Why are the leaves on my pin oak yellow-green?

FAQ
Question: 

Why are the leaves on my pin oak yellow-green?

Answer: 

In Iowa, the foliage of the pin oak (Quercus palustris) often turns a sickly yellow-green. The yellow-green foliage is due to a deficiency of iron. The problem is referred to as iron chlorosis. (A close examination of chlorotic leaves will show that while most of the leaf is yellow-green, the tissue around the major veins is a darker green.) Most soils in Iowa contain sufficient amounts of iron. However, in alkaline soils (those with a pH above 7.0), the pin oak is unable to absorb adequate amounts of iron because much of it is in an insoluble form. Since many soils in Iowa are alkaline, chlorotic pin oaks are common in the state. Wet soil conditions make absorption of iron even more difficult.

Correcting an iron chlorosis problem is difficult. Applying additional iron to the soil usually doesn’t help. The soil already contains sufficient amounts of iron. Adding more iron doesn’t overcome the problem. Lowering the soil pH to 6.0 to 6.5 would allow the roots of the pin oak to more readily absorb iron in the soil. Unfortunately, lowering the soil pH is extremely difficult, if not impossible.  As a result, homeowner efforts to treat iron chlorosis are often unsuccessful.

One strategy that sometimes works is to have an arborist or other tree care professional inject an iron containing compound directly into the trunks of chlorotic pin oak trees. The effects of a trunk injection may last three or four years.