Iron deficiency symptoms are common in California landscapes even though sufficient iron is present in most soils.
Azaleas, citrus, gardenias, rhododendrons, and other plants that are adapted to acidic soil are especially prone to iron deficiency when soil pH is above about 7.5 (alkaline). Iron deficiency is also common when soils are cool, high in calcium, poorly drained, or waterlogged and when root health is impaired by root decay pathogens, nematodes, or other biological or physical causes.
Iron deficiency in broadleaves causes young foliage to be bleached, chlorotic, or pale between distinctly green veins. Fading appears first around leaf margins, then spreads inward until only the veins are green on younger foliage. When severe, young leaves remain undersized, turn almost white, and develop black or brown spots, margins, and tips. Twigs may die back and leaves may drop prematurely.
In iron-deficient conifers, newer needles are commonly undersized, and the upper canopy becomes chlorotic while older, lower canopy foliage remains green. In iron-deficient palms, new leaves typically are uniformly yellow.
Tissue analysis may not be reliable for diagnosing iron deficiency for plants in the landscape. Diagnose this malady based on foliage symptoms and knowledge of existing cultural practices and soil conditions and whether iron deficiency is common in that plant species. Soil tests can be useful, for example if they reveal high pH or low organic matter content. Recognize that certain preemergence herbicides and manganese deficiency cause similar damage symptoms. Iron and manganese deficiency have mostly the same causes and remedies.
Remedy iron deficiency primarily by improving cultural practices and the soil environment. To improve aeration and reduce waterlogging, increase the interval between irrigations to the maximum extent that still provides adequate moisture to maintain plant health. If plants are small, consider digging them up and replanting them on a broad berm or mound of soil raised several inches to improve drainage. Amend soils to improve drainage, lower pH, and increase organic matter before planting or replanting species adapted to acidic soils.
If soil is alkaline, about 6 months before planting add 1 to 4 pounds of elemental sulfur per 100 square feet of soil surface. Mix or rototill the sulfur into the top 6 inches of soil and irrigate. Use the lower amount in sandier soils and the higher amount in soils high in clay. Around established plants use a soil probe or an auger to create holes around the drip line, then fill the holes with soil mixed with 2 to 3 teaspoons of elemental sulfur. Bacteria slowly convert sulfur to sulfuric acid; this lowers soil pH and increases iron availability slowly over months when soil is moist, warm, and well aerated. Acidification is not effective if soil is high in calcium carbonate as discussed in pH problems.
Alternatively, before planting, add acidic sphagnum peat or organic matter that has been well composted; mix it into the top 1 to 2 feet of soil at a rate not exceeding 20% of soil volume. Be aware that amended soils will settle as organic matter decomposes, causing new plants to settle in the planting hole and become subject to root and crown decay. When planting in amended soil, compost organic matter well before use, form soil into a broad mound, place the root ball on solid soil, and plant about 1/2 to 2 inches or more above the native soil line.
Another remedy is to apply iron chelate according to the product label, such as by spraying foliage. If used, apply chelate in combination with measures to improve the plant’s culture, environment, and soil conditions. Iron is also applied by trunk injections, but injections injure trunks and may mechanically spread plant pathogens. If applying inorganic fertilizers, switch from nitrate- to ammonium-based compounds, such as ammonium nitrate, which gradually lowers soil pH.
Regularly place composted organic matter as mulch on top of the roots of established plants. This eventually (slowly) remedies iron deficiency as organic matter decays and soil becomes more acidic. Mulching provides many benefits in addition to increasing nutrient availability. See "Soil and Fertilizer Management" in the California Master Gardener Handbook for more information.
Adapted from the publication above, Abiotic Disorders of Landscape Plants, UC Agriculture and Natural Resources, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Yellowing between veins indicates iron or manganese deficiency.
Severe iron deficiency yellows leaves except for distinctly green veins.
Sweetgum with iron deficiency (left) compared to a healthy leaf.
Iron deficiency in younger leaves of toyon.
Premature leaf drop and twig dieback due to severe iron deficiency from adverse soil conditions.