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What's Happening to our Oaks?

Dr. George Ware of the Morton Arboretum contributed information for this article. More information about oaks and protecting trees during construction may be found in the Tree Body Resource Files at the Palos Park Public Library.

Oak forests are an irreplaceable landscape legacy distinctly characteristic of Palos Park. What has taken centuries to develop can be destroyed by man and machine in twenty minutes. This is what we see going on during new housing constructed on wooded sites. Clearing the land means killing the oaks. Once damaged, it only takes two to five years for a two hundred year old oak to die.

In our area it is especially important to understand the interrelationships between forest trees and forest soils. Oaks thrive in acid soils produced by their fallen leaves. About two feet below the surface is a layer of clay which roots cannot penetrate. Due to these conditions, oak trees develop a mat or system of fine roots in the upper FEW INCHES of the soil. Because they are so near the surface, these fine roots are exceedingly sensitive to any changes in the surface layers of soil. We should also be aware that our forests provide favorable conditions for other trees and support a multitude of living things – mammals, insects, fungi, plants, and other forms of life. When trees are destroyed, our landscape heritage is profoundly altered.

Fortunately, there are places in Palos Park where large and small oaks are safe. It is possible to simulate the conditions of the forest in our yards. An ideal situation is a spacious, sunny area with an acidic soil developed by the decomposition of fallen leaves. Three inches of wood chips, humus, or other organic mulch should extend as far out as the drip line of the tree. As the tree grows, the mulch area should expand. Allow no mulch to touch the trees trunk. Ground covers such as Virginia creeper, purple wintercreeper, English ivy, periwinkle, and Japanese pachysandra are hearty , shade tolerant plants which collect leaves and protect the roots.

Oaks should not have to compete with grass for water and nutrients and be exposed to weed killers. Trees need much less fertilizer than grass. In natural systems, it takes along time for oak leaves to break down. If a tree shows signs of decline, Dr. Ware recommends fertilizing in September after the leaves are finished producing. Slow release pellets of a balanced fertilizer of nitrogen, potassium, and phosphorous can be absorbed by the roots up until Christmas and then in the spring.

In naturalized areas, it is important to kill grape vine which will shade the leaves and kill a tree. Also, clear away buckthorn and weeds from the trunks of the oaks as far out as the drip line to keep the roots from becoming soggy. This condition is an invitation to mushroom –shoestring root rot (the fungus Armillaria) which is infecting many of our red oaks. Dr. Ware said that a red oak that has lost its top or is dropping hollow limbs is probably diseased even though the tree may otherwise look healthy. He recommended cutting the tree down because of the unpredictability of if and where it may fall in a wind storm. The tree can be left standing if there is no danger of it hitting anything.

Many of our mature oaks are showing sins of stress and decline. In the spring, look at the top of the oaks for dead limbs and sparse branching. Dr. Ware says decline seems to be related to an imbalance of crown/root ratio. A reduction in the number of branches lessens the demand on the roots for water. Thinning of the crown and removing the dead branches is the most direct way of helping an oak in decline. (This is not topping a tree). Mulching and fertilizing may also invigorate a tree. A stressed tree with dead branches is an invitation to diseases and insect invasions.

While red oaks are more susceptible to mushroom rot, white oaks can occasionally be infected. In areas of diseased trees, it is best not to replant oaks. Dr. Ware suggested sugar maples, lindens, white ashes, and ironwood which tolerate some shade. In sunny areas, swamp white oaks and burr oaks are healthy species.

Before you compact the soil, raise or lower the surface of the earth, impede drainage, install thick sod, use nonorganic material around trees, park your vehicles, remove leaves or neighboring trees, use weed killers and fertilize lawns, consider your oaks. Are you killing them?

PALOS PARK TREE BODY RESOURCE FILES - Available at the Library!
Need help with you trees? Consult the resource files at the Library. The Tree Body currently has six volumes of articles about trees and a variety of subjects related to our environment. Information contained in these volumes covers a wide range of topics including tree species, wildlife, native plants and useful descriptions with pictures of diseases and insects to help our residents. We get information from the Morton Arboretum, National Arbor Day Foundation, EPA, etc. Marifran Peckenpaugh is collecting, organizing, and updating these materials.