Plant Depth and the Mulch Volcano Mistake

Two habits quietly shorten the lives of new trees: burying them too deep and heaping mulch up the trunk in a cone. Both look neat and feel caring. Both keep the bark wet, starve the roots of air and invite decay. The fix costs nothing and takes minutes.

Urban green space, from the I Hug Trees image library

Find the root flare first

Where a trunk meets the roots it widens into a gentle flare. That flare should sit at, or just above, the soil surface once planting is done. Many nursery trees arrive with extra soil piled over the flare, so the first job is to brush the top layer away until you can see where the trunk begins to widen.

Dig the hole as deep as the root ball is tall, no deeper, and about two to three times as wide. A wide hole with firm, undisturbed soil underneath lets roots spread sideways, where most of them live. A hole dug deeper than the ball lets the tree settle below the flare as the soil sinks.

What goes wrong when it is too deep

Roots need oxygen. When the flare is buried, bark that was never meant to stay damp is kept wet, and roots form a ring around the trunk instead of spreading out. Those circling, girdling roots can slowly choke the tree years later, often when it has grown large enough to be expensive to remove.

A tree planted too deep may look fine for several seasons and then decline. This is one reason new street trees fail long after the planting crew has left. Our guide to tree planting and long-term care covers what happens in those first years.

Mulch as a ring, not a volcano

Mulch is good for a tree. It holds moisture, moderates soil temperature and keeps mowers away from the trunk. The trouble starts when it is piled against the bark in a steep cone. Constant moisture on the trunk promotes rot, and sheltered mulch gives mice and insects a home beside tender bark.

Spread a layer of about 5 to 10 centimetres (2 to 4 inches) over a wide circle, as wide as you can manage, and pull it back a few centimetres from the trunk so the flare stays visible. Refresh it each year, and use wood chips or shredded bark rather than thick plastic or stones.

A simple check to repeat each spring

Walk around your trees once a year and look at the base. Can you see the flare? Is mulch touching the bark? Are any roots circling the trunk? If so, pull the mulch back and, for a young tree, gently loosen the soil to check.

Soil life matters here as well. Healthy ground full of fungi helps roots recover from mistakes, as we explain in the forest internet. Mulch that rots slowly feeds that life instead of smothering it.