Here's a commonly heard problem on the ISU Hortline: "I have a beautiful pin oak in the front yard--it's been there for almost 25 years. And now it's starting to die off and the leaves look terrible. What can I do?"
On most Iowa soils, probably the best course of action would have been to never have planted pin oak in the first place. While this homeowner thought they were planting for posterity, what they really planted was a problem tree that will cost them significant headaches for the life of the tree.
Pin Oak (Quercus palustris) does poorly on our clay-rich, poorly drained soils. Pin oak prefers moist, rich, well-drained, acidic soil, and is extremely intolerant of high pH soils. In appropriate soils, this tree can reach 70 to 100 feet and has an attractive pyramidal form with glossy dark green leaves.
When planted in soils with a pH greater than 7.5, however, the pin oak develops iron chlorosis, a nutrient deficiency symptom. (Other trees quite susceptible include silver maple, baldcypress, crabapple, and sweet gum.) Affected trees have yellow leaves with dark green veins, with some developing angular brown spots and brown, curled leaf margins. In severe cases, leaf color may change from yellow to white to brown. After suffering from chlorosis for a period of years, branches and twigs may begin to die. Chlorosis is often most severe in areas where topsoil has been removed, exposing clay subsoil, as in new housing developments.
Usually, there is plenty of iron in the soil, but high soil pH prevents the iron from being absorbed by the plant. Since iron is essential for production of chlorophyll, the tree fails to produce enough chlorophyll to maintain healthy green leaves. Lack of available iron can be magnified by low soil temperatures; high soil moisture; large amounts of copper, manganese, or zinc; or excessive application of phosphorus.
What are the alternatives to a lifetime of feeding a pin oak? First, choose plants suitable for your soil. There are several attractive oaks which do much better in typical Iowa soils. Recommended species include chinkapin oak (Q. muehlenbergii), English oak (Q. robur, cultivars 'Fastigiata' or 'Wandell'), or red oak (Q. rubra). Below are some common shade trees found in Iowa and their preferred soil pH ranges.
Preference of certain species of trees for various soil pH levels
|pH 5.5 or below||pH 5.0 to 6.5||pH 6.0 to 7.5|
|Bur oak||Douglas fir||Ash|
|Shingle oak||Flowering dogwood||Catalpa|
|Tulip tree (Yellow poplar)||Silver maple|
Source: Nebraska Cooperative Extension Service
If however, you have a yard with an established pin oak, there are some possible treatments for addressing soil chlorosis, though they can be expensive and/or time-consuming. There are three primary methods for treating iron chlorosis due to alkaline soil:
- Foliar application: While foliar application of iron sulfate or iron chelate can provide a quick response, the remedy is temporary since the iron does not move beyond the actual leaves that are treated. A rate of five pounds of iron sulfate in 100 gallons of water (2.5 oz. iron sulfate in 3 gallons) is recommended. Adding a tablespoon of detergent to the mixture will help to wet the foliage. Iron chelates are water-soluble forms of iron that remain in solution, therefore available to the tree. It is best to apply foliar applications during the evening or during periods of cool weather.
- Soil application: Adding iron chelates or acid drenches to the soil can make iron more available to the tree for two to three years, but won't have a lasting effect on soil pH. Iron sulfate or a mixture of sulfur and iron sulfate can be used to make iron more available to the tree roots. Treatments are typically applied by drilling holes spaced two feet apart and 15 to 18 inches deep in concentric circles around the tree, extending beyond the dripline by about 3 feet. Common chelates available at garden centers include the trade names "Tru-Green" and "Sequestrane." Consult the labels for rates.
Homeowners can also try to lower the soil pH by applying elemental sulfur (96%) at the rate of 2 lbs/100 square feet in April and again in September.
- Trunk injection: Tree injection systems include some available to the homeowner and others available only to professional tree services. Home injection systems include Medi-Caps and NutriBooster.In this treatment, iron sulfate, iron citrate or iron chelate is implanted in 3/8" holes 1/2" to 1" deep drilled in the tree trunk and then sealed with paraffin or grafting wax. Another method uses liquid ferric ammonium citrate applied in drilled holes or through a reservoir system. Injection treatments are most effective if applied in the early spring during bud break. However, homeowners should weigh the potential benefits from trunk injection with the potentially negative consequences caused by wounding the tree. This treatment should be reserved for high value trees.
Dealing with iron chlorosis in the home landscape can be a daunting task, but by doing a little extra research to place the right plants for the site in the first place, homeowners can ensure they'll enjoy attractive, healthy trees for years to come.
This article originally appeared in the July 11, 1997 issue, pp. 108-109.