on Azaleas and Rhododendron
Rosie Lerner, Extension Consumer Horticulturalist,
Department of Horticulture & Landscape Architecture, Purdue University
Chlorotic (a yellowing of the leaves) azaleas and
rhododendron in the
Indiana landscape are a common sight. The term chlorosis gets its
name from the lack of chlorophyll, the pigment responsible for
plants' green color. One major cause of chlorosis is a deficiency
either iron or manganese. Other landscape plants are also susceptible
to chlorosis, including pin oak, river birch, holly and sweet gum.
Iron deficiency leads to a pattern of yellowing called
interveinal chlorosis -- a yellowing of the tissue between the
veins while the
veins remain green. This striking contrast becomes apparent on
youngest foliage first. In extreme cases, the tissue may turn brown
and plants may be stunted.
Manganese deficiency symptoms are similar to those
of iron. Silver and
red maples are especially sensitive to manganese deficiency. However,
if manganese-deficient leaves are treated with iron, they become
Iron and manganese deficiencies usually are often
not caused by an
actual lack of these nutrients in the soil, but by soil that is
alkaline, which causes the iron and manganese to be chemically
the soil, making them unavailable for plant uptake.
Iron and manganese chlorosis can be corrected in
several ways. For a
long-lasting solution, make the soil more acidic to free up the
existing nutrients. Small areas can be made more acidic by applying
acidic organic matter, such as peat moss, to the soil. Larger areas
are more feasibly treated with soil applications of elemental sulfur,
iron sulfate or aluminum sulfate. The amount needed depends on
size of the area, the current soil pH and soil type. The soil will
have a tendency to return to alkaline, so it can be a never-ending
To bypass the problem of soil alkalinity, iron or
manganese can be
applied directly to the plant. The nutrients can be sprayed on
foliage, but such treatments generally give only temporary relief.
And, of course. you'll need sprayer equipment that can reach the
entire plant. Nutrients can be injected directly into the trunk
tree. Injections are very effective, however they are expensive
create wounds that can provide entry for insect and disease organisms.
Adding nutrients to the soil near the plant is yet
another option. Use
specially formulated nutrients, known as chelates, to avoid the
problem with soil alkalinity. These materials can be expensive
slow to work.
The best solution is to choose plants that are adapted
to your location. Avoid chlorosis-prone plants if your soil is