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COASTAL GARDENER
Franklin Laemmlen, Ph.D.
Q:
The leaves on my liquidamber tree, especially the newest leaves, are pale green to
yellow. The veins in the leaves are still green. What is the cause of this yellowing
foliage?
A:
Your tree has iron chlorosis. This is the term commonly used for iron deficiency.
Iron is one of the essential elements for plant growth. Iron is necessary to produce
chlorophyll, which is the substance which makes plants green. Chlorophyll is also
needed to allow the plant to convert carbon dioxide and water into complex
carbohydrate molecules in the presence of sunlight. A plant with iron chlorosis is
analogous to a person with anemia.
Iron chlorosis can have several causes. The first and most obvious is lack of iron
in the soil. High soil pH is another cause. Many plants have a difficult time
extracting iron from soils that have a ph (acid – alkaline balance) above 6.5 – 7
(common on the Central Coast). If the soil has a high calcium content, or is poorly
drained, or tends to waterlog, iron deficiency may result. Some plants need low
pH soils (5 – 5.5) in order to grow “normally.” These plants are very prone to iron
chlorosis here on the Central Coast. Azaleas, citrus, blueberries, gardenias,
rhododendrons, hydrangeas, all fit into this low pH loving group of plants. I have
seen specimens of Japanese maple, liquidamber, poplar, toyon, and rose from
local landscapes showing symptoms of iron chlorosis.
The quick fix for iron chlorosis is to spray the foliage of the affected plant with
iron. There are several products available which can be applied to the foliage to
help remedy iron chlorosis. Products vary in the amount of available iron, so
check the product label for iron content and dosage recommendations before you
purchase. To begin a long-term solution to iron chlorosis, you must work at
modifying the soil environment to make iron more available. Usually this means
lowering the soil pH. This can be done by (1) added compost, (2) adding sulfur –
sulfur reacts with elements in the soil to produce acid and in the process decreases
pH. Use (3) acid fertilizers like ammonium sulfate, urea, and/or ammonium
nitrate. Also (4) manage water to avoid waterlogging the soil. The incorporation
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of compost (organic matter) mentioned above also helps improve drainage and
helps loosen up tight soils. The long-term solutions may take several years of
repeated treatment to “fix” the problem. In the meantime help your iron chlorotic
plants with foliar sprays.
Send your landscape and garden questions to: Coastal Gardener, 624-A
West Foster Road, Santa Maria, CA 93455.
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