Mulch Volcanoes and Buried Root Flares

Drive through any Florida subdivision and you will see it — mulch piled a foot up the trunk in a neat cone. It looks tidy. It is slowly killing the tree.

Why it is damaging

Bark is not roots. The bark on a trunk is adapted to being in air. Buried against constantly moist mulch it stays wet, softens, and admits decay fungi and insects. The damage happens at the root collar, which is the structurally most important part of the tree.

Girdling roots. Roots grow into the mulch pile because it is moist and easy, then circle the trunk. As both thicken, the root strangles the trunk. This can kill a tree twenty years later and the cause is invisible unless somebody excavates.

Moisture and air problems. Deep mulch against the trunk can hold water where it does not belong and, when it is very deep, restrict oxygen to the roots underneath.

Rodents. Mulch against a trunk is habitat, and voles and mice girdle bark under cover.

The root flare should be visible

A healthy tree has a visible flare where the trunk widens into the buttress roots at ground level. If your tree goes into the ground like a post, the flare is buried — either by mulch, by added soil, or because it was planted too deep in the first place.

That is worth correcting. Carefully excavating down to the flare, with a hand tool or with compressed air, and removing any girdling roots found there can add decades to a tree.

Mulching correctly

1. Two to four inches deep. Not more.
2. Pull it back three to six inches from the trunk so the flare is exposed and air can reach the bark.
3. Spread it wide — out to the drip line where you can. The benefit is in the area covered, not the depth.
4. Use coarse organic mulch. Wood chips, pine bark, pine straw. Avoid rubber and avoid dyed products.
5. Top up rather than piling on. Rake and refresh; do not add a new layer over an old compacted one every year.

What good mulch actually does

Moderates soil temperature, conserves moisture, suppresses competing turf, adds organic matter as it breaks down, and — importantly in this climate — keeps mowers and string trimmers away from the trunk. Mower damage at the base is a genuinely common cause of decline.

Done properly, mulch is one of the best things you can do for a tree. Done as a volcano, it is one of the worst.