Iron Chlorosis
Nutrient Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
Iron is a cofactor in chlorophyll biosynthesis. When leaf cells cannot access sufficient iron, chlorophyll production falls behind leaf expansion and the new leaves develop interveinal yellowing. The veins remain green because they carry the iron that did arrive; the tissue between them bleaches first. In most landscape settings the total iron in the soil is not the problem. The problem is availability: alkaline soil chemistry, waterlogging, and high phosphate all tie iron up in forms roots cannot take up.
Quick Reference
Symptoms
The initial symptom is an interveinal chlorosis of the young leaves with the main veins remaining green. Affected leaves may remain small and fade from pale green to pale yellow to nearly white. In severe cases, the whitest leaves develop marginal necrosis, tip burn, and early drop. Older leaves on the same plant typically stay darker green because iron is not mobile within the plant and the deficiency shows up first in new growth. (Source: PNW Plant Disease Management Handbook, Azalea/Rhododendron - Lime-induced Chlorosis entry.)
Iron chlorosis is an interveinal yellowing on the youngest leaves with the veins holding their color. Nitrogen deficiency by contrast starts on the oldest leaves and produces a uniform pale yellow across the whole leaf blade. Manganese deficiency produces a similar interveinal pattern but affects older leaves first and the vein-bordering green band is narrower.
Timeline: Chronic rather than acute. Symptoms usually appear during flushes of new growth, worsen through summer, and may partially remit on new flushes after soil moisture normalizes. Severe deficiency can persist multi-season if soil chemistry is not corrected.
Triggers & Conditions
Soil conditions that lock up iron in plant-unavailable forms. Most commonly: alkaline soil pH above roughly 7, which converts soluble Fe2+ to insoluble Fe3+ compounds. Also contributing: poor drainage and waterlogging, which lower root respiration and limit iron uptake; high soil phosphate, which precipitates iron as iron phosphate; cold wet soils in early spring, which slow root function before the growing canopy needs iron; and root damage from compaction, construction, or girdling. (Source: PNW Plant Disease Management Handbook, Azalea/Rhododendron - Lime-induced Chlorosis.)
New growth flushes during the growing season. Spring flush on rhododendrons and azaleas is the most frequent window for symptom expression. Newly planted stock in alkaline backfill shows symptoms within the first growing season.
In the Puget Sound lowlands, iron chlorosis is uncommon on native soils because the regional pH baseline (5.0-6.0) keeps iron in plant-available forms. Risk sites are location-specific: beds adjacent to concrete foundations (calcium leaching pushes pH above 7), over-limed areas, sites with imported alkaline construction fill, and containers irrigated with municipal water (pH 7.5-8.5). The soil pH guide covers the general framework for diagnosing and correcting pH-induced nutrient lockout. Rhododendrons, azaleas, blueberries, and pin oaks are the most common local patients. Source: HFG soil series profiles (all 9); UC IPM pH Problems; HFG blueberry-selection guide.
Management
Prevention
- Match species to native soil pH before planting
- Keep susceptible species away from new concrete, limestone walls, and recently limed beds
- Mulch with acidic organic matter over the root zone
- Avoid overwatering and improve drainage on clay or compacted sites
Mitigation
- Adjust soil pH downward toward 4.5 to 6 for calcifuges
- Apply iron chelate as a foliar spray or soil drench
- Use ammonium sulfate as a nitrogen source on affected plants
- Use azalea, camellia, and rhododendron formulated fertilizers on acid-loving species
Group calcifuges together in beds amended with peat, pine bark, or other acidic organic matter, and keep those beds away from concrete foundations, masonry walls, and lime-heavy fill. Use raised beds or mounded backfill on native clay sites where drainage is marginal. Place drip irrigation rather than overhead to avoid surface crusting and evaporation concentration.
Plant Tolerance
Calcifuge (acid-loving) species show iron chlorosis most readily: Rhododendron, Azalea, blueberry, pin oak (Quercus palustris), holly, camellia, mountain laurel, and most Ericaceae. Calcicole (lime-tolerant) species rarely show it. Plants grafted onto lime-sensitive rootstocks can show symptoms their own roots would not.
More Tolerant
- Calcicole Mediterranean species (Cistus, Rosmarinus, Lavandula)
- Most Quercus species except Q. palustris
- Olea europaea
- Ceanothus species
More Sensitive
- Rhododendron species and hybrids
- Azalea (Rhododendron evergreen and deciduous)
- Vaccinium corymbosum (blueberry)
- Quercus palustris (pin oak)
- Camellia species
- Kalmia latifolia
- Pieris species
- Gardenia jasminoides
Native soil pH of the species origin is the strongest predictor. Plants evolved on acidic peats, granitic sands, or conifer forest duff expect pH 4.5 to 6 and fail to extract iron at pH 7 or above. Plants evolved on limestone, chalk, or alluvial clay tolerate higher pH because their root biochemistry releases organic acids and phytosiderophores that free bound iron.
Secondary Effects
Chronic iron chlorosis weakens plants over time, reduces photosynthesis, and predisposes them to secondary pests, winter injury, and drought stress.
Severe cases can cause branch dieback and whole-plant decline if soil chemistry is not corrected.