Understanding Chlorosis
When The Wrong Soil Can Be Deadly

You have a tree, shrub or perennial that was planted some years ago, which subsequently established well and has performed admirably and without cause for concern since that time. But this year, you’ve noticed that the leaves are yellowing as soon as they emerge, with dark green veins eerily standing out like a skeleton. They never fully reach their normal size, and the plant is starting to look sickly. Some novices might mistake these for beautiful golden-leaved variants of the plant, but no plant should suddenly change color one year - that’s always a bad sign!

Prematurely yellowing leaves are a sure sign of chlorosis, a nutrient deficiency problem that affects a number of landscape and garden plants. It occurs a result of two concurrent factors; an alkaline (high pH) soil and a particular plant that is intolerant to alkaline soils. Plants that are susceptible to chlorosis are unable to absorb essential nutrients, usually iron or magnesium, when the soil pH is too high. As these nutrients are essential to the formation of chlorophyll, the chemical that gives leaves their green color and which is absolutely essential for life, the leaves yellow.

If left untreated, the plant will invariably die. The problem tends to appear later in life because the early roots are usually planted in good soil, and only contact the native alkaline soil as they grow outwards and beyond the original planting hole. Some plants are highly susceptible to chlorosis, while others are quite tolerant of alkaline soils. Notoriously sensitive trees include the pin oak (Quercus palustris) and red maple (Acer rubrum), shrubs such as rhododendrons and azaleas (Rhododendron spp.) and blueberries (Vaccinium spp.), and perennials like false spirea (Astilbe spp.).

Chlorosis is a problem that is tremendously difficult to treat, but extremely easy to prevent, so the first line of defense must always be prevention. The key here is to understand the pH of your soil at planting time. If you know you have slightly to excessively alkaline soils, only select plants that are not susceptible to cholorosis. It is possible to amend or replace alkaline soils with acidic soils in a specially prepared bed for perennials and smaller shrubs. However, it is virtually impossible to amend the native soil for trees as their roots can eventually extend for hundreds of feet from the trunk.

It is possible to treat chlorosis on an established plant, but it is very expensive, extremely cumbersome and not always successful. The treatment usually involves adding iron chelate or magnesium (depending on the type of plant) to supplement the missing nutrients by increasing their concentration, and by adding aluminum sulfate and other chemicals to increase the pH of the soil. For trees, this involves complicated injections into the trunk that are best done by professional arborists. For shrubs, it is often easier to simply lift them, correct the soil or prepare a special bed, and then replant them.

This is one problem, however, where prevention is the best medicine by a long shot, so know your soil, and know your plants!