Mulch volcanoes around trees are becoming the new phenomenon in gardens as people become over zealous in their quest to care for their trees. On a gardening level, it’s becoming a dangerous practice as the tree will eventually suffer and incur problems and diseases.
Gardeners mulch around their trees in vast quantities in the belief that if they do they will stem weed growth, keep the moisture in and feed their tree for the duration of the season. This certainly is the benefit of mulching but creating these mounds, or “volcanoes” as they’re being colloquially referred to, can devastate a tree.
Scott Aker, from the Washington Post, wrote this;
The aim may be to preserve soil moisture in dry spells; the effect is exactly the opposite: No rain can penetrate a foot-thick layer of mulch, and because the mulch is sloped away from the tree, it runs off. Thick mulch also promotes the growth of mats of fungi that shed water. In effect, the mound functions as an umbrella over the root zone.
The correct way to mulch around your trees is to create a circle the width of the canopy with a weed mat or layers of moistened newspaper. This will keep the weeds at bay. Then apply a layer of slow-release fertiliser (I would recommend sheep manure) to feed the tree. Finally cover it with a layer of pea straw about 3cm (1in) thick. This will suffice for the whole growing season. Make sure that none of the materials used above are touching the trunk of the tree but give at least 10cm (4in) distance.