Burls on trees: rounded trunk swellings, as extension services describe them

This page describes what the named sources say. It does not diagnose a tree, say whether any tree or growth is safe or unsafe, or tell anyone to cut, keep or treat one. Each cause and claim belongs to the source that gives it.

What a burl is

The University of Maryland Extension (updated February 28, 2023) calls a burl a large woody, rounded swelling, usually found on a tree trunk, and says it can reach a diameter of up to 3 feet and may carry many buds or sprouts. It says the grain of the wood swirls around each bud trace, which gives the figured pattern that makes burl wood prized. The Northern Woodlands column by Joe Rankin (The Outside Story) describes the same grain as twisted, contorted and deformed, and compares normal grain to parallel strands of yarn and a burl to a ball of yarn.

University of New Hampshire Extension (Rockingham County Extension Forester Greg Jordan, January 6, 2022) says burls can be found on many kinds of trees, and that in southeastern New Hampshire he most often finds them on oaks, maples and cherries. It quotes Kevin Smith, a Supervisory Plant Physiologist with the United States Forest Service, who defines a burl as the result of hyperplasia, a greatly abnormal proliferation of xylem production by the vascular cambium.

What causes are named

Source	Cause it names
University of Maryland Extension	The cause of most burls cannot be explained. They may develop from insects, bacteria, fungi, mistletoes or environmental injury such as freeze damage.
University of New Hampshire Extension	Likely some infection by bacteria, viruses, fungi or insects, to which the tree responds with highly irregular wood growth. The exact mechanism is not fully understood.
Northern Woodlands, quoting the US Forest Service physiologist	The tree's growth hormones are disrupted when its metabolism is taken over by a virus, fungus or bacterium. The crown gall bacterium is named as responsible for many burls, many of them too small for woodturning.

Northern Woodlands adds a detail from the same physiologist: the bacterium that started a burl can be long gone by the time the burl is of any size, and burls are occasionally associated with dormant buds, but that does not explain why they switch on in one place and not another. In his words, verifiable proof of the cause of a turnable burl will be hard to come by.

What the sources say about harm

New Hampshire says that from the author's observations there is little to suggest burls cause damage to trees. Northern Woodlands quotes the physiologist saying burls do not seem to do much harm to the tree or shorten its life, because the vessels still work and starch is still stored in the healthy outer parts, so the burl is capable of normal function, though he is sure the function is diminished.

What the sources say about taking one off

All three sources speak to this. Maryland says sprouts arising from a burl may be pruned off, that otherwise no treatment is necessary, and that removal of the burl may damage the tree and is not recommended. New Hampshire says removing a large burl causes a large wound, like flush cutting, which may lead to serious decay, and that trees have trouble compartmentalizing wounds made this way, unlike proper branch pruning that protects the branch collar and branch bark ridge. Northern Woodlands says the same of cutting burls out, that it leaves large wounds likely to become infected.

Burls as a product

Northern Woodlands says burl wood has been valued by woodturners and furniture makers for hundreds of years, that burls are sometimes the target of thieves, and that a turnable size can take 30 to 40 years. It says attempts to induce burls by inoculating and wounding have not panned out. New Hampshire says woodworkers are interested in burls but are not likely to pay much, if anything, for them, and that it is hard to know what the inside looks like until one is cut open.

Other growth forms are on the odd tree growth page. Swellings with an insect or mite cause are on the tree galls page.