What are those?
What is the problem?
- Micronutrients are chemical elements
that are essential to a tree’s normal growth and development,
even though only a few atoms out of every million are needed.
- There are 7 known micronutrients. Iron and manganese
are the most important for ornamental trees. Crop trees have special needs
not discussed here.
- When trees (or shrubs) cannot get the micronutrients they require,
they do not grow or develop properly, and slowly die.
- Different micronutrients have different functions. Many commonly
affect specific cellular reactions such as chlorophyll synthesis, nitrogen
fixation, and respiration.
- Micronutrient deficiencies on urban trees are often due to a high
soil pH (7.8-8.2 on soils with significant lime content) that makes the elements
unavailable to the plant even though they are actually present in sufficient
Chlorosis on red maple from unavailable
What are the symptoms?
What common ornamental trees are particularly susceptible?
- Lack of iron or manganese in the leaf results in interveinal
chlorosis--a yellowing of the leaf blade between green veins--that
is worst on the youngest leaves. With extreme deficiency, the leaves become
completely yellow. Conifer needles turn yellow-green.
- Be aware that chlorosis is just a symptom: it can be caused
by many environmental (e.g., drought) and structural (e.g., girdling root)
problems that have nothing directly to do with micronutrients.
- Recent research has shown that "iron chlorosis" frequently involves
more than iron, even with a species like pin oak. Also, manganese is now
recognized to be very important.
How can I solve the problem?
- Iron: pin oak and sweetgum.
- Manganese: white oak, white pine, and red maple.
- A number of other common species (such as grey birch or hackberry)
show reduced vigor and health, but survive.
Where can I get more information?
- Do not plant susceptible species on soils with a high pH.
- Improve the drainage. In many cases, significant improvement
can be made by simply reducing soil saturation.
- Acidify the soil. Place small amounts of an appropriate
compound in 1-2" holes drilled 2' apart and 12-18" deep in a band around
the dripline. Good results have come with some comvination of a fast acidifier,
a slow acidifier (elemental sulphur), and organic matter. Repeat as
- Apply a chelate as a soil drench if one micronutrient is
involved. Repeat as necessary. The most durable iron chelate
on a high lime soil is EDDHA, found in several commercial products.
- Tree injection works well, but requires multiple and repeated
wounding. Spraying is ineffective, because crown coverage is difficult
and marginal leaf scorch is easy.
Koenig, Rich, and Mike Kuhns. 1996. "Control of Iron
Chlorosis in Ornamental and Crop Plants." Utah State University
Extension. For other information, advice and help on this topic, call
offices of your State Urban Forestry Coordinator or University Extension
service, or visit urban forestry web sites.