Mulching: the Doughnut, Not the Volcano

Mulch is the least glamorous element of a fine planting and one of the most consequential. Done well, it moderates soil temperature, holds moisture through dry spells, suppresses the weeds that would otherwise compete with new roots, and feeds the soil as it breaks down. Done badly — and it is done badly on streetscapes and front gardens all over the country — it slowly injures the very tree it was meant to protect.

The difference comes down to a shape, and the diagram above says most of it: build a doughnut, never a volcano.

The doughnut

A correctly mulched tree sits at the centre of a broad, flat ring of mulch:

• Depth: about 75 mm. Enough to shade the soil and hold moisture; not so much that rain and irrigation cannot reach the ground beneath. Deeper is not better — beyond about 100 mm, mulch begins to shed water and starve the soil of air.

• Clear of the trunk by 5–10 cm. The mulch ring stops short of the bark on every side, leaving the root flare open to the air. You should be able to see soil, and the flare, in a neat circle around the trunk.

• Wide rather than tall. Extend the mulch to the edge of the planting area — ideally out to the tree's drip line, and further as the canopy grows. The feeding roots of an established tree live in the top few hundred millimetres of soil across that whole footprint, and every square metre of mulch is a square metre of cool, moist, weed-free root run.

Use a coarse organic mulch — chunky hardwood chip, or a blended forest mulch that still has structure to it. Coarse material lets water and air through and lasts a season or more. Fine mulches, lawn clippings and anything that mats down should stay out of the tree ring.

The volcano

The volcano is mulch piled high against the trunk, tapering away like a cone. It looks tidy and generous, which is why it persists. What it actually does:

• Rots the collar. Trunk bark held permanently damp softens and admits decay — the same collar rot caused by planting too deep, arrived at from above. See Planting Depth & the Root Flare; the volcano undoes everything that guide asks of you.

• Sheds water. A cone of mulch behaves like a thatched roof. Rain and irrigation run off the mound and away from the rootball, so the tree stands in a dry pocket while everything around it looks well watered.

• Grows roots in the wrong place. Roots colonise the moist mulch pile itself, circling the trunk above the flare, where they later thicken and girdle it.

• Harbours pests. A warm, damp mound against the bark is an invitation to fungal disease and, in some districts, to termites exploring upward under cover.

If an established tree in your garden currently wears a volcano, the remedy is pleasantly simple: pull the mulch back from the trunk by hand until the root flare is visible, and spread the surplus outwards into the ring.

Mulching through the Queensland year

In the tropical north, mulch earns its keep in the dry months and needs watching in the wet: keep the ring clear of the trunk with particular discipline through the wet season, when everything stays damp for weeks. At the elevation of The Ravensbourne Estate, a full 75 mm blanket going into winter buffers the root zone against frost, and going into summer holds precious moisture through dry westerly spells. In both climates, top the ring up annually as the mulch breaks down — breaking down is the mulch doing its job.

From the growers — After spreading mulch, run your hand around the trunk. If your fingers cannot circle the base and touch bare soil the whole way round, the mulch is too close. It is a five-second check that prevents a ten-year problem.

If you would like a recommendation on mulch type and quantity for your planting — we are happy to calculate it from your tree size and bed dimensions — do get in touch, or see the mulch rings on our own stock during a by-appointment estate visit.