Learn About Tree Burls And What Causes Them

(Image credit: Kiyyah)

Have you ever admired the bumps or bulges growing from a tree trunk and wondered whether they were hurting the tree? The good news is that experts can’t find any evidence that a tree burl has any detrimental effect on the tree. What is a burl? What causes burls on trees? Read on to get more information on burls.

What is a Burl?

If you are wondering about tree burls, the best way to define them is as bumps, swellings, or bulges that grow on or from the trunk of a tree. Tree burls are not always smooth. The burl of a tree can be bumpy with a cauliflower texture, or it can appear in a donut shape, circling the base of a branch. The interior burl wood forms swirls that are particularly lovely. They are sought after by woodworkers.

Don’t mistake a burl for a gall. These are two very different things. Galls are fairly small and form along twigs and leaves. Burls are much larger and are found on major branches or trunks. Burls are a part of the tree itself, while galls grow outside and independent from the tree. 

What Causes Burls on Trees?

If you are wondering what causes burls on trees, you are not alone. Experts have not pinpointed a cause. It is generally known that tree burls form when bud growth cells develop abnormally. As the cells divide, they create a rounded shape rather than a normal tree branch. They divide in an abnormal way. As these dormant cells start dividing in many directions, they end up creating a rounded shape rather than forming the tree limb, which they originally intended to grow. 

Possible Stressors

What causes the cells to develop abnormally? Scientists don’t really have an answer. Many believe that the cells develop abnormally due to some stress the tree has suffered. However, the type of stress is as yet unidentified. Some of the suspects include bacteria, fungi, viruses, mechanical wounds, fire damage, or a combination of any of these factors. 

Since burls don’t harm trees, the fact that the exact cause of a burl remains a mystery is not serious. There is no reason to cut off that burl. In fact, leave it alone! Removing it might kill the tree. 

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.