How to Mulch Around Trees the Right Way

Mulching is one of the simplest and most beneficial things you can do for your trees. A proper ring of mulch conserves soil moisture, moderates soil temperature, suppresses weeds, and adds organic matter as it decomposes. Done right, it mimics the natural forest floor that trees evolved with. Done wrong, it can cause serious damage. The difference comes down to technique.

The Volcano Mulch Problem

Drive through any neighbourhood in Calgary and you will see it: mulch piled high against tree trunks in a cone shape, sometimes 30 centimetres deep or more. This is commonly called "volcano mulching," and it is one of the most widespread landscaping mistakes in the city.

When mulch is piled against the bark, it holds constant moisture against the trunk. Tree bark is designed to be exposed to air. When it stays perpetually damp, it begins to break down, creating entry points for fungi and bacteria. Over time, the inner bark (phloem) and cambium layer can decay, girdling the tree from the outside in. The tree may look healthy for years while the hidden damage accumulates, then decline suddenly.

Volcano mulch also encourages girdling roots — roots that grow up into the moist mulch pile and circle the trunk instead of spreading outward. These roots gradually strangle the tree as both the root and the trunk grow in diameter. Girdling roots are one of the leading causes of tree decline in urban settings, and improper mulching is a primary cause.

The Right Way to Mulch

Proper mulching follows a few simple guidelines:

• Depth: Apply mulch 5 to 10 centimetres deep. This is enough to retain moisture and suppress weeds without smothering roots or creating anaerobic conditions.
• Width: Extend the mulch ring as far out from the trunk as practical. A ring that reaches to the drip line (the outer edge of the canopy) is ideal, but even a ring of one to two metres in diameter provides significant benefit. Wider is always better than deeper.
• Distance from trunk: Keep mulch at least 10 to 15 centimetres away from the trunk. You should be able to see the root flare — the point where the trunk widens at the base and transitions into the root system. If the root flare is buried, the tree is either planted too deep or has accumulated too much soil and mulch over the years.
• Shape: Think donut, not volcano. The mulch ring should be flat or slightly concave, not mounded up against the trunk.

What Type of Mulch Is Best?

For trees, coarse wood chips are the best mulch material. They decompose slowly, improve soil structure as they break down, and support beneficial fungal communities in the soil. Arborist wood chips — the kind produced by tree service companies when they run branches through a chipper — are excellent and often available for free.

Other options include:

• Shredded bark: Widely available at garden centres. It stays in place well and looks tidy, but decomposes more slowly than wood chips and offers less soil benefit.
• Composted wood mulch: Darker in colour and partially broken down. It provides nutrients more quickly but needs replenishing more often.
• Pine needles: Excellent around evergreens and acid-loving plants. They decompose very slowly and allow good air circulation.

Avoid using landscape fabric under mulch around trees. Fabric prevents the natural cycling of organic matter into the soil and can girdle surface roots. It also creates a barrier that makes it harder for water and air to penetrate to the root zone.

When to Apply Mulch in Calgary

Late spring, after the soil has thawed and warmed slightly, is the best time to apply or refresh mulch in Calgary. Applying mulch too early can insulate cold soil and delay root growth in spring. A fall application before freeze-up provides insulation for winter, which is especially helpful for recently planted trees.

Refresh your mulch annually by adding a thin layer on top of the existing material. Before adding new mulch, rake the old layer to break up any matted or compacted areas. Over time, if the mulch has built up beyond 10 centimetres, remove some before adding more.

Mulching Newly Planted Trees

Mulch is particularly important for newly planted trees, which are more vulnerable to moisture stress and temperature extremes. A wide mulch ring around a new tree also keeps lawn mowers and string trimmers at a safe distance — mechanical damage from mowing equipment is a surprisingly common cause of tree decline and death in young trees.

For a newly planted tree, create a mulch ring at least one metre in diameter. As the tree grows, expand the ring to keep pace with the canopy. This approach not only benefits the tree but also reduces the amount of lawn you need to maintain around it.

A Small Effort With Big Returns

Proper mulching costs very little and takes perhaps 20 minutes per tree. The return on that investment — healthier roots, better moisture retention, fewer weeds, and a longer-lived tree — is enormous. Just remember: flat and wide, not tall and tight.