CORVALLIS, Ore. – Few landscapes are more inviting than a rolling grassland graced with large spreading oaks. The park-like beauty of an oak savanna attracts picnickers as well as developers who may hope that the old trees will lend a touch of grace to their designs.
But replacing the grassland with houses, lawns and sidewalks can have devastating effects on the trees, according to Rick Fletcher, Oregon State University Extension forester.
The few large, open-grown, heavy-limbed white oaks that remain in western Oregon are a remnant of a landscape that was once much more common. Many of these trees are 200 years old or older. They are hardy trees that have weathered fire and pestilence, flood and drought. But they may not survive being part of the modern suburban landscape.
"Homeowners should understand the natural ecosystem these big old trees represent," Fletcher said. "There is no need to rid trees of moss and lichens that droop from branches, nor to spray oaks to kill gall-forming insects."
Native oaks face new hazards in suburban life. Oregon white oaks are adapted to prolonged summer drought. Summer watering to maintain green lawns and lush flower beds under these trees predisposes them to root rot which will cause the trees to windfall and death, Fletcher said.
"In fact, the tree in your yard may have already suffered root damage during construction of your house. The roots of oak extend out from the trunk as much as four times the reach of the branches. Oak roots are susceptible to damage by heavy machinery, compaction of the soil and suffocation if extra dirt is spread across the yard to even out the landscape. In addition, many roots can be cut off when developers are installing underground utility lines.
Fletcher points out two basic steps needed to take care of oaks in your yard:
First, turn off the water. Don’t try to grow plants with high water demands under your oaks, particularly near the trunk. Even more important is to avoid frequent and shallow watering. You may have to choose between having healthy oaks or a green summer lawn in that part of your yard. Local OSU Extension offices can help you select drought-hardy native plants to use in the landscape beneath native oaks.
Protect the ground and tree roots from excavation and pavement. Avoid driving vehicles and tractors under the oaks. If you have an old oak in a new subdivision or where tree roots may have been damaged by nearby construction, contact a certified arborist to evaluate and help restore the health of your tree.
Majestic old oaks can add much to the value of your home and the pleasure of your yard. It is well worth understanding how to keep them healthy.