of Rhode Island GreenShare Factsheets
You Guilty of Over-Mulching?
Over-mulching is a waste of time and money and is quickly
becoming the number one cause of death of many common bushes
and trees. There are many reasons why over-mulched trees, with
mulch piled high against the stem or trunk, die:
This is the number one cause of death by over-mulching.
Repeated applications of mulch can cause a waterlogged soil
and root zone, causing root suffocation. Roots must respire
and take in oxygen, unlike leaves that give off oxygen. The
problems that are caused from yearly over-mulching are not
immediate. The symptoms may take three to five years to express
themselves, and sometimes longer, depending on the species
and soil type. When oxygen levels drop below 10 percent, root
growth declines. Unfortunately, by the time you recognize the
symptoms (off-color foliage, abnormally small leaves, poor
growth and die-back of older branches), it is generally too
late to apply corrective measures. At this point, the plant
has gone into an irreversible decline. When roots decline and
die, so does the plant.
Above-ground stem tissue of most trees, shrubs and perennials
is morphologically different from roots and must be able to
freely exchange oxygen and carbon dioxide. Mulch that is piled
onto the trunks decreases gas exchange, with inner bark tissue
eventually dying. When the inner bark dies, roots no longer
receive the energy produced by the leaves through the process
of photosynthesis, and the plant dies.
Another mortality factor that is
associated with the application of mulch next to stem tissue
involves fungal and bacterial "canker" diseases.
Most plant diseases require moisture to grow and reproduce.
These lethal trunk diseases are no exception and usually gain
entry into the stressed, decaying bark tissue. Once established,
these cankers will eventually encircle the tree, killing the
inner bark, starving the roots and ultimately killing the plant.
Thick mulch layers that are placed
against the stem will begin to decay and can produce excessive
heat. Similar to composting,
where inner mulch layers may reach 120-140 degrees F, the heat
may directly kill young trees and shrubs or may prevent the
natural autumn "hardening" process that plants must go through
to prepare themselves for winter. If trunk flare tissue does
not adequately harden before freezing weather arrives, the
tissue will die, the roots will starve, and again, the plant
will go into decline. The tree needs healthy roots to produce
the hormones and chemical growth regulators that allow it to
properly prepare for winter weather.
Placing piles of mulch adjacent to tree trunks and other
plants can kill plants by providing cover and habitat for chewing
rodents such as mice, voles, etc. With lots of cover from predators,
the critters will usually live under the warm mulch in the
winter and chew on the tender and nutritious inner bark. Often
you may not notice this chewing until the following spring
or summer when the tree no longer looks good. If the chewing
is extensive (more than 50 percent of the circumference) or
goes around the whole tree (girdles it completely), there is
little you can do to save the tree.
Continuously using the same type of mulch may cause plant
death by changing the soil's acidity level, commonly referred
to as soil pH. Acid mulches like pine bark may have a pH of
3.5 to 4.5, and when applied continuously, can cause the soil
to become too acidic for most plants to grow. Conversely, hardwood
bark mulch, although initially acidic, may cause the soil to
become too basic or alkaline, causing acid-loving plants to
quickly decline. Soil pH's above 6.5 usually create micronutrient
deficiencies of iron and manganese for many common landscape
plants. You can avoid this by periodically rotating the type
of mulch used.
Finally, non-composted, "fresh" or
non-aged mulches may cause nitrogen deficiencies in many
young trees, shrubs and
flowers. Decomposing bacteria and fungi that ultimately break
down mulch must have an ample supply of nitrogen to do their
job. Most landscaping mulches are comprised of bark or wood
that has very little nitrogen available for the decomposing
bacteria. Hence, the bacteria in the soil utilize the existing
nitrogen to break down the mulch. This process may cause nitrogen
deficiencies and yellow leaves as a result of the excessive
Knowing what kinds of plants you have, in addition to
knowing your soil's drainage, is imperative. If you have shallow-rooted
species growing on poorly-drained soils (clays), mulch depths
generally should not exceed 2 inches. On the other hand, if
you have more deeply-rooted species growing on better-drained
loams or sandy soils, your plants would benefit from 3-4 inches
of mulch. With coarser-textured mulches (large nuggets), you
can go a bit deeper due to the better oxygen diffusion through
the mulch and into the soil. However, be more cautious with
the finer, double-shredded mulches on the market. A 1-2 inch
layer may be all you need to keep weeds down and prevent unnecessary
soil drying in the summer. Also, a mixture of mulch particle
sizes is recommended for a more continual process of mulch
breakdown and nutrient release.
The best way to determine if you
have a mulch problem in your landscape is simply to dig through
the mulch layer
to see how thick it really is. Sometimes you need only to rake
lightly your existing mulch to give it that "finished" landscape
appearance. As a rule-of-thumb, for the optimum health of the
plant, keep the mulch a minimum of 3-5 inches away from the
trunks of young trees and shrubs and 8-12 inches away from
mature tree trunks, and spread the mulch evenly to the outer
branch tips of the tree or shrub. If done properly, mulching
is one of the best things that you can do to your landscape
plants, but if done improperly, it can stress and predispose
plants to a premature and untimely death.
By Dr. Brian Maynard, Associate
Professor of Horticulture, University of Rhode Island
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program