Now is the time of year when many pin oak (Quercus palustris) trees start to show their true Iowa colors. Unfortunately, those colors don't include dark green. Yellow or lime green are typical colors associated with pin oaks in Iowa. The pin oak performs poorly in most Iowa soils especially in areas where the topsoil has been removed, which exposes the clay subsoil. A typical Iowa soil has a pH higher than the optimum range for pin oaks, which prefer a well-drained soil with a pH between 5.0 to 6.5. When the soil pH reaches levels above 7.5, iron chlorosis, a nutrient deficient symptom develops. Iron chlorosis appears as yellow leaves with green veins. Some leaves may develop angular brown spots with brown curled leaf margins. Iron is essential for production of chlorophyll. Without enough available iron, the tree fails to produce enough chlorophyll to maintain healthy green leaves. Branches and twigs may begin to die after suffering from chlorosis over a period of several years.
Typically, iron is plentiful in Iowa soils. However, the high soil pH prevents the iron from being used by the plant. Large amounts of copper, manganese, or zinc, low soil temperatures, high soil moisture, or excessive amounts of phosphorus can intensify lack of available iron in the soil.
There are three methods in which iron chlorosis can be treated due to an alkaline soil situation. Incorporating elemental sulfur or peat moss is not an easy task. However, this will slowly help to decrease the soil pH making the iron in the soil more available to the tree. A second way to treat iron chlorosis is by applying a foliar application of iron chelate or iron sulfate. Th is approach is temporary since the iron does not move beyond the leaves that have been treated. A rate of 2.5 oz. of iron sulfate in 3 gallons of water is the recommended rate. Iron chelates are water-soluble forms of iron that remain in the solution making them available to the tree. The best conditions or time to make foliar applications are during the evening or in periods of cool weather.
Although beautiful under the right conditions, the pin oak is not a widely recommended tree to be planted in Iowa. Choosing plants more suitable for your soil will reduce disappointment and many headaches in the future.
This article originally appeared in the July 12, 2002 issue, p. 97.