Garden Detective

Danger overhead: When an old oak shows signs of rot

Garden Detective: What’s wrong with this oak tree? Wood rot has created a cavity where a limb was once attached.
Garden Detective: What’s wrong with this oak tree? Wood rot has created a cavity where a limb was once attached.

Experts tackle readers’ garden questions.

Q: We have a beautiful 50-foot oak tree in our front yard. Last year, I discovered a large mushroom growing from a tree knot where a large branch had been removed many years ago. I removed the mushroom, scraped the area “clean,” and sprayed it with a black tar tree heal purchased from Home Depot. Then, I noticed a color change to part of the black area and started digging it out. I found a large cavity (6 inches diameter, 4 inches deep) filled with moist sawdust. Although I removed the bulk of it, some sawdust still remains in the cavity and at the base of the tree. I do not know what to do now. Is this sawdust toxic to the tree or roots? How do I treat the cavity? We do not want to lose this tree! Your help and guidance are appreciated. The hole is about 4 feet above ground level.

Dave Crocker, Sacramento

Sacramento County Master Gardener Carmen Schindler: The most common cause of failure of trunks, large branches and roots of trees is decay from wood rot. More than a thousand different species of fungi can cause wood deterioration and decay that destroy the plant’s internal structural components – cellulose and lignin – potentially rendering them unable to support their own weight when stressed by heavy winds or rains.

Unlike redwood and some other conifers, hardwood (such as oak) decays rapidly both in the standing tree and fallen limbs. Although very strong when not decayed, oak wood quickly loses strength as fungi break down the cellulose and lignin in the cell walls of the wood.

Decay can be visible on the outside of the tree where the bark has been cut or injured, when a cavity is present or when the rot fungi produce reproductive structures (such as mushrooms). Wood decay can make trees hazardous when trunks and limbs are unable to support their own weight and can fall, especially when stressed by wind, heavy rain or other conditions.

It is important to have the tree analyzed by a certified arborist. Hazardous trees should be trimmed, cabled, braced or removed. Have a qualified expert assess the tree and recommend appropriate treatment. For information on hiring a certified arborist, visit the Sacramento Tree Foundation at

Wood decay usually appears in old, large trees and can be difficult to manage. However, a number of factors can reduce the risk of serious damage. First, trees should receive proper care to keep them vigorous. Minimize wood decay by protecting trees from injuries. Properly prune young trees to promote good structure. This will avoid the need to remove large limbs, which creates wounds at the pruning cut, as the trees grow older. Cut out dead or diseased limbs, but do not attempt to clean out or seal the cavity. Do not use wound dressing (such as black tar) because it has not been found to hasten wound closure or prevent decay.

For more information on wood decay fungi in landscape trees and how to properly remove branches or limbs, visit the University of California, Integrated Pest Management, website at

Carmen Schindler is a UC Cooperative Extension Master Gardener for Sacramento County.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h& Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call: