Q. I purchased a
house with mature trees that have mulch 12 to 18 inches deep at the
base of each of them. The mulch has been there for at least two
years. I’m afraid that the trees have been damaged. Would it be a
good idea to remove the mulch?
A. You are
absolutely right to be concerned about so much mulch around the base
of your trees. Excessive mulch holds moisture against the bark,
which can eventually cause it to rot. Also, the roots under this
mulch have been buried too deeply, depriving them of oxygen. Another
problem with excessive mulch is that it creates a safe haven for
small animals such as mice to feed on the bark unseen until you
notice dieback in the top of the tree. By then, it may be too late
to save it.
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 trunk and root
system for storage. When the cambium is damaged or destroyed by the bark
rotting or by animal feeding damage, this transport system is
disrupted. If damage is confined to one side of the plant, one side
of it may die.
If the cambium is destroyed around the circumference
of the trunk - a condition known as girdling - the plant is doomed.
Thick rings of mulch can also become hydrophobic – water repellent –
when they dry out completely during long periods of dry weather like
we experienced this summer. Rain that falls after thick mulch rings
have completely dried out may be lost to runoff rather than
benefiting the affected tree.
Mulching Large Trees
trees have thicker bark and do not experience severe damage as
quickly as young trees with very thin bark, you should remove the
excessive mulch as soon as possible. You will probably find that the
trees have produced roots in the mulch. These are known as
adventitious roots. They are not true roots since they are produced
by stem tissue rather than root tissue, and are not capable of
supporting the trees as their real root systems decline. Those
adventitious roots will die after you remove the excess mulch, but
that should not cause additional damage to the trees. Restoring the
balance of air and moisture in the soil will do more to help them
now than anything else.
What to avoid!
Leave about two
inches of mulch. This is sufficient to obtain the benefits of
mulching without endangering the health of woody plants. It may be
matted together, so be sure to rake out the remaining mulch to break
it up. Keep it a few inches away from physical contact with the
trunk. If possible, extend the mulch out to the ends of the branches
(drip line). The fine, hair-like feeder roots responsible for
absorbing water and nutrients extend past the drip line of most
Mulch has been applied at the proper 2-inch
depth and extends out to the drip line
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 often succumb to lawn mower or weed whacker