Why you shouldn’t make this big mistake when mulching around trees

Experts agree that mulch will do wonders for your garden, but there are certain things you should always avoid

adding bark mulch to a flowerbed and around a tree
(Image credit: ronstik/Alamy Stock Photo)

September is the ideal month to start thinking about mulching, but if you’re going to treat your trees as well as your beds and borders to a healthy dose of organic goodness, avoid the new trend of the ‘mulch volcano’ experts warn, where mulch is mounded high against the base of a tree to create a neat and uniform look.

So just what is it about this method that makes it a tree mulching mistake? 'A mountain of mulch, piled high against a tree trunk will not kill it immediately,' says Martha Smith, a horticulture educator at the University of Illinois. 

'Rather, it is a slow death, and homeowners don’t associate their actions with tree decline several years after they over-mulched a tree.'

high mound of mulch surrounding a young tree

When mulch is piled high around a tree trunk it can lead to problems

(Image credit: Ozgur Coskun/Alamy Stock Photo)

Why you need to avoid this mulching mistake

Landscaping around trees with mulch can offer benefits, but if done incorrectly or too much mulch is used, Martha Smith advises it can lead to problems.

Piling mulch onto the bark exposes it to dark and moisture, says Martha. 'The bark will start to rot, and rotted bark cannot protect the tree from insects and diseases, which grow better in this type of environment.' 

If mulch is too heavy, it can deprive the roots of oxygen and reduce the soil’s ability to dry out. The mulch will also produce heat which will over-protect the tree and prevent its natural cycle of preparing for the seasons. 

And, too much mulch promotes the growth of secondary tree roots which encircle the trunk and can choke off the tree’s vital main roots.

Mulch does matter when used correctly

Used correctly, mulch helps the soil transition between seasons and can bring benefits such as improving quality and helping to get rid of weeds

BBC Gardeners' World presenter Adam Frost, whose new book How I Garden (BBC Books), available on Amazon (opens in new tab), is out now, says his mulch of choice is well-rotted manure. 

'Mulches come in handy as they suppress new weed growth, help hold moisture in, make the garden or pots look tidy and depending on what you use, can also feed the soil.'

person mulching a flower bed

When used correctly, mulch can have many benefits for your beds and borders

(Image credit: Elizabeth Whiting & Associates/Alamy Stock Photo)

Using the right materials

Natural leaf droppings gathered from woodlands or your garden cost nothing and makes an amazing mulch. 

To learn how to make leaf mold, simply leave leaf mulch in loosely tied bags for a year or so to degrade and when you apply it to your soil it will appear to disappear in no time. 

hands holding leaf mould

Leaves that have rotted down can make a fantastic mulch for your garden

(Image credit: Deborah Vernon/Alamy Stock Photo)

Other organic mulches include wood chips, pine needles, hardwood or softwood bark and cocoa hulls. 

Some organic mulches decompose quicker than others, however. Be warned; mulches of evergreen leaves, such as cypress, take years to break down and can look unsightly.

Jayne Dowle
Freelance writer

Jayne Dowle is an award-winning gardening, homes and property writer who writes for publications including Sunday Times Home, Times Bricks & Mortar, Grand Designs, House Beautiful and The Spectator. She was awarded the Garden Journalist of the Year accolade at the Property Press Awards in 2021.