Mulch Volcanoes Are Killing Florida Trees

Drive through any neighborhood in Volusia County and count the trees with mulch piled a foot up the trunk in a neat cone. That practice — mulch volcanoes — is close to universal, it looks deliberate and maintained, and it is slowly killing a large number of the trees it is applied to.

Mulch is one of the best things you can do for a tree. Applied wrong, it is one of the worst.

What Mulch Actually Does For a Tree

Done correctly, mulch:

• Holds moisture in sandy soil that drains fast
• Moderates soil temperature, which matters in Florida summer
• Suppresses turf and weeds that compete with tree roots for water
• Breaks down slowly and adds organic matter to poor soil
• Keeps mowers and string trimmers away from the trunk, which prevents a very common source of fatal bark damage

That last one alone justifies mulching. Trimmer damage around the base of a tree, repeated over seasons, girdles the trunk and kills the tree, and it is entirely avoidable.

What a Volcano Does

Bark rot. Trunk bark is adapted to being exposed to air. Root bark is adapted to being in soil. They are different tissues. Burying trunk bark under constantly moist mulch causes it to break down, opening the tree to decay organisms.

Girdling roots. Roots grow into the mulch because it is moist and easy, and mulch piled against the trunk means those roots grow around the trunk. Years later they strangle the tree.

Oxygen deprivation. Deep mulch limits gas exchange at the root zone.

Rodent and insect harborage. Mulch against the trunk gives rodents cover to chew bark, and holds moisture that attracts pests.

Water shedding. Thick mulch, particularly finely shredded material, can mat and become water-repellent, shedding rain away from the root zone instead of letting it through. The tree ends up drier under a foot of mulch than it would be with none.

Hidden problems. You cannot inspect a trunk you cannot see. Girdling roots, decay, insect damage and mechanical wounds all go unnoticed under a mulch pile.

How to Do It Right

Two to three inches deep. That is it. Deeper is not better.

Wide, not tall. The goal is a broad ring, as far out toward the drip line as you can reasonably manage. Root systems extend well beyond the canopy edge, so more area covered is more benefit. A three-foot ring around a mature oak accomplishes very little compared with a ten-foot one.

Pull it back from the trunk. Several inches of bare soil around the base, with the root flare fully visible. If you cannot see where the trunk widens into the roots, the mulch is too close or too deep.

Flat, not conical. Think doughnut, not volcano.

Refresh rather than reapply. Mulch breaks down, which is the point. When it thins, add enough to return to two or three inches — do not add a fresh layer on top of the old one every year, which is how depths creep up over time.

Choosing Material

For most Florida landscapes, coarse organic mulch works best: shredded hardwood, wood chips, pine bark. Coarse material allows better water and air movement than fine shredded material, which mats.

Arborist wood chips — the material produced when a tree service chips brush — are excellent tree mulch and often free. They are variable in size, which is actually a virtue for water movement, and they break down into good soil.

Pine straw is the regional standard and works well, particularly on slopes where it stays put.

Avoid rubber mulch, which adds nothing to the soil, plastic sheeting under mulch, which blocks gas exchange, and very fine shredded material applied thickly, which mats and sheds water.

Cypress mulch is widely sold in Florida and worth considering carefully given sourcing questions around it.

The Turf Problem

Grass growing right up to a tree trunk is a genuine competitor. Turf is very effective at capturing water and nutrients in the top few inches of soil — exactly where a young tree's fine roots are.

A mulched ring eliminates that competition and eliminates the trimmer damage that comes from mowing right up to the trunk. For a newly planted tree, expanding the mulch ring is one of the most effective establishment practices there is.

Fix an Existing Volcano

If you have trees with mulch piled up the trunk, pull it back now.

Remove the excess, exposing the root flare and several inches of bare soil around the trunk. Then look at what you find. Check for girdling roots, for bark that has softened or discolored, and for insect or rodent damage. Spread the removed material outward to widen the ring rather than throwing it away.

If the trunk shows significant decay where it was buried, that is worth having assessed.

Tell Your Landscaper

Mulch volcanoes usually come from landscape crews doing what they have always done, not from malice. A specific instruction — two to three inches, pulled back from the trunk, spread wide — is generally all it takes.

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ArboristRX assesses tree health across Edgewater, New Smyrna Beach, Port Orange, Oak Hill and Volusia County, and we produce arborist wood chips that make excellent mulch. Call (386) 444-5959.