It is my understanding that too much mulch around the base of trees
is harmful to them, but I see piles of mulch around the trunks of
trees everywhere I go. What is up with this "volcano mulching?"
A: I do not know how
this fad started, but you are correct that it is not a good
practice. I imagine that home gardeners see it in professionally
managed landscapes and mistakenly believe it is the proper way to
against the trunks of woody plants can cause the bark to rot. It is
also a great place for certain
insects and small animals such as
mice to damage the bark without being noticed. Once the plant shows
signs of injury, it may be too late to save it.
mulching against the trunk is bad for a tree
The cambium - the thin layer of light green wood just under the bark
– is responsible for transporting water and nutrients from the roots
to the leaves. It also transports the products of photosynthesis
from the leaves to the root system for storage. When the cambium is
destroyed, this transport system is disrupted. If damage is confined
to one side of the plant, that side of the plant may die. If the
cambium is destroyed around the circumference of the trunk - a
condition known as girdling - the plant can die altogether. Young
trees and shrubs have very thin bark, and are much more vulnerable
to the damaging effects of excessive mulch.
with excessive mulch depths
Another problem is that excessive mulch limits gas exchange in the
soil. We do not often think about it since roots are underground,
but they need oxygen as surely as you and I. I have seen declining
trees make miraculous recovery once the excessive mulch is removed.
Trees often try to survive excessive mulch and being planted too
deeply by growing adventitious roots – roots that are growing from
stem tissue rather than true root tissue. They are not sufficient to
support the tree, and often become girdling roots. These are roots
that grow around the trunk in such a way as to “strangle” the tree.
Certain species of trees such as maples (Acer spp.), sweet gums
(Liquidambar styraciflua) and lindens (Tilia spp.) have a tendency
to develop girdling roots, and excessive mulch exacerbates that
Mulch Volcano or not?
This Magnolia appears to have a
but it may have been
planted partially above ground.
downside of excessive mulch is that it may become water-repellent
once it dries out completely during hot, dry summer weather. Any
rain we get during such weather may simply run off the surface of
the mulch rather than benefiting the plants.
organic mulches benefit all plants by conserving soil moisture,
moderating soil temperatures and controlling weeds that compete for
water and nutrients. As an added bonus, they add organic matter to
the soil as they decompose. Mulching around the base of woody plants
also keeps lawn mowers and weed whackers at a safe distance from
their trunks. Young trees and shrubs are especially vulnerable to
mower or weed whacker “blight” because their bark is very thin.
The right mulch depth
Two or three
inches of mulch provide the benefits without endangering the health
of woody plants. Mulch can be placed within a couple inches of the
trunk or stems as long as it does not make physical contact. It is
ideal to mulch trees out to the drip line (the ends of the branches)
The right mulch
Some of the best materials to use as mulch include shredded bark,
wood chips, mushroom compost, composted sawdust, cocoa bean hulls
and leaf mold. Avoid peat moss, whole leaves and uncomposted grass
clippings because they tend to mat together, and once they dry out,
they are difficult to rewet.
Fine-textured materials such as leaf mold, mushroom compost, cocoa
hulls and finely shredded bark look better with herbaceous plants
than coarse bark or wood chips because they are more in scale with
the plants. Coarser textured bark and wood chips are better suited
to woody plants.