You’ve probably seen it: a neat cone of mulch piled high around a tree trunk, looking tidy and intentional. It’s everywhere—parks, gardens, car parks, new housing developments.
It’s also slowly killing the tree.
These “mulch volcanoes” (mulch piled against and up the trunk) are one of the most common—and most damaging—mistakes in tree care. They look professional, but they cause rot, disease, pest infestations, and structural failure.
Here’s why mulch volcanoes are so harmful, how to recognise the damage, and how to mulch trees properly.
What Is a Mulch Volcano?
A mulch volcano is mulch piled directly against a tree trunk, often in a cone or mound shape. Instead of a flat, shallow layer, the mulch is stacked high—sometimes 15-30cm deep—right up to (or even covering) the base of the trunk.
It’s usually done with good intentions:
- “More mulch = better protection”
- “It looks neat and finished”
- “Everyone else does it this way”
But those good intentions lead to serious, often irreversible harm.
Why Mulch Volcanoes Kill Trees
1. Bark Rot and Decay
Tree bark is designed to be exposed to air, not buried in damp organic matter.
When mulch is piled against the trunk:
- It traps moisture against the bark
- Bark stays wet for extended periods
- Fungal pathogens colonise the softened bark
- Rot spreads into the living wood beneath
This is especially bad in UK climates where mulch stays damp for months. The tree’s natural defences (designed for open air, not buried conditions) can’t cope.
2. Girdling Roots
Mulch piled high creates a false “soil line.” The tree responds by growing new roots into the mulch layer.
These roots:
- Circle around the trunk instead of spreading outward
- Eventually encircle and constrict the trunk (like a wire pulled tight)
- Cut off water and nutrient flow to the upper tree
- Weaken structural stability
Girdling roots can take years to cause visible symptoms, but by the time you notice (dieback, yellowing leaves, poor growth), the damage is often irreversible.
3. Pest Infestations
Thick mulch against bark creates ideal habitat for:
- Voles and mice – Nest in the mulch and gnaw bark for food and moisture (especially in winter)
- Insects – Bark beetles, borers, and other pests thrive in the damp, protected environment
- Slugs and snails – Though less harmful to mature trees, they can damage young saplings
Rodent damage is particularly insidious. A complete ring of gnawed bark (girdling) cuts off the tree’s vascular system and kills it, sometimes within a single growing season.
4. Excess Moisture and Anaerobic Conditions
Deep mulch layers:
- Hold excessive moisture around the root collar (where trunk meets roots)
- Reduce oxygen availability to roots
- Create anaerobic (low-oxygen) conditions that favour harmful bacteria
Tree roots need oxygen. Burying them under 15-30cm of decomposing mulch suffocates them.
5. Temperature Extremes
While mulch is supposed to insulate, too much against the trunk causes problems:
- In winter: Keeps the trunk too warm, preventing proper dormancy and making tissue more vulnerable to freeze damage
- In summer: Decomposing mulch generates heat, cooking the bark and root collar
Signs Your Tree Is Suffering from Mulch Volcano Damage
Damage often takes years to become obvious. Watch for:
- Soft, discoloured bark at the base of the trunk
- Fungal growth (mushrooms, conks, or brackets) on or near the trunk
- Adventitious roots (roots growing from the trunk above ground)
- Dieback in the canopy (dead branches, thinning foliage)
- Poor annual growth (short twig extensions, small leaves)
- Visible girdling roots circling the base
- Rodent damage (gnaw marks, stripped bark)
By the time you see canopy dieback, the damage is advanced. Early intervention (removing excess mulch) is critical.
How to Mulch Trees Properly
Proper tree mulching is simple and looks just as tidy as a volcano—without the damage.
The Doughnut Method (Not the Volcano)
Think doughnut, not volcano:
- Clear the area
Remove weeds like dandelions, grass, and debris from around the tree base. For persistent invaders such as bindweed or ground elder, tackle them before mulching—mulch won’t kill established perennials. - Expose the root flare
The root flare is where the trunk widens at the base and roots begin to spread. It should be visible above the soil. If it’s buried, carefully excavate until you can see it. - Leave a mulch-free zone
Keep a 7-15cm gap between the mulch and the trunk. The trunk should never touch mulch. - Apply mulch in a flat layer
Spread mulch evenly in a circle around the tree, 5-10cm deep (no deeper). Extend it as far as you can—ideally to the drip line (where the branches end), but even a 60-90cm radius helps. - Keep it flat
No mounding. No cones. Flat and even, like a doughnut with the trunk as the hole.
Best Mulch Materials for Trees
- Wood chips (composted or fresh)—long-lasting, good weed suppression
- Shredded bark—tidy appearance, breaks down slowly
- Leaf mould—free if you make it yourself, excellent for soil
- Compost (well-rotted)—adds nutrients, but needs topping up more often
If you’re comparing different mulch materials for your garden, our guide on bark mulch vs wood chips vs gravel explains the pros and cons of each option.
Avoid These Materials
- Fresh grass clippings—mat down, heat up, and smell
- Gravel or rock—doesn’t improve soil, can raise soil temperature excessively
- Rubber mulch—doesn’t decompose, can leach chemicals, doesn’t feed soil
How Often to Refresh Mulch
- Check depth annually
- Top up if it’s dropped below 5cm
- Don’t pile new mulch on top of old indefinitely—rake old mulch back and redistribute before adding fresh
What to Do If Your Tree Already Has a Mulch Volcano
Good news: you can fix this.
- Carefully remove excess mulch
Pull mulch away from the trunk until the root flare is visible. Use your hands or a rake—don’t damage bark or surface roots. - Inspect for damage
Check for soft bark, rot, girdling roots, or rodent damage. If you find serious issues, consult an arborist. - Redistribute mulch properly
Spread it in a flat, even layer (5-10cm deep) with a gap around the trunk. - Monitor the tree
Watch for signs of recovery (new growth, improved vigour) or continued decline.
The sooner you act, the better the tree’s chances.
Why Do Mulch Volcanoes Keep Happening?
If mulch volcanoes are so harmful, why are they everywhere?
- It looks intentional – The cone shape looks “designed” and tidy
- Misunderstanding of “more is better” – People assume extra mulch = extra protection
- Landscapers repeat the mistake – Some companies do it because clients expect it (or don’t know better)
- It’s fast – Easier to pile mulch high than spread it properly over a wide area
Education is the fix. Once you know what proper tree mulching looks like (and why it matters), you can’t unsee mulch volcanoes.
Quick Reference: Mulch Volcano vs Proper Mulching
| Feature | ❌ Mulch Volcano (Wrong) | ✅ Proper Mulching (Right) |
|---|---|---|
| Shape | Cone/mound against trunk | Flat doughnut with gap at trunk |
| Depth | 15-30cm+ (often excessive) | 5-10cm (consistent depth) |
| Trunk contact | Mulch touches or buries trunk | 7-15cm gap around trunk |
| Root flare | Buried and invisible | Visible above mulch |
| Coverage | Often concentrated at trunk | Spread wide to drip line if possible |
| Long-term effect | Rot, girdling, pest damage, death | Healthy roots, moisture retention, weed suppression |
Common Questions About Mulch Volcanoes
Can a tree recover from mulch volcano damage?
It depends on how long the volcano was in place and the extent of damage. If caught early (before significant rot or girdling), removing the excess mulch and proper care can help recovery. Severe cases may be irreversible.
How deep should tree mulch be?
5-10cm (2-4 inches) is ideal. Never more than 10cm, and always kept away from the trunk.
Should I mulch newly planted trees?
Yes—but properly. Mulch helps retain moisture and suppress weeds while the tree establishes. Just follow the doughnut method and keep mulch away from the trunk. For more detailed guidance, see our article on mulching techniques for newly planted ornamental plants.
Can I use mulch around fruit trees?
Absolutely. The same rules apply: flat layer, 5-10cm deep, gap around the trunk. Fruit trees benefit from mulch (moisture retention, weed suppression, nutrient cycling).
Final Thoughts
Mulch volcanoes are a perfect example of a well-meaning practice causing long-term harm. They look tidy and professional, but they rot bark, encourage pests, and kill trees slowly.
The fix is simple:
- Keep mulch away from the trunk (7-15cm gap)
- Keep it flat (5-10cm deep, no mounding)
- Spread it wide (as far as practical, ideally to the drip line)
- Expose the root flare
Think doughnut, not volcano. Your trees will live longer, healthier lives for it.
