Chlorosis is the term used to describe tree or shrub leaves that turn yellow due to the lack of chlorophyll. When tree or shrub leaves lose chlorophyll their ability to produce nutrients is reduced. This condition leads to a loss of vigor, stunted growth, and the possible death of the plant.
Iron Chlorosis usually occurs because certain plants are placed in soils that are too alkaline (soils with a pH above 7.0). Higher soil pH levels limit the ability of roots to absorb iron, which is necessary for the formation of chlorophyll.
In the early stages, the leaf yellowing will only be noticeable between the veins of the leaf. As the condition progresses the entire leaf will become yellow and brown spots will begin to develop on the leaf. As Iron Chlorosis advances entire leaves will turn brown and entire branches will die. The condition is usually worse at the end of tree branches and in the top of the tree.
Iron Chlorosis inhibits the ability of the tree or shrub to make food for itself. The tree or shrub will try to survive by using its stored food reserves. If corrective action is not taken Iron Chlorosis will continue to weaken the plant and it may become vulnerable to attack by various insect and disease pests.
Iron Chlorosis on Oak. Photo: Robert L. Anderson, USDA Forest Service, Bugwood.org
Certain conditions such as construction damage, exceptionally low soil organic matter, poor drainage, or soil compaction can aggravate, or imitate, the symptoms of Iron Chlorosis.
Iron Chlorosis is most often seen on trees or shrubs that prefer to grow in lower pH soils, such as:
- Northern Red Oak
- Pin Oak
- River Birch
- White Oak
- White Pine
- Willow Oak
SYMPTOMS OF IRON CHLOROSIS
Initially, leaves will be yellow between the leaf veins and the leaf veins will be green. Leaf size is stunted. Eventually, the entire leaf will become yellow and brown spots will develop on the leaves. In severe cases tree or shrub branches will begin to die.
HOW TO CONTROL IRON CHLOROSIS
Prevention is the best way to avoid having to deal with Iron Chlorosis. Trees and shrubs susceptible to Iron Chlorosis should not be planted in soils with a pH above 7.0. Unfortunately, the vast majority of homeowners (and most landscapers) don’t test the soil pH before planting trees.
With that being said, if the trees or shrubs are only mildly chlorotic it may be possible to incorporate sulfur or other soil acidifiers into the soil to lower the pH. This is not a permanent fix and it may take months or years to see significant improvement. Continuous treatments will be necessary, with the goal being to reduce the soil pH level to below 6.5.
Tree trunk injections of iron are another method that can be used to control Iron Chlorosis. The major benefit of trunk injections is that they provide a quicker response and the treatment may provide 2 - 3 years of relief for the tree. For most tree species it is best to have trunk injections done in the fall, as spring trunk injections may blacken leaves and cause some temporary defoliation.
Just as with soil acidification, trunk injections are not a permanent solution and will need to be periodically repeated. The effects of either method can often be enhanced by improving the root health of the tree or shrub. An arborist or tree service can improve root health with soil injections of mycorrhizae, root stimulants, and rhizobacteria. Mulching around trees with organic mulches will also improve root health and prolong the effects of soil acidification products. Also, avoid applying lime (which is used to raise soil pH) near trees or shrubs that are susceptible to Iron Chlorosis.