Walk through almost any new housing estate, retail park or office car park and you will see it: a tree sitting in the middle of a tall cone of bark chippings, piled high against the trunk like a small volcano. It looks neat, and it is meant well, but it is one of the most common ways that young trees are harmed. Mulch is one of the best things you can give a tree. The problem is not the material; it is where and how deep it goes.
What mulch actually does for a tree
A layer of organic material spread over the root zone does several useful jobs at once:
- It holds moisture. Mulch slows evaporation from the soil surface, which matters most for newly planted trees during their first few dry summers.
- It suppresses competition. Grass and weeds are surprisingly effective at stealing water and nutrients from young tree roots. A grass-free circle makes a real difference to how quickly a tree establishes.
- It evens out soil temperature, keeping roots cooler in summer and buffering them from sharp frosts.
- It feeds the soil. As organic mulch breaks down, worms and soil organisms pull it into the ground, improving structure and fertility over time.
- It protects the trunk from mower and strimmer damage, simply by keeping machinery further away.
Why the volcano causes harm
The base of a tree trunk, where it flares out into the main roots, is not designed to be buried. Bark on the stem needs air; piling damp material against it for months at a time creates the problems the tree has no defence against.
Bark decay and disease
Constantly wet bark softens and can begin to rot. That opens a route for fungal and bacterial infections into the living tissue just beneath the bark, which carries water and sugars around the tree. Damage that runs all the way round the stem can kill it.
Roots growing in the wrong place
Roots grow where moisture and air are available. A deep mound encourages them to grow up into the mulch rather than out into the soil. These shallow roots dry out quickly in summer, and some may circle the stem and gradually constrict it as both thicken.
Shelter for pests
A deep, loose heap against the trunk gives voles and mice a sheltered place to gnaw bark through the winter, out of sight.
Too much depth, too little air
Very thick layers, particularly of fine material such as grass clippings, can mat down and restrict the movement of air and water into the soil, which is the opposite of what you want.
How to mulch properly
Think of the right shape as a doughnut or a shallow saucer, not a cone.
- Clear the ground first. Remove grass and weeds from the area you plan to cover. For a young tree, aim for a circle at least a metre across.
- Water if the soil is dry. Mulch locks in whatever moisture is already there, so apply it to damp ground. Avoid spreading it over frozen soil.
- Spread to roughly 5–8cm deep. That is enough to suppress weeds and hold moisture without smothering the roots.
- Leave a gap around the trunk. Pull the mulch back a hand’s width from the stem so the root flare is visible and the bark stays dry.
- Go wide rather than deep. For mature trees, the feeding roots spread well beyond the trunk, often past the edge of the canopy. A wider, thinner layer helps far more than a tall heap.
Which material to use
Good options include composted bark, wood chip, leaf mould and garden compost. Wood chip from tree work is often available cheaply or free, and is close to what trees receive naturally in woodland. A mix of particle sizes knits together and resists blowing away.
A common worry is that fresh wood chip “robs” the soil of nitrogen. Some nitrogen is tied up where the chips meet the soil as they begin to decompose, but because the mulch sits on top rather than being dug in, the effect on tree roots below is generally small. If you are mulching vegetable beds or very young bedding plants, use well-rotted material instead.
Use well-rotted manure sparingly and keep it well away from the trunk. Avoid plastic sheeting under mulch around established trees: it stops organic matter reaching the soil and can upset the balance of air and water at the surface. Stones and gravel look tidy but give none of the soil-building benefits.
When to mulch, and how often
Late winter to mid-spring is the ideal time, once the soil has been moistened by winter rain but before it starts to dry out. Autumn mulching also works, particularly for newly planted trees. Organic mulch breaks down steadily, so top it up each year to keep the depth roughly consistent rather than adding a thick layer all at once.
Fixing an existing volcano
If you have inherited a tree with mulch, soil or compost piled against its trunk, it is worth correcting. Gently pull the material away from the stem by hand or with a small rake until you can see where the trunk widens into the roots. Spread the excess outwards into a broad, even layer. If you find thin roots circling tightly around the stem, or soft, discoloured bark, make a note and keep an eye on the tree; our guide to the warning signs of an unhealthy tree explains what to look for next.
The same principle applies to soil. Raising ground levels around an existing tree – for a new patio, a raised bed or simply a pile of spoil – can do similar damage on a larger scale. Keep the root flare at its original level.
A simple habit with long-term rewards
Getting mulch right costs nothing extra, takes a few minutes and pays off for years. If you are planting something new this season, our guide to choosing a tree for a small garden covers the planting itself. Wide, shallow and clear of the trunk: remember that, and you will give your trees exactly what they need.