic feet at a time. Bags win below that on convenience and on the absence of a delivery fee, and they let you buy exactly what a small bed needs. Above it the arithmetic turns: the same volume delivered loose is usually cheaper per cubic foot before the fee, and it arrives in one pile rather than in a trunk that has to be emptied, split open and flattened. The price panel on this page settles it with your own numbers, because the crossover moves with what your yard charges to send a truck.

Only in the top half inch where wood touches soil, and only if you dig it in. Microbes decomposing woody material do pull nitrogen out of their immediate surroundings, but on the surface that zone is a thin skin at the mulch-soil boundary, well above the root zone of an established plant. The problem is real when fresh chips are tilled into a bed, which puts that competition throughout the root zone at once. Leave wood mulch on top and there is nothing to correct; till it in and there is.

Just under nine cubic feet, which is five bags, at four inches deep with an eighteen inch bare circle. The arithmetic is worth seeing once: the full six foot circle is 28.3 square feet, the bare middle takes out 1.8 of them, and 26.5 square feet at four inches deep comes to 8.8 cubic feet. Widen the same ring to eight feet and it goes to 16.2 cubic feet — nearly double, because the area of a circle grows with the square of its diameter. That is also the argument for making the ring wide while the tree is young and the mulch is cheap.