Mulch helps moderate soil temperature and slows evaporation, but piling it against a trunk can hold moisture against the bark and invite decay. A thin, wide layer with the trunk exposed gives a young tree the benefits without creating a damaging wet collar.
Clear the planting area
Remove grass and weeds carefully without cutting shallow roots. Check that the soil drains and that the tree is not planted too deeply. Do not use mulch to hide a trunk that is already buried or damaged.
Build a wide ring
Spread clean, appropriate mulch in a broad ring over the root zone. Keep it several inches away from the trunk and avoid a volcano-shaped pile. A moderate depth is usually enough; excessive material can keep the soil saturated and reduce air movement.
Protect the surface
Keep mulch away from sidewalks, siding, and drainage grates. Do not mix fresh woody material deeply into the soil around a young tree. Water the root zone slowly when needed and check beneath the mulch before watering again.
Maintain seasonally
Rake the ring lightly to break crusts and remove weeds. Add only enough new mulch to maintain the layer rather than stacking year after year. Inspect the trunk flare, roots, and soil moisture whenever you refresh the bed.