Thinking of Mulching Around Your Trees?
Mulch is one of the best things you can do for trees — if you get it right. Done wrong, it can damage bark, suffocate roots and encourage disease. Here’s how to do it properly.
Quick Answer: Tree Mulching Dos and Don’ts
Mulching around trees mimics what happens naturally in woodland. Fallen leaves and organic debris accumulate on the forest floor, decomposing to create a rich, moisture-retentive layer that feeds the soil and suppresses competing plants. When we mulch garden trees, we’re simply recreating this process with materials that look tidier. The key benefit for weed prevention is that it eliminates the bare soil around the base where weeds typically establish.
Why Mulch Volcanoes Are Dangerous
A “mulch volcano” is the cone-shaped pile of mulch you see heaped up against tree trunks in parks, front gardens and commercial landscapes everywhere. It looks deliberate, so people copy it — but it’s genuinely harmful:
- Bark rot: Tree bark is designed to be exposed to air. Damp mulch pressed against it keeps the bark permanently wet, creating ideal conditions for fungal pathogens to attack and rot the bark tissue
- Stem-girdling roots: When the root flare is buried under mulch, the tree can develop roots that grow around the trunk instead of outward. Over years, these girdling roots tighten like a belt, cutting off the tree’s own vascular system
- Pest habitat: Mulch piled against trunks creates a sheltered, moist environment favoured by bark beetles, voles and other organisms that can damage the tree
- Root suffocation: Excessively deep mulch (15cm+) can prevent adequate oxygen reaching the root zone, suffocating fine feeding roots
The fix is simple: always leave a 10-15cm gap between the mulch and the trunk. The root flare — the point where the trunk widens as it meets the ground — should always be visible. Think doughnut, not volcano. To learn more about this critical mistake, see our detailed guide on why mulch volcanoes kill trees.
The Root Flare: Why It Must Stay Exposed
When you look at a healthy tree, you should see the trunk gradually widen at ground level where the main structural roots emerge. This is the root flare, and it serves as the transition zone between above-ground trunk tissue and below-ground root tissue. The bark in this area is different from the bark higher up — it’s thinner and more vulnerable to moisture damage.
If your tree looks like a telephone pole stuck straight into the ground with no visible widening, the root flare is probably already buried — either by mulch, soil that’s been piled up during landscaping, or gradually accumulated debris. Carefully scraping back material to expose the flare is well worth the effort for long-term tree health.
How to Mulch a Tree Correctly
- Clear the area. Remove any existing weeds from around the tree. For stubborn perennial weeds like bindweed or ground elder, carefully spot-treat with glyphosate using a paintbrush or weed wiper to avoid getting any on the trunk or exposed roots
- Define the area. Ideally, mulch out to the tree’s drip line (the outer edge of the canopy). For a mature tree with a 3m canopy spread, that means a 3m diameter mulched circle. For young trees, a minimum 1m diameter ring is a good starting point
- Apply 7-10cm of mulch. Composted bark, wood chip or leaf mould are all excellent choices. Spread it evenly across the whole area
- Create the gap. Pull mulch back 10-15cm from the trunk in all directions. The root flare must remain visible and exposed to air
- Check annually. Top up each spring to maintain depth, and always re-check that the gap around the trunk hasn’t been filled in by mulch migrating inward
Best Mulch Types for Trees
For fruit trees and soft fruit, composted bark or well-rotted garden compost are particularly good choices. They add nutrients as they decompose, supporting healthy fruit production. Avoid very acidic mulches (pine needles, ericaceous compost) around trees that prefer neutral to alkaline soil unless you’ve tested your soil pH first. To compare different materials, see our guide on bark vs wood chips vs gravel.
Trees in lawns present a particular challenge. Grass competes directly with tree roots for water and nutrients, and mowing close to trunks risks bark damage. Creating a mulched circle around lawn trees — eliminating the grass competition — is one of the single best things you can do for tree health. It’s far more effective than any fertiliser.
Frequently Asked Questions
How wide should the mulch ring be around a tree?
As wide as practical — ideally to the drip line (the outer edge of the canopy). For mature trees, even a 1-1.5m diameter ring is beneficial if you can’t mulch the full area. For newly planted trees, aim for at least 1m diameter and expand as the tree grows. The wider the mulched area, the less root competition from grass and weeds. Before buying, check how much mulch you’ll need for the area.
Can mulch cause tree root rot?
Not if applied correctly. A 7-10cm layer of organic mulch actually improves drainage and soil structure. Problems only arise when mulch is piled too deep (15cm+) or pressed against the trunk. The trunk-contact issue causes bark rot, not root rot. Keep mulch at the right depth and away from the trunk and your tree will thrive.
Should I put landscape fabric under mulch around trees?
Generally no. Fabric restricts gas exchange and can prevent the beneficial decomposition of organic mulch into the soil. It can also interfere with surface root development. For trees, organic mulch directly on soil is better. Cardboard under mulch is an alternative if you need extra weed suppression initially — it decomposes naturally within a year.
Can I use weed killer around trees instead of mulch?
With extreme care. Using herbicides near trees risks damage to roots, bark or foliage from spray drift. Glyphosate is relatively safe when applied carefully to weed leaves only (it deactivates in soil), but broadleaf herbicides can damage trees. Mulch is the safer, longer-lasting approach — it prevents weeds rather than treating them repeatedly.
How long after planting should I mulch a new tree?
Immediately. Mulching at planting time is ideal. It conserves moisture around the developing root system during the critical establishment period and prevents weed competition. Apply 7-10cm of composted bark in a circle at least 1m in diameter, leaving the gap around the trunk. This single step significantly improves survival rates for newly planted trees.
Does mulch attract pests to trees?
Organic mulch can harbour slugs, woodlice and other invertebrates, but these rarely harm established trees. The exception is mulch piled against the trunk, which can attract bark-damaging organisms. If rodent damage is a concern (voles chewing bark in winter), keep mulch 15cm away from the trunk and consider a spiral tree guard for young trees.
Weeds Around Your Trees?
Mulching is the safest way to control weeds near trees. For persistent weeds that need treating first like dandelions or ground elder, our mulching guide and weed killers range cover all the options.
