Falling for a Maple Burl

I’ve had my eye on this maple in my woods for some time. Not because it’s a beautiful timber tree – it’s only about eight inches in diameter after all. But it has an interesting burl about 14 feet up the trunk. As a woodturner, I love the twisted wood grain found in most burls. A burl is a surprise package on a tree.

Yes, straight-grained wood is beautiful. I love the open grain of red oak, the milky brightness of birch, the rich burnished glow of cherry. But they are predictable. A burl is anything but. I’m not the only one who loves burls. Burl wood has been valued as an element in wood turnings and furniture for hundreds of years.

The Mysterious Origins of Burls

According to Kevin Smith, a plant physiologist at the US Forest Service’s Northern Research Station, they are so valued that they are sometimes the target of thieves. In 2012, Smith published a piece on burl biology in the newsletter of the Massachusetts Tree Wardens and Foresters Association. Next to his article was a report detailing several Massachusetts burl thefts, including one where “arboreal bandits” climbed 30 feet into a very old sugar maple to steal two burls from either side of the trunk. The theft of redwood burls has also made the news in California and Oregon.

What makes burls so sought after is the way that the grain of the wood is generally twisted, contorted, and deformed, producing what’s called “figure.” Visualize a normal grain pattern as parallel strands of yarn. A burl would be a ball of yarn. “It’s as though the tree’s cells went haywire and decided to tie themselves into a knot,” Smith said. In burl formation, the tree’s growth hormones get disrupted when the metabolism of the tree is “hijacked” by some other organism – a virus, fungus, or bacterium.

The crown gall bacterium is responsible for many burls, although many are too small for woodturning. “That common bacterium is especially interesting,” said Smith. “It carries within it a little extra DNA called a plasmid which infiltrates the tree’s genetics. The plasmid prompts the tree to make special amino acids and growth regulators to produce the burl, which apparently is the preferred habitat for the bacterium.”

Figuring out exactly what prompted the formation of a particular burl, however, isn’t always easy, explained Smith. “The bacterium that started the process can be long gone by the time the burl is of any size,” he said. Burls are occasionally associated with dormant buds, but even that does not explain why they get “turned on” here and not there. “So verifiable proof of the cause of a turnable burl will be hard to come by.”

The Unpredictable Nature of Burls

Burls don’t seem to do much harm to the tree or shorten its life, said Smith. “The xylem, twisted and contorted though it is, still seems to do its job of transporting water and nutrients. The vessels are still working, and there’s still starch storage in the healthy outer parts of the burl, so it’s capable of normal function, though I’m sure that function is diminished.”

Cutting out burls, however, does damage a tree, since it leaves large wounds that are likely to become infected. “If I decide I want to turn my maple burl rather than admire it from the ground, I’ll take the whole tree, turn the burl into a bowl or bowls, and use the rest for firewood. It’s in the shade of larger trees anyway, and the stand needs to be thinned.”

With burls being so prized, you’d think someone would have figured out how to induce their formation and produce them commercially. “Different methods of inoculating and wounding have been tried, but haven’t panned out,” said Smith. “It’s one thing to know how gall plasmids work, but to induce that relationship between the bacterium and the tree and favor that over time… well, we don’t know how to do that. Another consideration is the long time span required. It can easily be 30 to 40 years before a burl gets to turnable size. It’s not a get-rich-quick thing,” Smith said.

Perhaps the beauty of burl wood turnings and furniture is enhanced by the fact that it’s something we can’t mass-produce on a whim, even with all the science at our command. As Joe Rankin writes, “The xylem twisted and contorted though it is still seems to do its job of transporting water and nutrients. The vessels are still working, and there’s still starch storage in the healthy outer parts of the burl, so it’s capable of normal function, though I’m sure that function is diminished.”

The Wonders of Stabilized Burl Wood

I’ve also seen burls that have grown around insects, and the burl itself was a crumbly mess. It was in an English walnut tree, I believe, and the insect was some sort of large, pale-colored grub that I never even knew we had around here. I had also thought of using resin to hold the burl together, but it basically disintegrated in my hands.

In case you’ve never heard of the process, look up a technique known as stabilization, which uses a vacuum to allow thoroughly dedicated material to wick water-thin epoxy into itself. Once the epoxy cures, the material can be turned, cut, drilled, chiseled, and even polished to a mirror finish. Stabilization also renders the material basically impervious to the expansion and contraction normally caused by environmental factors like getting it wet or too hot.

This technique has seen much use as of late for creating knife scales, handles, fountain pen barrels, and plenty of other stuff. Some of my favorite examples are knife scales made from corn cobs and also pine cones. I have a box full of flower petals from my daughter’s wedding that I plan on turning into something special someday soon, most likely using a fairly-easily home-made vacuum chamber and a brake-bleeding hand pump or my buddy’s actual vacuum pump.

Unique Burls and Deformations

In Texas, we get Cedar Elm deformations. Not sure if they should be considered burls, but I think what happens is when the mistletoe soaks the wood along the limb and makes it soft, various insects – ants, grubs, beetles, etc. – take advantage and make it their home. The grains then grow in knots around the hollow inside. I’ve cut into it before, and it’s beautiful, but also riddled with holes from the insects. I’ve thought about casting resin into it to try and fill the gaps.

Burl wood is not the only “figured” or deformed timber out there. I’ve seen some amazing things in my time as a hardwood sawyer. Tiny little nodules, almost like a fungus, that produce pure burl figure and grain. Though most are way too small to turn, they are quite spectacular. It’s quite obvious that it’s the tree’s way of repairing itself.

I’m quite fond of studying these characteristics and have some other thoughts on neurological or hormonal deformities. My home was built in 1908, and my staircase and downstairs trim are completely covered with burled pine. I can only imagine how long it took to gather enough to decorate my home. Just makes me appreciate it even more.

The Mysterious Role of Wasps

Yes, certain wasps, most or all in the family Cynipidae, can induce swellings or galls in plant tissues. The most commonly seen example is probably the so-called “oak apple.” Yes, the wasp locally stimulates tree metabolism, which provides habitat for the wasp larvae.

Now, this won’t sound very scientific, and it might just be my personal usage, but I use the term “gall” for smaller, fleshier, or softer growths, and “burl” to mean something large enough and firm enough for a woodworker’s saw. I know we still say that large burls are caused by the crown gall bacterium, honoring several centuries of usage. Nature doesn’t always fit into our neat categories, but here is a good example of convergence by very different organisms to affect tree physiology to meet their own needs.

I have often heard that certain non-stinging wasps can induce burls when they lay their eggs in or under the bark. Is this true, and does it correlate with the viruses, fungi, or bacteria mentioned in the article? It’s a fascinating area of study, and I’m always eager to learn more about the hidden wonders of the natural world, especially when they manifest in such beautiful and captivating ways.