📐How deep should your mulch go?

For weed control, only just. Two inches works over shallow-rooted shrubs like azaleas and rhododendrons and in beds you replant every season, where a deeper layer would be in the way. On open ground it lets enough light through the thin spots that annual weeds still come up, so three inches is the depth where suppression becomes reliable.

Two to four inches over the root zone, and nothing at all against the trunk. Leave a bare collar three to six inches wide around the base — the mulch volcano that buries a trunk keeps bark permanently damp, invites rot and encourages roots to circle in the mulch instead of anchoring in soil. Depth matters far less than that gap.

Yes, and it is the most common mistake in an established bed. Past four inches, rain has to saturate the mulch before any reaches the soil, the layer stays wet against stems, and fine roots migrate up into material that dries out completely in a heatwave. Deep mulch also goes anaerobic and can sour, which smells of vinegar and burns foliage it touches.

Four inches of shredded or double-shredded mulch, never nuggets. Shredded material knits into a mat that holds through a storm; bark nuggets and chips float and end up in the gutter. On anything steeper than about one in four, add jute netting under the mulch for the first season.

Push a screwdriver or a stick straight down until it hits firm soil, mark it with your thumb and hold it against a tape. Do it in three or four places across the bed and use the shallowest reading — mulch drifts, and the strip along the edging is always deeper than the middle.

A little. Fine, fluffy mulches like double-shredded hardwood pack down and can be laid at the 3-inch mark; coarse bark nuggets hold more air, settle less and are fine at 4 inches. Straw and pine straw are laid deeper still — 4 to 6 inches — because they compress to a fraction of their loose depth within weeks.