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University
of Rhode Island GreenShare Factsheets
Maintaining
Healthy Rhododendrons
and Azaleas in the Landscape
Rhododendrons
and azaleas, which are closely related, are both popular flowering
shrubs in the Northeast. These shrubs thrive in many urban environments,
with relatively few serious health problems once they are established.
They do, however, have some special requirements that must be met
in order to ensure good health. Essentially these require matching
the environmental conditions to those of the areas to which these
shrubs are native. Rhododendrons and azaleas grow on forest floors
in many parts of the world, in shaded habitats with acidic soils
rich in organic matter. Soils are often covered with a surface layer
of decaying leaf litter. Matching these conditions where native
rhododendrons and azaleas thrive is the key to their good health
in the landscape. Plant in areas with good soil drainage, low pH
and partial shade, sheltered from direct afternoon sun and winter
winds. Some of the more common problems that can occur on rhododendrons
and azaleas include iron deficiency, winter injury (burn), black
vine weevil and Phytophthora root rot. Yellowing
of rhododendron leaves is typical of iron deficiency. The yellowing,
which occurs between the veins, is more severe on younger leaves.
This problem generally results from plants growing in soils of improper
pH. Rhododendrons must be grown in acidic soil that is high in organic
matter. If the pH is above 6.0, soil amendments such as sulfur,
iron sulfate or ammonium sulfate must be incorporated into the root
area to lower pH. It will be difficult to overcome the deficiency
problem in soils high in lime or calcium, even with soil amendments.
In such cases, mulch the plant heavily with a good grade of sphagnum
peat. Bark mulch mixed with the peat provides a mulch with good
aeration and drainage, as well as suppresses root rotting organisms.
If kept moist, plants will root into this mulch. Use a complete,
acid fertilizer that contains iron. Leaf drying and browning
can occur on rhododendron leaves as a result of winter exposure.
The leaves, even though they may be "rolled
up" at times, are subject to drying out in dry winter air. The
solution is to protect the plant from the drying wind. Plant rhododendrons
behind buildings or other plants that can serve as wind shields.
Put wind shields in place around the plants during the winter months.
Mulching as described above is critical to preventing winter injury.
The adult weevils feed on rhododendron leaves producing a C-shaped
notching in the leaf margin. These insects can be quite damaging;
most of the damage is a result of weevil larvae feeding on the roots.
Affected plants lose vigor and may die eventually. See GreenShare
Factsheet on the Black Vine Weevil for identification and life
cycle information, as well as control recommendations.
This disease is caused by a soil-borne fungus (Phytophthora spp.).
It is usually a problem where wet (saturated) soil conditions occur
frequently. Early symptoms of the disease consist of retarded
growth, drooping of foliage (perhaps on one or two branches only)
and yellowing of leaves. Infected roots appear dark and "mushy." As
the disease progresses, a browning discoloration of the wood may
extend upward from the base on affected branches. Plants in
poorly drained soils are subject to waterlogging, which makes them
highly susceptible to this disease, and plants may die quickly.
Although infected plants cannot be cured, root rot may be tolerated
by the plant if improvements in soil drainage and aeration are made
as soon as possible. Young plants can be lifted and replanted. Before
replanting, improve the drainage and aeration of the soil. Use tile
drainage or add porous materials in a layer beneath the root zone.
Plant in a raised bed and do not mound the soil up around the crown.
Mulching with tree bark provides biological control. The mulch must
be applied to a depth of two inches and reapplied as it decomposes.
If plants cannot be lifted and replanted, try to improve drainage
and lessen the occurrence of over watering by redirecting rain runoff,
placing drain tiles or changing irrigation programs. If the plants
die from root rot, it would be unwise to replant another rhododendron
in the site without considerable improvement in the soil conditions.
Caroline and English Roseum are rhododendron cultivars with some
resistance to this disease. Adapted
from Harry A. J. Hoitink, Stephen Nameth and Jim Chatfield, Ohio
State University Extension, 2000
Pesticides
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.
For more
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.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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