Spring’s arrival is ushered in by warming temperatures and verdant greens. But amongst the landscape, you spot some very yellow leaves. Very. Yellow. That particular tree with the extremely yellow leaves is not a species known for that particular hue. Instead, it should be a typical tree green. Just what is going on?
To sum it up, iron chlorosis. This malady generally strikes during the spring. The reason that it may be more noticeable in spring is that the chlorosis tends to be exhibited in new growth at branch tips.
The characteristic look of iron chlorosis is a yellow leaf with green veins. If this is left untreated and it worsens, the leaf color will become a pale yellow to even whitish in appearance. However, the leaf veins will remain green. You may even notice angular brown spots between the veins and a scorched look along the leaf margin. Other symptoms of an untreated tree are poor growth (compared to a healthy tree). The leaves themselves may curl, dry, and drop from the tree. And if the tree bears fruit, the fruit itself may be smaller than normal and have a bitter flavor. In severe cases (those left untreated for years), limbs may die or perhaps the entire tree.
The reason that chlorosis is so damaging to a tree is that iron is needed in order for chlorophyll (which is responsible for the green leaf color) to form as well as for photosynthesis to take place. Once the plant is stressed from chlorosis, it loses vigor, may become prone to winter injury, and becomes susceptible to insects or other diseases.
Causes of Chlorosis
- high soil pH can cause chemical reactions that make iron unavailable to plants
- conditions that restrict air movement through soil, such as plastic sheeting laid down over a soil surface as a manner of weed suppression or soil compaction
- cool soil temperatures
- eroded soils (or top soil scraped away) that reveal a lime enriched subsoil
- low pH loving plant material planted in high pH soils
- regular overwatering
To rule out other culprits that may look similar to iron chlorosis, such as certain cultivars bred to have yellow foliage, improper herbicide application, fungus, or insect infestation, have your soil tested. Iron deficiency is usually found in soils with a high pH (7.0 or greater).
Types of Available Treatments
- trunk injection – holes are drilled into the tree trunk and iron compounds are placed in the holes. The compounds may be liquid or dry based upon the manufacturer. If this is the selected treatment, go with a brand that requires small holes which will minimize damage to the tree as well as lessen the area for insects to infiltrate. Some brands will specify that the holes should be made just above the soil surface, made at a certain angle, and at a specific depth. (Professional arborists may be able to perform trunk injections for very large trees.) For best results, apply in early spring and follow label instructions.
- foliar treatment – a compound is mixed with water and sprayed directly onto the affected leaves. For best results, spray on a cool, cloudy day or in the evening. This will reduce the likelihood of leaf burning from the application. Of all treatment types, foliar typically shows the quickest response, though is may be spotty and temporary. Iron additives are typically in the form of iron sulfate or iron chelates. Repeat applications are generally needed for lasting results. Please note that on large trees, it may be difficult if not impossible to reach all affected leaves.
- soil treatment – a compound is mixed with water and applied directly to the soil, typically at the drip line (the outer reach of the branches) of the chlorotic plants. This type of treatment is quick and easy to apply, whether it is for an individual plant or a group. NOTE: there are treatments in a stake form which can be pounded into the soil, but those are falling out of favor due to the iron not traveling as well through the soil compared to compounds that are mixed in water and applied. Iron additives for treatment can be in the form of iron chelates, iron sucrate, or even iron sulfate plus sulfur. For best results, consult with a nurseryman and read the label for proper application rate.
- lowering the soil’s pH – this may be the most impractical method, especially if the soil contains free-lime (calcium carbonate). According to Colorado State Extension Service, you can test for free-lime by placing a “rounded tablespoon of dry, crumbled soil in a small cup. Moisten the soil with vinegar. (The soil needs to be thoroughly moistened, but not swimming in vinegar). If the soil-vinegar mixture fizzes or bubbles, it has free-lime. High lime content is typical of soils with a pH above 7.5.” If your soil does not contain free-lime, you can use sulphur products to lower the soil pH. Be aware though that sulphur treatments are typically effective over several years. Read label instructions and retest soils in a few years.
If you have heard the stories about using metal shavings or rusty nails as treatment for iron chlorosis, ignore them. According to Utah State University Forestry, high soil pH causes a chemical reaction that makes “iron solid and unavailable to plant roots. Such iron will be tied up indefinitely unless soil conditions change. Iron released by these materials immediately forms solids that are unavailable to plants”.
While there are no quick and easy ways to treat iron chlorosis, there are steps you can take:
- test your soil’s pH
- plant material that is suitable for the pH
- do not overwater
- do not compact the soil
- begin treatment at the first signs of chlorosis
With proper treatment and patience, your plant(s) can return to their normal color and back to a healthy state.