Symptoms
The term chlorosis means a general yellowing of leaves. Many factors contribute to chlorosis.
Iron chlorosis refers to a yellowing caused by an iron deficiency in the leaf tissues. The primary symptoms of iron deficiency are interveinal chlorosis, a general yellowing of leaves with veins remaining green. In severe cases, leaves may become pale yellow or whitish, but veins retain a greenish cast. Angular shaped brown spots may develop between veins, and leaf margins may scorch (become brown along the edge). [Figure 1]
Iron is necessary for the formation of chlorophyll, which is responsible for the green color in plants and necessary for photosynthesis (sugar production in plants). Any reduction in chlorophyll during the growing season reduces plant growth, vigor, and stress tolerance. Plants with reduced vigor from iron chlorosis are more prone to winter injury, and winter injury may aggravate an iron chlorosis problem. Weakened plants are also more susceptible to other diseases and insect infestations.
Iron is not very mobile within plants. Plants use their stores of iron in new leaves as they create them, so iron chlorosis shows first and more severely on the newer growth at branch tips. Leaves may be smaller than normal and may eventually curl, dry up, and fall. Fruits may be small with a bitter flavor. Mildly affected plants become unsightly and grow poorly. In severe cases individual limbs or the entire plant may die.
It is common for iron chlorosis to show on a single branch or on one side of a tree. This is particularly common for plant species with marginal winter hardiness following winter injury. Plant species and varieties vary in their susceptibility to iron deficiency.
On junipers, pines, and other evergreens, chlorosis usually develops as an overall yellowing of needles.
Similar Symptoms
Iron chlorosis symptoms can be confused with other problems. In the high pH soils of Colorado, a suspected iron chlorosis problem may be a combination of iron and manganese deficiencies. It is common for chlorotic trees to show a response to both iron and manganese treatments.
Zinc and manganese deficiencies result in similar leaf symptoms, but typically appear first on older, interior leaves. Iron chlorosis appears first on the younger or terminal leaves but may progress into older and lower leaves under severe conditions.
Nitrogen deficiency shows as a uniform yellowing of the entire leaf including the veins. It appears first in the older leaves, while iron chlorosis appears first in the newer growth.
Damage from soil sterilants (e.g., Pramitol, Atrazine, Simazine, Ureabor, and Diuron) used to prevent weeds results in similar symptoms. With these weed killers, the leaf tissue along the vein remains green. With iron chlorosis, just the vein itself remains green.
Natural aging of tissues may create similar symptoms in some plants. Root and trunk damage, some viruses, phytoplasmas, and vascular wilt diseases may cause similar leaf symptoms.
Causes and Complicating Factors
The factors leading to iron chlorosis are complex and not fully understood. Several chemical reactions govern iron availability and contribute to the complexity of iron chemistry in soils.
Many environmental factors also create or contribute to iron deficiency which need to be evaluated and alleviated to the extent possible. In many situations, attention to watering and soil conditions will satisfactorily correct minor iron chlorosis problems.
Calcareous Soils
Many Colorado soils are naturally high in lime (calcium carbonate and other calcium compounds) which raises the soil pH above 7.5. In these calcareous soils, iron chlorosis is common on susceptible plants.
Colorado soils are abundant in iron, as evidenced by the common “red rock” formations. In alkaline soils (pH above 7.0), iron is rapidly fixed through a chemical reaction into insoluble, solid forms that cannot be absorbed by plant roots. Such iron will be tied up indefinitely unless soil pH changes. Soil applications of iron alone are ineffective, as the applied iron will quickly be converted to these unavailable solid forms.
Over-Watering
Iron chlorosis is a common generic symptom of overwatering. Overly wet or dry soils predispose plants to iron chlorosis. It is more prevalent following wet springs, or when
gardeners overwater in the spring. In western calcareous soils, iron chlorosis can be moderated by eliminating springtime overwatering.
Overly dry soils can also lead to nutrient deficiencies since many nutrients are absorbed in solution with water. Severe cases of iron chlorosis involving “acid-loving” plants may not be corrected through improved irrigation practices.
It is common for gardeners to allow sprinkler control settings to remain unchanged from the high summer water needs to the lower water needs of spring and fall. In this situation, the yard could receive as much as 40% more water than needed in the spring and fall. Such overwatering can contribute to iron chlorosis.
Soil Compaction
Soil compaction and other conditions that limit soil air infiltration (like surface crusting and use of plastic mulch) predispose plants to iron chlorosis by limiting effective rooting area and soil oxygen levels. Plants that have smaller root systems have less chance of “finding” available iron. These are key contributing factors in clayey soils. Using organic mulch (like wood or bark chips) helps prevent and reduce soil compaction. Avoid the use of plastic under rock mulch around landscape plants.
Trunk Girdling Roots
Iron chlorosis is a common early symptom of trunk girdling roots in trees. The primary cause of trunk girdling roots is planting trees too deep. Trunk girdling roots can lead to decline and death some twenty years after planting.
In tree planting standards, the top of the root ball should rise slightly above grade (e.g., one to two inches above grade) for newly planted trees. At least two structural roots should be in the top one to three inches of the root ball.
On established trees, the trunk-to-root flare should be noticeable. If the trunk goes straight into the ground, expect planting problems and possible development of trunk girdling roots over time. To check, perform a root collar excavation (carefully removing the soil around the base of tree) and examine the trunk/root flare.
Other Contributing Factors
Plant Competition – In susceptible plants, competition from adjacent lawns or flowers may aggravate iron chlorosis. Replace the grass under the tree canopy with wood/bark chip mulch.
Winter Injury – Trees with cankers and other winter injuries are prone to iron deficiency. (Winter bark injury on tree trunks is caused by winter drought.)
Soil Organic Matter – Organic matter is essential to successfully gardening in Colorado’s soils. Ideally, the soil’s organic content should be increased to 5%. However, excessive amounts may aggravate iron problems.
Excessive Salt Levels – High soil salt levels adversely affect uptake of water and nutrients, including iron. For details, refer to CMG GardenNotes #224, Saline Soils.
Soil Temperature and Light Intensity – Extreme soil temperatures and high light intensity may increase iron chlorosis problems. Use organic mulch to moderate soil temperature. Shading may help some crops.
Acid-Loving Plants – Acid loving plants are highly susceptible to iron chlorosis and not suited to Colorado’s soil conditions. These include blueberries, azaleas, rhododendron, flowering dogwood, and heather.
Nutrients – Excessive levels (from over-application) of phosphate, manganese, copper, or zinc may aggravate iron chlorosis.
Plant Selection – Right Plant, Right Place
In Colorado’s high pH soils, the best method to prevent iron chlorosis is to select plant species tolerant of high soil pH and less affected by low iron availability. Avoid planting the more susceptible species on soils prone to iron chlorosis problems [Table 1] (pH above 7.5, compacted, clayey, or wet soils).
| Table 1. Examples of Woody Plants with High Susceptibility to Iron Chlorosis |
|---|
| Amur maple | Dawn Redwood | Maple |
| Apple and crabapple | Douglas fir | Oak, pin and red |
| Arborvitae | Elm | Peach |
| Aspen | Flowering dogwood | Pear |
| Azalea | Grape | Pine |
| Beech | Honeylocust | Raspberry |
| Birch | Horsechestnut | Red maple |
| Boxelder | Juniper | Rhododendron |
| Bumald Spirea | Linden | Spruce |
| Cherry | London planetree | |
| Cotoneaster | Magnolia |
Iron Additives
Unfortunately, there is no easy, inexpensive, or long-term correction for iron chlorosis. Treatments may be expensive and give disappointing results. Since plant and soil conditions vary, there is no single approach that is consistently best. Reducing springtime overwatering and soil compaction along with knowledge of other contributing factors can be effective in mitigating iron chlorosis in some situations.
The first step in using iron additives is to know the soil pH and free lime (calcium carbonate) content. These factors directly affect the success of any approach.
Determine soil pH by a soil test. When the pH is above 7.5, effective approaches are limited.
To check for free lime, place a rounded tablespoon of dry crumbled soil in a small cup and moisten the soil with vinegar. (The soil needs to be thoroughly moistened, but not swimming in vinegar.) If the soil-vinegar mix fizzes or bubbles, it has free lime. High lime content is typical of soils with a pH above 7.5. A standard approach in treating iron chlorosis is to lower the soil’s pH, but lowering the pH is impractical to impossible if the soil contains free lime.
There are four general approaches to iron treatments: 1) lowering the soil’s pH, 2) soil iron treatments, 3) foliar sprays, and 4) tree injections. Each has advantages, disadvantages, and gives variable results depending on plant species and soil conditions.
The two principal types of iron-containing products used for iron application include iron chelates and inorganic iron compounds (such as iron sulfate and ferrous sulfate). Several types of iron chelate are marketed under a variety of trade names. Soil pH dictates the type of chelate to use. Treatment with any iron product made mid-season may not produce satisfactory results.
Lowering Soil pH with Sulfur Products
A standard approach used in many products is to lower the soil pH. This approach merits consideration only if the soil does NOT have free lime (high calcium carbonate) and may show effectiveness over a period of years.
Due to the high pH and lime content of many Colorado soils, this approach seldom merits consideration. If irrigation water is hard, the calcium carbonate (lime) in the water will counter any acidifying effect. (As a side note, it has been observed that in some older gardens the pH has dropped below natural levels as the lime content is slowly leached out with decades of irrigation.)
The pH is lowered by soil applications of sulfur products. See the product labels for specific application rate. (Use of aluminum sulfate to lower soil pH is not recommended due to a potential for aluminum toxicity.) For details on lowering pH, refer to the CMG GardenNotes #222, Soil pH.
Soil Applications of Iron Sulfate Plus Sulfur
A simple approach is to apply a mixture of equal amounts of iron (ferrous) sulfate and sulfur to the soil. Examples of products include Copperas, Jirdon Super Iron Green, Hi-Yield Soil Acidifier Plus Micros, and Fertilome Soil Acidifier Plus Iron. Over a period of months to years, an improvement may be noticed. When it is effective, treatments may last up to three or four years, depending on soil conditions.
This approach merits consideration only on soils without free lime.
For trees, apply the mixture in holes around the dripline of the tree, as described for chelates (see below). Over time, the sulfur reacts to lower soil pH in a localized area. Broadcast applications, which dilute the material over a larger area, are less likely to give satisfactory results. Treat rows of berries or small shrubs by placing the mix in a furrow four inches deep and twelve to twenty-four inches away from the plant. See specific label directions for application rates. For the best results, treat the soil in spring.
Soil Applications of Iron Chelates
Soil application of iron chelates may give a rapid response if the correct chelate is used and other contributing factors are minimal. Applications after May 1st are less likely to show results. Treatments may last less than a season to two years.
Treat trees by placing the iron product in rings of holes in the ground beneath the dripline (outer reaches of the branches). Make holes one and a half to two inches in diameter, six inches deep and twelve inches apart in rings two feet apart. For smaller trees, make two to three rings of holes. For large trees, create four to five or more rings of holes which may need to extend beyond the drip line. No holes should be made within two and a half to four feet of the tree trunk on established trees. [Figure 2]
Drill holes in the soil with a power or hand auger, bulb planter, or small trowel, removing the soil core. Using a punch bar that makes holes by compacting the surrounding soil may be less effective. To avoid damage to shallow utility lines, call 811 before starting to have any buried lines marked. [Figure 2]
In soils with a pH above 7.5, only special chelates formulated for a high pH are effective. Examples include EDDHMA (Miller’s Ferriplus®) or EDDHA (Fe Sequestrene™ 138). Due to their higher cost, these products have limited availability. See product label for specific application rates.
In acid to slightly alkaline soils, try other chelates like EDTA (Fe Sequestrene® 330, Fertilome Liquid Iron) and DTPA (Miller’s Iron Chelate DP). They lose effectiveness quickly as the pH rises above 7.2 to 7.5. See product label for specific application rates.
Soil Applications of Iron Sucrate
Iron sucrate, a relatively new iron source, is manufactured from iron oxide and molasses to form an iron-containing organic complex with limited water solubility. It is less prone to staining due to its low solubility.
Iron sucrate merits consideration in high pH soils, and additional scientific evaluation is warranted for Colorado soils. It is marketed as Lilly Miller Iron Safe.
Foliar Sprays
Foliar sprays of iron sulfate or iron chelates may provide a quick response, often in a matter of days. However, the treatment is often spotty and only temporary. Multiple applications per season may be needed and the effects will not carry over into subsequent years.
Both types of products are equally effective, but iron chelates are more expensive. See product labels for specific application rates and instructions. With foliar applications, spray in the evening or on cloudy days when drying time is slower. A few drops of liquid dishwashing soap or commercial wetting agent will enhance sticking properties.
Foliar applications to large trees are not recommended due to application limitations. Complete coverage of all leaves is essential, individual leaves not treated may remain chlorotic. Coverage on large trees is impractical to impossible.
There is a small margin between an iron concentration that will green up the leaves and a concentration that will cause leaf burn. Leaf tissues are prone to turn black from an iron burn. Following an iron sulfate foliar treatment, it is common to see leaves that remain chlorotic, leaves that green up, and leaves with black burn spots on the same plant. Spray hitting the sidewalk, house, and other objects may leave a permanent rusty discoloration. Chelated iron sprays are inactivated by sunlight.
Trunk Injections
Professional arborists have trunk implant or injection methods available for treating iron chlorosis on large trees and they may last from one to five years. Refer to product information for application details. Injections may create pathways for decay organisms to enter a tree.
This publication, reference GardenNotes #223, is developed as part of the Colorado State University Extension Master Gardener Program.