What makes a burl

That said, burls might — just might — indicate that the tree has been under stress. All sorts of things have been implicated as the cause of burls. For instance, a burl could grow in response to a limb rubbing against the bark, to chewing by insects or some other physical injury. Perhaps the tree experienced or is experiencing some environmental stress — temperatures too cold or too hot, not enough food or not enough sunlight, for example.

Diseases have also been held responsible for burls. However, no pathogens are found inside burls when they are cut Still, a pathogen could have done its job of inducing a burl, and then skipped on to other adventures.

We could also blame genetics, because some tree species are more prone to developing burls than others. Redwoods are renowned for their burls, which are often sold as souvenirs.

Odd stem growth

We do know, though, how burls develop. Along any stem are buds that can elongate to become shoots, and each of these shoots similarly has buds that can become shoots themselves. Obviously, not all buds on a plant stretch out; some remain dormant, at least for a while.

In the case of a burl, instead of dormant buds expanding into straight shoots, they grow inward, twisting and turning under the bark and never emerging as branches.

On the rare occasion when a shoot does elongate from a burl on a mature tree, the shoot usually expires from lack of sufficient light. Interestingly, redwood burls frequently sprout when cut from the tree and placed in a pan of water, almost as if the water reminds the cells that they can be shoots. On a tree, though, cells within a burl generopen.