Garden Tips: Are yellowing tree leaves a sign of sickness?

On recent visit to a local park I noticed some yellow-green oak and maple trees.

There are a number of normal cultivated varieties of plants with yellowish or yellow-green leaves. But the yellowish leaves on these oaks and maples are not normal.

The abnormal yellow discoloration is a symptom of a problem called chlorosis.

The most common cause of chlorosis in oaks is a lack of iron caused by alkaline soil conditions. When the soil is alkaline, certain nutrients, especially iron, become less available for absorption by the roots.

Pin oaks (Quercus palustris) are intolerant of alkaline soils and prone to iron chlorosis, while other oaks are more tolerant. Silver maple, red maple, sweetgum and dawn redwood are also predisposed to iron chlorosis in strongly alkaline soils.

Soils in our region tend to be slightly alkaline to strongly alkaline. In landscape situations, soil may be more alkaline due to the leaching of limestone from nearby concrete walls and walkways. Increased alkalinity can also be a problem when topsoil is removed and replaced with more alkaline subsoil during construction activities.

Plants with iron chlorosis are not able to produce enough carbohydrates needed for plant growth. As a result, the plant’s health is compromised, leading to a reduction in plant growth and vigor. When severe, the plant will eventually die unless the problem is corrected.

While there are various soil and foliar applications which can help alleviate a tree’s iron deficiency, these practices are not permanent and will need to be repeated for the entire life of the tree. It would be much wiser to check the soil pH before planting. If the soil is alkaline, plant trees that are not as vulnerable to iron chlorosis.

Take note that there are other problems not related to soil alkalinity that can cause chlorosis. Excessively wet soil conditions, root damage, girdling roots, and compacted soil can all lead to chlorosis.

I like a variety of green colors in my landscape, including plants with normal yellow-green foliage. They can be real showstoppers.

There are numerous deciduous trees with yellow or golden leaves available for planting including, Princeton Gold Norway maple, Hearts of Gold eastern redbud, Cloud weeping birch, Aurea catalpa, Aurea alder and Wredei elm. The most widely planted tree with yellow-green leaves is the Sunburst honey locust.

The species or common honey locust is a large tree growing up to 70 feet tall. It is tolerant of alkaline soil, drought, compacted soil and road salt, as well as being bothered by few insect pests.

It initially found popularity in urban parks for its adaptability, fast growth, lacy compound leaves and filtered shade. However, it was disliked for its nasty thorns and the messy excessive seed pods it produced.

Gardeners and cities rejoiced when a thornless honey locust that did not produce seed pods was introduced. They were excited to plant the smaller (35- to 40-foot) thornless and podless Sunburst honey locust with yellow-green leaves.

While well liked at first, it has also lost popularity once it was noted that they tend to develop prolific surface roots that lift sidewalks and driveways and make mowing difficult.

If your tree has yellow leaves that are not normally yellow, there is a problem. Check it out.

Note: Imperial honey locust, is the smallest (35 feet) of the thornless honey locusts. It has green leaves and a compact crown. As a somewhat smaller tree it should not produce as many troublesome surface roots, especially if it is planted in good soil that is not compacted and if it is encouraged to develop deep roots with proper watering practices.

This story was originally published June 30, 2017 at 2:51 PM with the headline "Garden Tips: Are yellowing tree leaves a sign of sickness?."