During my hike in the Ouachita National Forest in Arkansas last December, I came across an exceptionally large burl on an oak tree. Although I frequently spot these growths on my hikes, this one was noteworthy for its size. I even stood next to it to showcase its magnitude.
Burls are distinct growths on the trunk or branch of a tree characterized by their knobby appearance. They develop as a protective response to disease, usually in reaction to injury or infection caused by insects, viruses, fungi, or bacteria. While not inherently harmful to the tree itself, they are believed to sap a tree’s vigor. Though most burls do not impact plant health, some types, like the horned and gouty oak galls, can prove fatal to younger trees. Woodworkers prize burls for the unique grain found within them.
The erratic growth of a burl forms a mass of shoot and bud tissue, creating a distinctive bulge on the trunk resembling a benign tumor. Burls are thought to arise in response to stress, often at wound sites, to shield the tree from further damage.
To discern whether a trunk protrusion is a burl or a gall, consider these differentiating traits:
Burl:
- Large, rounded, knobby outgrowth with smooth surface
- Color matches surrounding bark
- Exhibits complete, swirling grain pattern
- Protective response to disease
Gall:
- Abnormal shape, from tiny bumps to massive growths
- Formed by insect or disease activity
- Surface may be rough with knots and bumps
- Discolored due to infection/inflammation
In summary, burls are smooth, rounded accretions that develop to protect compromised trees, while galls are irregular outgrowths induced by parasites and pathogens.