Burl on Tree? What Causes Burls on Trees? ID Burl Value Based on Species

What causes a burl on tree trunks? Most people have noticed these large knots that can appear on trunks or branches or even roots, but have you ever wondered why they happen?

The most common species in North America with these features is the coastal Redwood.⁸ Because most often, deformed-looking tree knots are burls.

Despite what one may assume, a burl on tree is usually not harmful at all. In fact, these knots are prized by woodworkers.

They represent a change in the grain pattern of the wood which means they can be formed into unique and visually appealing items.

This complete guide explains what a burl on tree actually is, what causes them, what to do if you notice a burl, and how woodworkers ID the burl tree value based on species.

Tree Burls: What Is a Burl on a Tree?

Simply put, burls (or burrs, in British English) are growths on a tree where the wood grain has deformed.

They are commonly round outgrowths visible on branches or trunks.

They are often filled with small knots from suppressed buds.

Burls are similar to the standard kind of callous tissue which trees use to regenerate and protect themselves. For example, a tree will self-seal pruning cuts off.

However burls are not regulated by the same chemicals as other callous growth, this allows for unique growth shapes.¹

What Is a Burl?

A burl is a growth of tree wood on the trunk or branches that do not follow the regular grain path. It can twist, fold or roll around itself and look like a tumor on the tree.

Burls are thought to be a type of stress response and are not a threat to tree health.

A burl is usually covered in bark, but unlike a pruned branch, a removed burl will not simply heal over, and the wound will leave the tree exposed to fungi and other infections.

Burl Tree: What Causes a Burl on a Tree?

Modern research doesn’t yet have a perfect understanding of the burl on tree.

If we did, we probably wouldn’t need to cut apart wild trees to harvest them. Instead, they could be farmed.

However, the basic mechanism which produces burls is fairly well-understood.

They are produced when a tree undergoes stress. It can come in many forms, such as some type of infection or parasite infestation.

When this happens, the normal growth is upset and tissue begins to build in unpredictable ways.

Burls may continue building for years, layering on itself. Burls are covered by bark, so they are protected from the outside and usually do not increase a tree’s vulnerability to non-human attackers.

As the burl grows it incorporates diminished buds along with any minerals, air, or dirt that find themselves trapped in it.

Burls store the genetic information of a tree and can therefore produce clones. For example, forest Redwoods are known to clone themselves out of burls.

A fallen or cut tree can re-sprout and regrow itself with new wood and buds simply from a burl.¹

Cancer in Trees

Because burls are basically growths that do not follow the regular structure of a tree, they have been compared to non-malignant tumors in humans.

Some other similarities between burrs and tumors are the mysterious but likely stress-related causes.

(Image: Tom6667¹¹)

Also, burls can grow at all times of the year and come in unpredictable shapes and sizes. Unlike tumors, it is not suspected that burls can be or become malignant.

Perhaps a better analogy would be to warts!

Burl on Tree: Underground Burls

Wood swelling of a tree root’s crown is commonly thought of as an underground burl, although it is classified as a lignotuber. The two are formed in much the same way and are often confused.

Some plants may develop basal burls after coppicing or other such stress.

Unlike burls, however, lignotubers are actually part of normal tree growth and stress adaptation. They often develop early in a plant’s life for defense.

The root crowns contain buds for new sprouting and storage for starch that can sustain the tree during diminished photosynthesis or increased energy need.

Usually, they grow significantly larger than burls. One of the largest ever recorded was around 45 feet in diameter with a weight of over 500 tons.

The term “lignotuber” was minted in the early 90s.³

Burl Wood: What Is Wood Burl?

When walking through the woods, or looking at a tree in someone’s yard, one might see a burl on tree trunks as unfortunate deformities in the trees’ usual appearance.

But to woodworkers they look like precious geodes; from the outside a plain rock, but containing highly precious material. A burl is simply wood with a mixed grain, and sometimes people call trees with them, burl trees.

If you cut into one, the cut angle will determine different grain patterns. They are usually complex and can be difficult to discern from the outside.

(Image: tdfugere¹²)

As mentioned above, burls can be considered a protective tissue; they can morph their grain according to the perceived need of the tree. Some trees have enormous burls.

The largest usually comes from redwoods.

They can wrap around an entire section of the trunk (which is just the aerial tree stem of a mature specimen). It should be noted that burls only occupy the woody above-ground parts of a tree.

Roots can also have abnormal growths too, but these are different than burls.

In addition to the complex grain pattern, a burl on tree trunks continue producing tree buds as it grows. These buds get rolled and folded into the burl which creates a unique and beautiful growth design.

Another component that adds to the beautiful burl patterns will be anything that gets trapped inside as it grows; pockets of soil, air, and minerals are common additions. They get rolled in and become added features, although they can present unfortunate difficulties for woodworkers.²

Burl on Tree: Types of Burl Wood

Some tree types typically produce beautiful burls. These include the Cherry Tree, Walnut Tree, Elm Tree, and Ash Tree, to name a few.

In addition, a species you have to look out for is the Sugar Maple Tree. It is a very common carrier of a wood pattern called bird’s eye.

The pattern has swirls, similar to a tiny bird’s eyes, interrupting the straight lines of the wood grain; such a pattern is prized in woodworking.

(Image: extremis¹³)

Like burls, the cause of bird’s eye is unknown. It was once thought that bird pecks into the wood or a fungal infection causes the initial deformation.

However, research has found little support for these theories.

The bird’s eye pattern can be found in other tree species as well, but they especially occur in trees in the Great Lakes region of North America and are most commonly found in the Sugar Maple.

Here are some of the popular types of burl wood:

Heartwood Burls

Heartwood burls are considered the most prize and will fetch the highest price. Heartwood is wood that,¹⁰ by virtue of its age, has become more resistant to decay.

This is older wood closer to the center of the tree, hence the name heartwood. The outer wood is called sapwood which performs functions like conducting water.

Redwood Burls

The coastal redwoods in California are probably the most common species of burl producers in North America. They are very strong and long-lived trees. 

Redwood Tree burls make up a large percentage of burl poaching,⁷ especially in US national forests. An interesting fact about redwoods is that they continue growing as long as they are alive, and can accumulate an enormous amount of biomass.

For other tree varieties, you should consult specific sources to answer how long does it take for a tree to grow.

Oak Burls

The Oak Tree is another North American species that commonly grows burls. Even though they inhabit similar regions, it should not be hard to distinguish oaks from redwoods.

Oaks have distinctive spiral set leaves and produce acorns in the fall. By contrast, redwoods have short needles and cones, with reddish-brown bark.

Burl Tree Value

Because of its unique and interesting grain patterns, burl wood is highly prized by woodworkers, the trees themselves carry the implied value of that wood. This is what makes them targets of poaching.

However under ethical harvesting conditions, burling on tree trunks can provide large profits to their owner.

Burl wood is prized for countless woodworking applications including framing, furniture, musical instruments, vases, car paneling, and doors.

Almost any application of wood can be turned into a unique art piece when produced from wood. In fact, various artists and sculptors seek burl wood for their pieces.

There are several online tree value calculator available for general estimates.

Burl Wood Prices

The price of a burl or of burl wood depends on an enormous number of factors. These generally include location, tree type, age, and condition of the burl, as well as other factors.

Usually, burl wood is priced by weight and can be $2.50 a pound or higher. You can check out eBay or a local lumber dealer for more specific information.

Why Is Burl on Tree Valuable?

Burls are considered valuable because of their unique shape and grain pattern. Usually, wood crafts have relatively standard grain patterns, however, something made from burl wood will display its unique and visually interesting grain pattern.

Additionally, the shape of burls allows for interestingly shaped wood pieces such as bowls or vases. Many videos can be found online of woodworkers turning burls into stunning pieces.

Buying Burl Wood

When buying burl wood you will want to make sure that the burl is in good condition, the right size and shape, and of the grain pattern you desire.

You also need to know the wood’s hardness. For example, a lot of Redwood Burl is not very hard but they could be improved through wood stabilizing techniques.

For a primer on wood hardness, consult this wood hardness scale.

If the burl has not been dried, that is another consideration.

In all cases, you should ask the supplier where the burl came from and how it was obtained to ensure ethical and legal sourcing. Legal burl sources include burls obtained from private land with the land owner’s permission and from lumber companies permitted to engage in salvage logging practices.

The carbon footprint of timber can also be reduced with ethical sourcing.

How To Cut Burl Wood

If you notice a burl on tree branches and trunks, don’t be tempted to cut it. As discussed, burls are not a threat to your tree and in fact are a product of its defense response.

Removing them usually results in death because burls are not regulated in the same way as other tree tissue.

For example, normally if a branch is pruned, the tree will simply start regenerating and seal off the area. This doesn’t happen with burls, so if you try to cut it off, you will simply expose your tree to infections.

The good news is that, although to an untrained eye, a burl might look like an issue, to an arborist or woodworker it looks like a unique feature that holds value.

If the entire tree needs to be removed for whatever reason, be sure to have your tree care professional preserve the burl so that you can sell it to a woodworker. Otherwise, it would be turned into wood chips that can be used for mulching around trees.

(Image: tdfugere¹⁴)

The standard way to keep and preserve it is simply to cut around the burl, making sure to include a margin of at least a few inches. Often, the desirable grain pattern will extend further than the visual indication of the burl itself.

Apply some sort of grain sealer to protect the cut surfaces. Then, the burl can be stored to begin the drying process while evaluations are solicited from potential buyers.

You might want to include some documentation or visual proof that the burl was obtained ethically (not poached). This will likely increase the buying price.

Once the burl is in the woodworker’s shop it can be very hard to work with. Since the grain is folded, twisted and/or rolled, it can be very dense.

Under both hand and machine tools, burl wood has been known to chip, crack, and even shatter. Often, woodworkers will prefer to carve burl when it is still green, and then let it dry as a finished piece.

One example of a process might involve using adzes to get the material into an unpolished shape of the original design. It is recommended to leave extra material because the wood can morph and twist further as it dries.

Once drier it can be led into its finished shape. For this step, rasps are more effective and would be the best choice instead of knives.

Lastly, sand and finish.⁴

Burl Poaching

Due to the prized value of burls, they have become a target for poachers, especially in North America. A great deal of poaching takes place at Redwood National and State Parks.^5,9

To access burls sitting high up a tree trunk, poachers will often just cut down the tree. More commonly, when it is within reach, poachers will cut the burls directly off the tree branches or trunks with chainsaws.

This causes diverse risks for the tree. First and probably most importantly, it leaves the interior of the tree exposed to infection.

Secondly, it often weakens the structural integrity of the tree, making it more susceptible to weather elements, like snow, wind, etc.

Third, since the burls contain buds, cutting the burl deprives a redwood of critical means of reproduction; it’s possible for a 2000-year-old tree to generate another tree that will live millennia from a burl.

Solutions to burl poaching can take many different forms. Since the burls are a precious resource available in largely unguarded areas, the task is a difficult one.

There are basically two approaches. The first is stepping up enforcement.

This is difficult as many US national parks are already significantly understaffed. The idea that an official is going to stumble across nighttime poachers in such a vast area is not really viable.

However one strategy to use is for law enforcement to go to abandoned logging roads. Poachers often use these roads to move in because they will need vehicles to transport the stolen wood covertly.

Targeting these areas could yield some results.

Another strategy that could help is to invest in highly sensitive acoustic equipment. Poachers will almost always use chainsaws to tackle the difficult problem of how to fell a tree.

Besides improved enforcement,⁶ another approach to reduce, if not prevent, burl poaching comes from the buyer side. Most wood buyers tend to be conscientious people.

So if they are aware that they should be asking about its origin and are aware of how to identify suspicious sourcing, this can reduce the incentive to poach. Buyers can also ask sellers for a copy of a document from organizations that certify the ethical sourcing of the burl.⁴

Can I Cut Off a Burl I Found?

Whether you can remove a burl you found depends on a number of factors, assuming you are not a lumber company with a salvage permit, the answer is probably no. Certainly not if the burl is from a tree on land that does not belong to you.

However, sometimes even on your own land, it would not be allowed. States like California have strict rules governing the destruction of trees and other wildlife in order to preserve the natural habitat.

The best way to find out is to contact a local tree service provider or relevant state authority for any guidance. Even if you are in a state that permits you to remove a burl, you should know that removal will likely cost the life of the tree.

Also, the burl may be rotted or otherwise unusable, so an expert should still be consulted before taking any action for burl on tree.