Burls: what makes the big knobs on a trunk, and do they harm the tree?

A burl is a hard, rounded, knotty swelling on a trunk or a big limb, wood the tree laid down too fast and in every direction at once. It rarely harms the tree, because the twisted wood still carries water and stores starch. What starts one is seldom known. Cutting it off does more harm than leaving it. The lookalike that matters is a canker, a patch of dead bark that can girdle a limb.

Why burls form

A burl begins in the cambium, the thin layer under the bark that makes wood, when its cells divide faster, for longer each season and in no set direction. Bacteria, viruses and fungi are the usual suspects. Insects, mistletoe and freeze injury are named too. Any tree can grow a burl, and oaks, maples, cherries, elms and willows often do.

Proof is rare. The organism behind a burl can be long gone by the time it is large, and no one has found a way to grow one on purpose. Some burls bristle with epicormic shoots from dormant buds, but Kevin Smith, a Forest Service plant physiologist, says the buds do not explain where a burl forms:

“Burls are occasionally associated with dormant buds, but even that does not explain why they get ‘turned on’ here and not there.” (Northern Woodlands, Go Figure: How Tree Burls Grow)

Not every such swelling is abnormal. Blue Whale Trees grows New Jersey tea, a knee-high shrub whose thick, burl-like root sends up new shoots after fire.

Burl, gall or canker

A gall is the tree’s own tissue grown around an insect, and the common oak galls are sorted in wasp galls on oaks. Where a growth sits and what it shows tell the three apart:

Should you cut a burl off?

No. Unlike a branch, a burl has no protection zone at its base. Taking one off works like a flush cut and leaves a wide, shallow wound, as if machinery had skinned the trunk. Decay usually follows into the stem and keeps going, even if woundwood closes the wound. Sprouts from a burl may be pruned off, and nothing else needs doing.

Burl wood is prized for grain that swirls around each bud trace, yet a burl on a standing tree is worth less than it looks:

“And although woodworkers are interested in burls, they are not likely to pay much, if anything, for them.” (University of New Hampshire Extension, The Mystery of Tree Burls)

Nobody sees the grain until the burl is cut open.

When a swelling needs a second look

Three signs call for a closer look, or an arborist’s:

• Dead bark. A sunken, dark or split patch is likely a canker, which can kill a branch or weaken it enough to break in a storm.
• A bulge beside a seam. Swelling on both sides of the seam where two trunks meet means the union is under strain, as multistem or single stem explains.
• Conks at the base. Shelf fungi at the foot of a trunk can mean rot in the roots, as fungus on a tree explains.

We take questions about trees you already have on our support page. A photo of the whole tree, a close-up of the swelling and a note on when it started help us answer quickly.

Common questions

What causes burls on oak trees?

Infection or injury usually gets the blame, on oaks as on any tree. What started a particular burl is seldom known.

Is a burl bad for the tree?

Rarely, because the wood inside still carries water and stores starch. Cutting one off is what harms the tree.

Is a burl on my tree worth money?

Seldom much, because woodworkers pay little for burls and no one can see the grain until one is cut open. Take the wood only if the tree comes down for another reason.

Sources

• The Mystery of Tree Burls: University of New Hampshire Extension (Greg Jordan, 2022): burls most often on oaks, maples and cherries, little sign that they harm trees, removal as a large wound like a flush cut, the grain unseen until a burl is cut open, woodworkers paying little for burls, and waiting until the tree comes down for another reason.
• The biology of burls: USDA Forest Service (Kevin T. Smith, in Bark 3, Massachusetts Tree Wardens’ and Foresters’ Association, 2012): the cambium dividing faster, longer and in disordered directions, infection as the suspected cause, no way yet to induce a burl, no chemical protection at a burl’s base, removal as a shallow wound like a skinned stem, and decay before and after woundwood closes it.
• Go Figure: How Tree Burls Grow: Northern Woodlands (Joe Rankin, 2015): Kevin Smith on burl wood that still carries water and stores starch, the starting organism long gone, dormant buds that do not explain where a burl forms, and attempts to induce burls that have failed.
• Burls: University of Maryland Extension (2023): the cause of most burls unexplained, with insects, bacteria, fungi, mistletoes and freeze injury among the causes, sizes up to 3 feet, buds and sprouts that may be pruned off, and grain that swirls around each bud trace.
• Burls: Texas A&M Forest Service: burls on any tree, most on maples, oaks, elms and willows, on stems and branches, and seldom harmful.
• Canker Diseases: The Morton Arboretum: cankers from fungi and bacteria that often enter through wounds, sunken, dark bark on trunks, branches and twigs that splits from the living bark, branches killed or weakened for storms, and limbs and trunks girdled over months or years.
• Galls on Oak: Penn State Extension: galls as plant tissue grown around an insect, usually on leaves or twigs, small wasps as the gall makers on oaks, and the horns of the horned oak gall.
• Ceanothus americanus: USDA Forest Service, Fire Effects Information System: New Jersey tea’s thick, burl-like, deep woody roots and its sprouting after fire kills the top.