Iron Chlorosis in Oak & Birch

It is common for pin oak and river birch trees to exhibit yellow leaves in late spring through summer.  These trees are often suffering from a micronutrient deficiency called iron chlorosis. Chlorosis symptoms can range from a slight yellowing of some of the leaves to severe yellowing and dieback of the entire crown.  Severe iron chlorosis can lead to the death of leaves, dieback of branches, decrease in tree health, and eventually tree decline and death.

For less severe cases, iron chlorosis may affect only a few branches, while portions of the tree remain green and healthy. Trees do not necessarily show symptoms every year for mild chlorosis. Trees with more severe symptoms will often decline slowly over a few seasons.

Typically, one of two conditions exist:

1-The soil is lacking iron or

2-The soil is not lacking iron, but the iron is not available for trees because of high soil pH

*This is an important distinction because there is either a deficiency or soil issue.

Chlorosis is typically associated with one of three site conditions:

1.     Soil with a pH above 7 (an alkaline soil)

2.     Heavy wet, clay soil

3.     Soil where there has been severe disturbance such as compaction, construction, soil fill or removal of soil

Other conditions which can accelerate the symptoms of iron chlorosis include:

• Improper planting depth (too deep)

• Extreme heat (trees near parking lots and streets)

• Lack of watering / drought conditions

Multiple pin oak trees in summer suffering from chlorosis in a compacted soil with a high pH.

There are several options to attempt to treat iron chlorosis depending on the severity of the problem.  However, it must be noted that it usually means “wrong tree for the soil conditions”. Iron chlorosis treatments often include a combination of:

1.     Deep root fertilization

2.     Trunk injections of iron directly into the trunk

3.     Sulfur application to soil to help lower pH

4.     Foliar sprays of iron

Typically, all of the treatments are short-term solutions which can help minimize the impact of iron chlorosis.  However, none of these solutions eliminate the true problem, which is wrong soil situation for the tree.  For severe cases, it is often better to replace the stressed tree with a better suited tree.

Chlorotic pin oak (beyond recovery) in mid-July in Minnesota.

Recommendation from The Big Oak =

Unless the suffering trees are sizable and very important, I would typically lean towards replacement of pin oak trees with better options. Pin oak trees can be great-performing trees, but NOT in heavy clay soils with a higher pH. River birch trees are a tougher decision because they are well suited for wetter, clay soils. The soil pH is typically the determining factor if chlorosis will become an issue.

Replacement option for pin oak = Swamp white oak

Replacement option for birch = Elm

A Note Regarding Red Maples

Very similar to iron chlorosis, many red maple trees suffer in the same soil conditions as pin oak and birch trees. However, the difference is typically associated with manganese (not iron).

Summary from The Big Oak = Generally stay away from river birch, pin oak and red maple in heavy clay soils with a high soil pH.

A red maple tree suffering from manganese deficiency in mid-summer in Minnesota.