Preventing and Treating Iron Chlorosis in Trees and Shrubs

by Michael Kuhns, Extension Forestry Specialist, and Rich Koenig, Extension Soils Specialist

Note: See What is Iron Chlorosis and What Causes It? (click on link) for information on causes of iron chlorosis.

Diagnosis
As noted above, go to What is Iron Chlorosis and What Causes It? for more information on what iron chlorosis is and its causes. Briefly, iron chlorosis is a yellowing of plant leaves caused by iron deficiency, usually in high pH soils (pH above 7.0). Other causes of yellowing need to be ruled out first, however. For example, leaf yellowing can be due to insect or disease problems (pathogenic diseases caused by fungi or other organisms), herbicide misuse, or a history of over watering. Some tree cultivars have even been developed to have yellow foliage on purpose -- an example is the 'Sunburst' honeylocust ('Gleditsia triacanthos' var. inermis 'Sunburst').

If you have looked for these other problems and still suspect iron chlorosis, have your soil tested to see if the pH is above 7.0 to 7.5. If pH is high and you have ruled out other problems then iron deficiency is likely. Leaves with iron chlorosis will develop a yellow color with a network of dark green veins. In severe chlorosis even the veins may turn yellow or the leaf may even turn white. The outer edges also may scorch and turn brown as the cells die. Chlorosis can show an a few leaves, an individual branch, half of the crown, or the entire tree. Not all plants in a ladnscape will be affected.

Though iron deficiency is more likely, high soil pH also can cause manganese deficiency with similar looking chlorosis. Though a soil test may be helpful in ruling out such problems, often treatment for suspected iron chlorosis ultimately ends up ruling out other problems.

In the west soil pH tends to be highest where precipitation is the lowest. Therefore, look for iron chlorosis to be worse at low elevations away from the mountains.

Iron Chlorosis Prevention
Control of iron chlorosis is not easy and can be expensive, so prevention is better than treatment. Select plant species and cultivars that are tolerant of high soil pH and less likely to be affected by low iron availability. Table 1 describes the susceptibility of common landscape plants to iron chlorosis (some non-woody plants are included there as well). Avoid planting highly susceptible selections in Utah and other places with high soil pH, since recurring chlorosis will weaken the plants, predisposing them to other problems and/or shortening their life span.

Popular trees in Utah and throughout the interior West that have serious iron chlorosis problems and should be avoided in high pH soils are silver maple (Acer saccharinum) and quaking aspen (Populus tremuloides). Even though aspen is a native, it is native to higher elevation, cooler, wetter sites in the mountains with lower soil pHs. It is not well adapted to low elevation sites where it is typically planted.

Even trees that do well on soils with a moderately high pH, like Norway maple (Acer platanoides) and Scotch or Scots pine (Pinus sylvestris), can show chlorosis on especially bad sites. If such trees start to show chlorosis, the problem tends to get worse over the years as carbonates build up in the soil from irrigation.

If a tree is young and constantly chlorotic consider removing it and planting a better-adpated species. But what should be done about large, established, valuable trees that are exhibiting chlorosis symptoms?

Plant culture is also important in the control of iron chlorosis. Avoid saturated soil conditions by reducing watering or by installing drainage, especially with susceptible trees and shrubs. Aerate compacted areas around the base of affected vegetation. Also, avoid using plastic sheeting as a mulch for susceptible plants, since it restricts oxygen movement into the soil. High soil phosphorus also can make iron chlorosis worse.

Iron Chlorosis Treatment
Several methods are available for treating iron deficiency. These are: 1) soil application of elemental sulfur combined with ferrous (iron) sulfate; 2) soil application of iron chelates; 3) foliar sprays containing ferrous sulfate or chelated iron; or 4) trunk injection of ferric ammonium citrate or iron sulfate (trees only). Foliar treatments produce a rapid but incomplete response, while a soil or trunk treatment will last longer. Soil treatments require considerable work, but generally do not injure trees and can be more economical. Table 2 lists some advantages and disadvantages of different iron chlorosis control methods. Often, one method will work well in one area but not in another due to variations in soil conditions and species susceptibility. Try different methods until you find the one that works in your situation.

Soil treatment -- Use soil applications to treat individual trees and shrubs, or small areas in a landscape, in the fall or early spring. A mixture of equal parts iron (ferrous) sulfate (Table 2) and elemental sulfur can produce lasting results and is relatively inexpensive. Select an inorganic iron source with a high concentration of iron and one that is derived from iron or ferrous sulfate. Read labels to determine iron concentrations and forms in different products.

It is not practical or desirable to blanket an entire landscape with the elemental sulfur-ferrous sulfate combination. Instead, treat small areas by making holes 1 to 2 inches in diameter and 12 to 18 inches deep. Space the holes 18 to 24 inches apart around the area within the drip line (outer edge of crown) of affected trees and shrubs. Fill each hole with the iron sulfate-elemental sulfur mixture to within 4 inches of the soil surface. Table 3 provides recommendations for the number of holes and quantity of the ferrous sulfate-elemental sulfur mixture required to treat plants according to their size. Make holes with an auger or soil probe that removes soil to reduce compaction. Avoid damaging large, woody roots when making holes. Also, check with local utility companies if making holes in the vicinity of underground utility lines.

Areas of small shrubs in a garden also can be treated with equal parts ferrous sulfate and elemental sulfur. Use a hoe to excavate a small trench approximately 4 inches deep, 12 to 24 inches away from the base of plants. Apply one inch of the ferrous sulfate-elemental sulfur combination to the bottom of the trench and then fill in the remainder of the trench with soil.

Over time, the concentrated sulfur in the holes or trenches reacts to form acid which neutralizes lime and lowers soil pH in a small zone around the treated areas. The acidification of soil in combination with the iron sulfate maintains iron in a form that can be absorbed by plants as roots grow into the treated areas. One soil treatment with iron sulfate-elemental sulfur may last 2 to 4 years depending on conditions.

Some iron chelates can be used as a soil treatment; however, the effect is temporary (one year) and chelates are relatively expensive. Check label instructions for application guidelines. The only chelate that works well under high pH soil conditions is one containing the FeEDDHA molecule (Table 2). All other chelates currently on the market are ineffective at pH greater than 7.2 and therefore are not very effective as soil treatments in much of Utah.

Use chelates in spring before growth begins. Sprinkle dry chelate on the soil surface and irrigate in, or dissolve in water and apply to soil around the base of plants. Chelates can also be applied in holes around the drip line of affected vegetation.

Foliar treatment -- Foliar applications are made directly on the leaves of affected plants during the growing season. These treatments produce a quick response, often in a matter of days. Response to foliar sprays, however, is often incomplete (spotty control) and temporary. Repeated applications of foliar sprays may be required if chlorosis symptoms persist or as new foliage
appears. Foliar sprays are difficult to apply to large trees.

Iron chelates (Table 2) are quite effective as foliar sprays. Follow label recommendations that come with these products. A 0.5% solution of ferrous sulfate applied to foliage also provides some control and is less expensive. A 0.5% solution is formulated by dissolving 2 ounces of ferrous sulfate (20 to 22% iron) in 3 gallons of water. Foliage should be sprayed in the evening or on a cool, cloudy day to prevent leaf burning. Add a few drops of liquid soap or wetting agent (available at farm supply stores) to help the solution adhere to the leaves. Repeated applications of foliar sprays may be needed if chlorosis symptoms persist or as new foliage appears.

Trunk injection or implantation -- Iron compounds in dry or liquid form can be placed directly into holes drilled into a tree's lower trunk. Systems also are available that use plastic tubing and tees, capsules of various types, or a hypodermic-like tool to place iron materials into the tree. Though these techniques can be quite effective, they injure the tree's trunk and should be used with care. Minimize injury by using methods and formulations that require small holes (some systems use holes as small as 1/8 inch diameter), and avoid any treatment that would require injecting a tree more than once every few years.

Commercial injection formulations are available as liquids or powders and should be used according to directions. Look for formulations that contain ferric ammonium citrate (iron citrate) or ferrous sulfate. Holes should be made with a sharp brad-point bit to ensure quick uptake and reduce injury. Pay particular attention to manufacturer recommendations on hole placement, angle, depth, and diameter. Studies have shown that uptake is better and more evenly distributed if holes are drilled near the soil surface on the outside of root flares. Covering or capping holes can be done for cosmetic reasons, but will not reduce the chance for decay or speed healing. Wound dressings should not be used.

Injection treatments generally are most effective if applied in the early spring during bud break, but follow label directions for particular products. Treatments later in the year often will not be as effective and may not last as long. Effects can be expected to last for two or more years, after which retreatment probably will be necessary. Avoid injecting materials on hot, dry, windy days since leaves may blacken or burn, though such damage is usually temporary and not serious. Make sure the tree is well-watered for several days before and several weeks after injection treatments.

Product Availability
Ferrous sulfate, iron chelate and elemental sulfur products can be purchased at larger garden supply stores or agricultural chemical dealers. Chemicals and apparatus for injecting or implanting trees can be found at, or may be ordered by, some nurseries and garden centers.