www.arborage.com Arbor Age / April 2014 13
and even the optimal ultra-violet light
refl ection coeffi cient of said mulch.
For practical use, three things are most
important: material, depth and width.
For material, all mulch will provide
benefi ts over not having mulch but,
as mentioned earlier, organic mulches
provide more in terms of nutrient and
microorganism value than an inorganic
source material.
For depth, 2 to 4 inches seems to
provide the most benefi ts, while a lit-
tle more or less is still acceptable. The
economics story we started with is in-
terpreted in two ways: the cost/benefi t analysis we spoke of and
another theory — the law of diminishing marginal utility. Simply
put, charge too much for the toll and the number of users will go
down. Mulch has a similar law of diminishing marginal utility —
use too much mulch, and the value of mulch goes down. We are all
too familiar with the bright orange mulch stacked three feet high
around newly planted trees in shopping mall parking lot. Long story
short: don't volcano mulch.
Recommendations for the optimal width of mulched area
around a tree will differ from source to source. Although a ring
of mulch a few feet in radius around
the tree will provide benefi ts, most
sources recommend mulching trees
out to the dripline. While you can
certainly over-mulch a tree in terms
of depth, it is impossible to over-
mulch a tree in terms of width. For
the optimal width, I'll defer to the
quotable Dr. John Ball who once said,
"How wide should the mulch ring
be? Show me the property line and
we'll negotiate inwards."
A short list of mulch downsides are
mostly related to improper use, such as too deep or too close to the
base of tree, both of which can be avoided at the time of applica-
tion. Given the large number of known benefi ts, the short list of
drawbacks, and relatively inexpensive start-up costs, it is easy to see
why mulch is one of the smartest investments you can make for the
health of urban trees.
Brandon Gallagher Watson is director of communications at Rainbow
Treecare Scientifi c Advancements, and is an ISA Certifi ed Arborist (#MN-
4086A).
Volcano mulching (DON'T DO IT).
