Question
We have had a 12" round bulging growth on the trunk of our
oak tree, I didn't like the looks of it, so I put decorative
eyes, nose and mouth on it that I saw in a gift store.  But
now we are noticing a new one beside it that wasn't there
before and new ones starting on neighboring oak trees.  What
is going on???  It's not funny anymore. Thank you.

Answer
These are called burls. Burls are abnormal growths on the trunks and branches of trees. The tumor-like growths are caused by a number of factors, some of which are unknown. Some disease organisms, such as the Phomopsis fungus, cause a swelling of woody tissues, rather than a sunken canker typical of Nectria and other common tree diseases. A grown-over branch collar on the trunk of a tree will leave a bump, but that抯 not really a burl. A true burl is not caused by a pathogen (as far as we know). The uncontrolled growth of tissue in a burl leaves a hemispherical bump on the tree without typical growth rings and wood structure.
Apparently, little research has been done into the causes of burls. Computer search engines yield all sorts of information for woodworkers, and sources for buying burled wood, but next to nothing about the biology of burls on trees. Burls may serve as infection courts for diseases, but as a rule they do not appear to be harmful to the trees. They certainly should not be removed, since that would expose a large wound that would surely allow decay or diseases into the tree.


The burls are looked at by wood worker as prizes that can be made into beautiful wood art. Do not worry about the burls they pose no health threat to the trees. Enjoy them!!