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March Nature Notes: What Causes a Burl?

Insects, viruses and injuries responsible for unusual growths on trees.

What causes those peculiar, woody lumps we see on trees occasionally? Burls, as they’re commonly known, appear as woody growths that look almost cancerous—as if the tree has been infected with some malignant pathogen. The answer is less alarming, according to forestry experts. Yet there’s a grain of truth in our suspicions: Insect burrowing, viruses and injury can cause a tree to develop unusual growth responses. And, once they develop, burls often continue growing for as long as the tree is alive. Some burls, such as this oak burl spotted within Crab Orchard National Wildlife Refuge in Williamson County, can measure a few feet across.          

“They’ll grow on any tree” reports Eric Holzmueller, assistant professor of forestry at Southern Illinois University-Carbondale. Holzmueller adds that, despite their creepy appearance, burls pose no risk to the tree—or us. “They’re safe to touch.”          

Woodworkers also love burls for their unique grain, a crazy, swirling network of dense loops. Their one-of-a-kind, burl creations are yet another reason to hug a tree, lumps and all.

By: Joe McFarland