Mulch should sit 2 to 3 inches deep across the bed and never touch the trunk of a tree or shrub. That's the entire rule. Nearly every mulch problem we get called about breaks one half of it.
Under 2 inches, mulch doesn't do its job. Sunlight reaches the soil, weed seed germinates, and the bed needs pulling again by June. Over 3 inches, rain can't get through - the mulch soaks it up and the soil underneath stays dry. Shrub roots start growing up into the mulch layer looking for water, which leaves them exposed when it finally does dry out.
The volcano is the worst version. Mulch piled 8 or 10 inches up a trunk holds moisture against the bark around the clock. Bark isn't built for that - it rots, insects move into the soft spots, and roots start circling inside the pile instead of spreading out. The tree declines so slowly that nobody connects it to the mulch job from four years back. You can see volcano mulching in half the parking lot islands in Wake County. Crews do it because it looks tidy for a month.
What we do instead: pull mulch 3 to 6 inches back from every trunk so the root flare stays visible, taper up to full depth at the edge of the ring, and refresh at 1 to 2 inches a year rather than burying the old layer.
Ordering math: one cubic yard covers about 100 square feet at 3 inches. Walk your beds with a tape measure, multiply length by width, and divide by 100. We install double-ground hardwood at $85 per cubic yard inside Apex, delivery included.
If a previous crew volcanoed your trees, don't strip it all in one go in summer heat. Rake it back to the drip line, expose the root flare, and let the tree adjust. Text a photo to (919) 555-0187 and we'll tell you whether it's a quick fix or a bed rebuild.