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Causes of Burls

By Contributor ; Updated September 21, 2017

Prized for veneer, inlays and unusual turnery, burlwood's unusual grain and great beauty places it in great demand. Some causes of this strange growth pattern are easily understood, but the possibility of commercial production is still elusive. The acquisition of solid high quality burls remains as much a product of luck as finding a gold nugget. Hard work and skilled observation yield the best results.


Some of the best burls are solid wood growths that do not appear to focus on an obvious injury to the tree. The similarity to cancerous growths in animals is obvious, but the connection is only speculative. Growth rings of burls do not show random uncontrolled patterns. The structure of burls is orderly but contorted, and the contortions give the burl beauty and value. Burls do not appear to harm the tree. Approximately five per cent of sugar maples in a northern stand will show birds eye figure, very similar to burl patterns, but no precise cause has been observed. Environmental stresses or hormonal imbalances are suspect.


Scar tissue often gives the appearance of burl wood, but is rarely of commercial value. If a limb dies and rots away, the tree's living tissue covers the wound and creates a bulge that does contain a burl figure grain. The shape of the burl is hollowed, and drying the severed burl without damage from checking is unusual. Occasionally a burl of this type will be usable for bowl stock or small inlays.


Some of the most unusual burls form around parasitic invasions by other plants, specifically mistletoe. Rather than enclosing dead tissue, the living tree strives to overgrow the living parasite. The burl that results is not strong nor continuous, but it may contain enough solid material to be artistically useful. These burls occur on major limbs of trees, not on trunks, and are of no commercial value.


Older trees become subject to many diseases including heart rot. Fungal infections weaken the heartwood but do not kill the tree immediately. Such trees become hosts to many other living creatures including birds, animals and insects. One of the more common invaders in North America is the carpenter ant. This insect does not feed on rotting wood but does remove it in order to build a nesting network of tunnels and chambers. Trees react to the infestation by growing around damaged limbs. These burls are small, but can be of interest to artists and woodworkers.


Since the cause of commercially valuable solid wood burls is not as yet clearly understood, finding these important trees becomes a speculative enterprise like prospecting. Understanding the known causes -- disease and injury -- may give some insight into likely locations of burl trees. Older trees in mature stands, growing under stressed conditions, may be more likely to produce highly figured wood of high quality.


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