Are your trees sporting yellow leaves at the start of spring or in the height of summer? Yellow leaves indicate a lack of chlorophyll – the green pigment responsible for photosynthesis in plants. Or they could be a sign of infection or infestation.
If the leaves are yellow and spotty, it could be a leaf disease. Is there a sticky residue on the leaves? Then it’s probably a pest infestation. If you’ve ruled out pests and disease, then it could be iron chlorosis.
Iron Chlorosis and soil acidity
The appearance of yellow leaves often indicates a lack of iron. Either your soil lacks sufficient iron or your trees are unable to absorb the iron in the soil.
In soils that are alkaline (pH greater than 7.0), iron chlorosis is common and affects both plants and trees. Even though the soil is not lacking in iron, the high soil pH affects the chemical reactions within the soil, rendering the iron solid and hard for plant roots to absorb.
Conditions that contribute to iron chlorosis include:
Cool soil temperatures
Lack of air movement
Bad mulching practices.
Overwatering is another contributing factor that causes nutrients in the soil to wash away before the roots of the tree have time to absorb them.
Tree topping (the practice of cutting all the branches on a tree right down to stubs) may also cause iron chlorosis. Trees respond to severe pruning by putting out multiple shoots.
This is a survival mechanism as the tree needs to find a way to increase its food production. As a result, the tree depletes its store of nutrients, including iron. There are lots of other good reasons why you shouldn’t top trees.
Prevention and treatment of iron chlorosis
While the condition can be treated, it is easier and cheaper to prevent iron chlorosis. Before adding a tree to your garden, get to know the condition of your soil. If you know that your soil tends to be alkaline, choose trees that can tolerate these conditions.
Avoid overwatering your trees by following our tips on how to water your trees the right way. Remember that the soil around your trees should be moist, not soggy. You can also use mulch to help the soil retain moisture so you don’t have to water as often.
It is possible to treat existing, mature trees for iron chlorosis. Your local nursery should be able to recommend an iron sulphate soil additive. Treating a large area with this supplement is expensive, so concentrate your efforts on the drip line (the area directly under the crown).
Make small holes in the soil, 2.5 – 5cm wide and 30-45cm apart and fill each hole with the iron sulphate mixture. You should also loosen any compacted soil around your trees to increase the flow of air to the roots. The concentrated soil will react with the other chemicals in the soil and lower the pH in the zone around the tree.
The effects of an iron sulphate soil treatment will only last for a year. It’s best to treat the soil in spring before your trees start growing again. With regular treatment and effort, it is possible to restore your trees to full health and eliminate the effects of iron chlorosis.
If you’d like a tree care professional to assess the health of your trees, contact Brands Tree Fellers and schedule an appointment with our horticulturist.