Nursery Notes: Trees don’t like to be surrounded by ‘mulch volcano’

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What on earth is a mulch volcano? It is a simple term used to describe the excessive piling of mulch up against the base of a tree — like a little bark mulch volcano with a tree growing out the top. “Too Much of a Good Thing Can be Bad” and “Go Wide, Not Deep” are two more International Society of Arboriculture continuing education articles I’ve just read. The latter one asks the arborist to persuade his local newspaper editor to run an article on the mulch volcano. Here it is.

Two basic choices in mulch are inorganic rock materials and organic types like pine needles, straw or bark mulch. The benefits of the rock are its longevity, while the organics decompose and need to be replenished more often. This actually adds to the soil in a good way.

How much is too much? This depends on your soil type. Overmulching a tree or shrub can promote excessive soil moisture and promote subsequent root rots, even damaging the inner bark layer. It can promote fungal, bacterial, insect and even rodent damage to the root and crown areas of the plant material. Not good!

Again, how much is too much? Four to six inches of mulch is too much. Fine textured mulches tend to pack down and don’t breath as well as more coarse textured mulches so the coarse mulches can be applied deeper than the fine stuff. A depth of two inches is considered by most to be a deep enough layer to inhibit weed seed germination. In poorly drained clay soils, this two-inch layer is tops. One could get away with a two- to four-inch layer of mulch on really well drained soils.

Keep in mind that in either case you don’t want to pile the mulch up against the base of the tree or shrub. That is to say, pull the bark mulch away from the trunk to give it some extra breathing space.

Several excellent benefits of a good mulching program include less summertime watering due to reduced evaporation, less weed competition, moderation of extreme soil temperatures, improved plant growth and vigour, improved soil aeration and soil structure and increased soil fertility (organic mulches).

In nature, trees naturally mulch themselves out to the edge of their branches (dripline). These layers of mulch are applied a little at a time as seasons pass.

Research has shown that an eight-foot circle of mulch properly applied under young trees can quadruple the root densities when compared to trees grown under competition from turf grass. Better root means a healthier and better growing tree.

Evan Davies owns Beltane Nursery at 2915 Highway 3 in Erickson.

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