HomeMy Public PortalAboutMulch Out Not Up
Proper Mulching Techniques and
Problems Caused by Over-mulching
Information in this publication was borrowed extensively from articles by Chris
Carlson, Dir. Hort. Tech., Kent State U.; Diane Relf, Environmen. Hort. VA Coop
Ext, VA Tech; Dr. Robt. Nuss, Hort. Penn State U.; Donald Rakow, Cornell Univ.
Drawings copied from International Society of Arboriculture Training Manual or
drawn by Barbara Lupfer, Certified Arborist..
Compiled and edited by Barbara Lupfer, Certified Arborist
Edited and Produced by Jay Banks, Certified Arborist, Town of Leesburg, VA.
Urban Forester.
Symptoms of Decline
Death from over-mulching is gradual, with symptoms
sometimes taking 3-5 years to express themselves. It
starts with the decline of plant vigor and rate of growth.
Off-color leaves (pale or marbled)
Abnormally small leaves
Poor twig growth
Die-back of older branches
Rotting, pealing trunk bark under the mulch
are classic signs which get worse every year, and at
which point they are recognized, it is too late to apply
corrective measures.
Benefits of Proper Mulching
Good tree maintenance is common sense – it is what
trees need to flourish in nature. In the wild, the forest
floor is naturally covered with a layer of decomposing
leaves, twigs and other plant material. In urban settings,
the most common mulch is made of wood chips of varying
types and sizes. Mulch:
Impedes growth of weeds and grass that compete
with tree roots robbing them of water and nutri-
ents.
Conserves soil moisture by slowing down the evap-
oration of water from the soil surface helping to
retain more water for root use for longer periods
of time.
Protects the trunk from mower/ weed whacker
damage by eliminating the need to mow or trim
immediately around the trunk.
Reduces soil compaction by reducing foot and ve-
hicle traffic allowing roots to breathe.
Moderates soil temperature keeping the roots
cool in the summer and warm in the winter there-
by reducing stress.
Improves soil fertility as it decomposes.
Prevents erosion.
Incorrect Mulching
No higher than the heal of your hand, generally
2-4 inches. If using finely textured or double
shredded mulch, use 1-2 inches because these
materials allow less oxygen to the root zone.
Not against the trunk – keep all mulch 3-4
inches away from the trunk of the tree or
shrub, allowing the root flare zone to show just
above ground level.
To the tree’s drip line if possible. Remember
that the drip line moves out as the tree grows.
(see “How Roots Really Work” drawing.)
Other Tips:
If a “fresh” look is desired each season, take
some of the old mulch away before adding a new
layer to reach the 2-4” depth. Just lightly rak-
ing the existing mulch can achieve a finished
look. Applying new material over old in succes-
sive years is the same as applying a too deep
layer all at once!
Remember: Keep the bark dry and the roots moist.
Root Flare
Zone
2-4” Deep
Drip
Line
Proper Mulching Method
Problems Associated With Over-Mulching
Incorrect mulching is a waste of time and money
and is quickly becoming the number one cause of
death of trees and shrubs. Over-mulching, with
mulch piled high, directly against the stems or
trunks, smothering the root flare zone; or with
very deep mulch covering part or all of the root
area cause:
Root Suffocation/ Root Rot - Repeated or deep
applications of mulch cause waterlogged soil by
slowing water loss through evaporation. Roots
must “breathe”, taking in oxygen. When oxygen
levels drop, root growth declines then they die,
making it impossible for the plant to take up water
and nutrients, leading to death.
Inner Bark Death - The living tissue (phloem)
just inside the outer bark must be able to freely
exchange oxygen and carbon dioxide. Mulch piled
high onto the trunk decreases gas exchange, killing
the inner bark and then the roots which can no
longer receive food from the leaves.
Rodent/ Insect Chewing – Deep layers of mulch
against the trunk provide a perfect habitat for
these pests. If chewing is extensive (more than
50% of the circumference) or “girdles” the entire
tree, little can be done to save the tree.
Fungal and Bacterial Diseases – These can grow
and reproduce in the thick, moist mulch next to
the trunk, gaining entry into the stressed, decay-
ing bark. Once established, cankers caused by
these diseases, can encircle the tree, killing the
inner bark, starving the roots and killing the plant.
Excessive Heat – Thick layers of mulch begin to
decay and can produce heat (similar to compost-
ing).
Waterproof Layers – Thick layers of certain
mulches can create impervious surfaces that do
not allow water to reach to roots, especially during
drought. Roots dehydrate and die, killing the tree.
Because Roots Need
Oxygen in order to grow
they don’t normally grow in
the compacted, oxygen
poor soils under paved
streets.
Note: A few species have a Tap-
root that grows straight down
three to seven feet or more until
they encounter impenetrable soil
or rock layers, or reach layers with
insufficient supplies of oxygen.
Between 4 and 11 Major Woody Roots originate from
the root collar and grow horizontally through the soil.
These major roots branch and taper over a distance of
3-15 feet from the trunk to form an extensive frame-
work of long, rope-like roots which as 1/4 to one inch
in diameter. These are important structural roots,
supporting the tree against wind, etc.
Illustration from International Society of Arboriculture
The Root Collar is usu-
ally at or near the
ground-line and is identi-
fiable as a marked swell-
ing of the tree trunk.
The Framework of
major roots usually lies
less than 8—12 inches
below the surface and
often grows outward to a
diameter one to two
times the height of the
tree.
A complex network of
smaller non-woody Feed-
er Roots grow outward
and upward from the
framework roots. These
smaller roots branch 4 or
more times to form fans
or mats of thousands of
fine, short, non-woody
roots. These slender
roots, with their tiny root
hairs, provide the major
portion of the absorption
surface of a tree’s root
system. They compete
directly with the roots of
grass and other ground-
covers.