By Crystal Kim, Red Butte Garden Horticulturist
If your garden plants produce healthy green leaves in spring, that fade to yellow as the season progresses, they may be suffering from iron chlorosis. Iron chlorosis is the most common micronutrient problem in Utah and is caused by a deficiency of iron in the plant. Chlorosis is the term for loss of the chlorophyll, or the green pigment in leaves. Iron is an essential micronutrient for plant health and is important in plant development and production of chlorophyll, which is essential in photosynthesis. Because chlorosis can also be caused by other micronutrient deficiencies, a soil test is important for determining the cause and treatment.
The primary symptom of iron chlorosis is intervienal chlorosis, or leaves that are light green, yellow or white, with dark green veins. In summer heat, leaves may become scorched at the margins or even die. Chlorosis can occur on the whole plant or be localized to a single branch. If iron chlorosis persists for several years, it can cause decline in health or even death of the plant.
Iron chlorosis occurs when a plant is unable to extracts iron from the soil. Iron is plentiful in Utah soils; however, the high pH of our soils affects its availability to plants. In high pH, or alkaline soils (above pH 7.0), iron rapidly forms solids that are not available to plants. Some plants are more tolerant of low iron conditions, as plants vary in their ability to extract iron from alkaline soils. Iron chlorosis may persist year to year or may vary due to environmental conditions. Environmental conditions that increase likelihood of iron chlorosis in alkaline soils are consistently waterlogged or poorly drained soils, as these contribute to changes in soil chemistry that decrease iron availability.
Management of iron chlorosis
Plant selection: Choosing plant species and cultivars that are tolerant of high pH soils is the most important preventative thing you can do. Plants that are native to Utah or areas with alkaline soils will naturally be tolerant. For some plants, their susceptibility to iron chlorosis will also depend on the growing conditions.
Culture: Improve the drainage of compact, poorly drained soils and avoid excessive irrigation that keeps soil consistently wet. Also avoid use of plastic sheeting as this restricts movement of oxygen to the soil.
Soil test: Learn more about your garden soil, including its pH, by having your soil tested. Utah State University’s Analytical Laboratory performs inexpensive soil tests for home gardens.
Treatment: For highly susceptible plants that are worth growing, you’ll need to apply iron fertilizer or soil amendments regularly. Keep in mind that any treatment, whether to the soil or plant, will be temporary, require extra effort, and may be costly. Attempting to lower the soil pH is a losing battle because soil is resistant to change. Applications of iron to the soil will only offer temporary relief as the iron will quickly become bound and unavailable. With those caveats in mind, here are some treatment options:
- Inorganic iron sulfate: good results when applied to turf but usually unsuccessful in other garden situations.
- Iron chelates: Chelated iron consists of an organic molecule bound to iron which makes the iron more available to plants. It must be applied in the root zone, and lightly worked or watered into the soil. Apply in spring when the first flush of growth appears. Yearly application is required.
- Foliar application: Iron fertilizer applied directly to foliage produces the most rapid but temporary response. Repeated applications must be made as new foliage appears.
- Artificial soils or raised beds: This option is ideal for plants like strawberries and raspberries which are very prone to iron chlorosis.
For information on the susceptibility of different landscape plants to iron chlorosis, as well as effectiveness of different treatment methods, see the USU Extension article “Preventing and Treating Iron Chlorosis in Trees and Shrubs”.