Yellow Leaves With Green Veins: Iron Chlorosis in Montana Trees

What is iron chlorosis?

Chlorosis simply means yellowing. Montana State University Extension describes it as a leaf that goes from light green early in the season to yellow later, often with the veins staying bright green while the tissue between them fades. In advanced cases leaves are smaller, the margins brown, and they drop early.

Iron chlorosis is the version caused by too little iron inside the leaf. Colorado State University Extension explains why it matters: iron is needed to make chlorophyll, so a chlorotic tree makes less food, grows less and tolerates stress poorly. Because iron does not move easily within a plant, it shows first and worst on the newest growth at the branch tips. An MSU Extension alert names insufficient iron or nitrogen uptake as the most common nutrient deficiency in Montana landscapes.

Why does soil pH cause it?

Because the iron is usually there, just locked up. MSU puts it plainly: chlorosis does not mean the nutrients are missing from the soil, since a high pH can limit a tree's ability to reach them, and adding more will not necessarily fix it. Colorado State describes the chemistry: in alkaline soil, above a pH of 7, iron is quickly converted into insoluble forms that roots cannot absorb. Utah State University Extension identifies lime, or calcium carbonate, in the soil as the most important predisposing factor.

Pouring ordinary iron on the ground therefore tends to disappoint. Colorado State says soil applications of iron alone are ineffective in such soils, because the added iron is converted to the same unavailable forms.

Are Missoula and Bitterroot soils really alkaline?

Often, but do not assume. MSU's soil testing MontGuide says surface soil pH in eastern Montana is typically between 7 and 8 and that western Montana soils tend to be slightly more acidic. A valley bottom, an old river terrace and a hillside cut can differ a great deal over a short distance.

Town soils add their own complications. The City of Missoula's urban forest plan notes that the valley floor is sand, gravel and cobble left by glacial Lake Missoula, that it drains very rapidly, and that a large share of the area's soil is classed as disturbed urban land. Fill, concrete, compaction and hard irrigation water all change what a tree's roots are living in. A soil test settles it, and MSU recommends one if chlorosis keeps coming back.

What else makes it worse?

Almost anything that stresses roots. MSU lists drought, overwatering, compacted soil, limited planting space and poor drainage as factors that raise a tree's vulnerability, and says chlorosis is more commonly the result of several of these together than of one deficiency.

Overwatering deserves special mention. Colorado State calls iron chlorosis a common symptom of it and notes that sprinkler timers left on summer settings through spring and fall are a frequent cause. Utah State calls water management probably the most important consideration when growing plants in alkaline soil. Colorado State adds that chlorosis is a common early sign of girdling roots on trees planted too deep, and that lawn growing up to the trunk competes with a struggling tree. If your system runs on a fixed schedule, a seasonal adjustment as part of irrigation system service is a simple place to start.

How do you know it is iron and not something else?

Look at where the yellow is. Colorado State's guidance is that iron chlorosis appears first on the younger leaves at the tips with only the veins green, while a nitrogen shortage yellows the whole leaf, veins included, starting with older leaves. Zinc and manganese shortages look similar to iron but typically start on older, interior leaves. Red maples are a known special case: the City of Missoula's street tree list notes that 'Red Sunset' red maple can show manganese deficiency.

Other problems imitate it. Colorado State lists root and trunk damage, wilt diseases and injury from soil-applied weed killers, a subject covered in our guide to herbicide damage on trees. MSU stresses ruling out pests and pathogens first. It also offers a simple test: spray chelated iron on one affected branch, and if those leaves green up, iron is the problem. A tree health and risk assessment covers the same ground with the whole tree in view.

What actually helps?

Start with the free things. MSU's management advice is to water during drought, remove grass from the base of the tree to cut competition, and mulch with two to three inches of compost to hold moisture. Colorado State says attention to watering and soil conditions will satisfactorily correct minor cases, and that reducing springtime overwatering and compaction is effective in some situations. Our guide to watering trees through a Montana drought covers how much and how often.

If that is not enough, the next step is iron in a form that stays available. Both Utah State and Colorado State point to chelated iron, and both say that where soil pH is above about 7.5 only the chelates made for high pH, such as the EDDHA type, are effective. Utah State says chelates must be worked or watered into the root zone, applied in spring with the first flush of growth, and usually repeated every year. Colorado State notes that soil applications made after about the first of May are less likely to show results, and that arborists also have trunk implant or injection methods for large trees, with the caution that injection wounds can let decay in. Read and follow the product label whatever you use. We are licensed for fertilizer application, so ask us at an assessment whether a treatment makes sense for your tree.

What does not help?

Trying to change the soil's pH across a yard. MSU's MontGuide says Montana's well-buffered soils make lowering pH very difficult and generally expensive, and that even heavy sulfur applications have produced only a small drop. Colorado State says the approach merits consideration only in soil without free lime. It describes a kitchen test: moisten a spoonful of dry soil with vinegar, and if it fizzes, the soil contains free lime.

Leaf sprays work fast and fade fast. Colorado State and Utah State both describe foliar iron as a quick but temporary response that must be repeated and does not carry over to the next year. Colorado State does not recommend it for large trees because complete coverage is impractical, and warns that iron spray can burn leaves and leave rust stains on walks and siding. Utah State adds that iron sulfate worked into the soil is not effective in most alkaline soils. Extra general fertilizer is not the answer either. Colorado State notes that excess phosphate can aggravate the problem.

Which trees should you avoid planting?

MSU says that choosing trees tolerant of alkaline soil is the most important step in managing chlorosis. It names apple, maple and poplar as more prone to it. Colorado State's list of highly susceptible woody plants includes maple, red maple, Amur maple, aspen, birch, pin and red oak, apple and crabapple, pear and peach. Utah State's advice for a tree that never recovers is to replace it with a more iron-efficient species.

On the tolerant side, MSU lists linden, ash, boxelder, oak, elm, hawthorn and honeylocust as less likely to show chlorosis. Two cautions apply. Colorado State's susceptible list includes several of those same trees, which shows how much local soil and variety matter. And ash should not be planted now for another reason, explained in our guide to emerald ash borer and Montana ash trees. Our list of the best trees to plant in western Montana takes soil into account.

When is a chlorotic tree worth an arborist's time?

When it has been yellow for more than a season or two, or when dieback has started. Utah State and Colorado State both note that persistent chlorosis can kill individual limbs or the whole plant, and Colorado State says weakened trees are more prone to winter injury, disease and insects.

An assessment looks at planting depth, the root flare, watering and soil before anyone reaches for a product. It is part of our tree services, and the Montana Tree Guide has guides to the other common causes of off-color leaves.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a tree assessment and an ISA Certified Arborist will look at the tree with you.

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