Tree burls beautiful as well as a bit mysterious

As you glance up into tree limbs, perhaps searching for some sign of spring in a swelling flower bud, your sight might be arrested by fat, rounded growths on the bark.

On some trees, these hard, woody outgrowths — called burls — stand out on an otherwise clear trunk like a goiter. On other trees, the whole trunk might be covered with these masses.

But don’t be alarmed: They cause little or no harm to the tree.

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, chewing by insects or some other physical injury. Perhaps the tree experienced temperatures too cold or too hot, not enough food or not enough sunlight, for example.

Diseases have also been held responsible. However, no pathogens are found inside burls when they are cut open.

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

Along any stem are buds that can elongate

to become shoots.

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 generally just keep dividing with no obvious purpose or benefit to the tree.

Benefit to us humans is another story, though. Cut open a burl and instead of straight grain you find waves and swirls of wood, marbled and feathered wood, perhaps some “eyes” staring back at you.

Woodworkers lust for burls, especially for turning on a lathe, where the appearance of the swirling tissue need not be constrained within the straight and sharp-edged geometry of sawn lumber. Burls are also frequently sliced into veneer.

No one has been able to reliably produce burls in a tree on purpose.