Tree galls and burls are both abnormal growths on trees, but they form for different reasons, look different inside, and affect the tree differently. A gall is triggered by an outside organism, usually an insect or mite, while a burl grows from the tree’s own disrupted bud tissue. Telling them apart matters whether you’re assessing tree health, identifying a find in the woods, or evaluating wood for crafting.
What Causes Each Growth
Galls form when insects, mites, bacteria, or fungi introduce chemicals into plant tissue during feeding or egg-laying. These chemicals mimic the tree’s own growth hormones, hijacking normal cell development and causing tissue to swell into unusual shapes. The gall essentially becomes a shelter and food source for the organism inside it. Galls usually start forming in late spring or early summer when insects become active, often appearing just below buds as the stem grows. Common culprits include tiny wasps in the Cynipidae family (responsible for the familiar “oak apple” growths), eriophyid mites, gall-forming midges, and adelgids on spruce trees.
Burls have a murkier origin. They develop when dormant buds beneath the bark begin to sprout but never fully emerge. These aborted buds, called adventitious buds, trigger surrounding cells to multiply and grow in chaotic directions. Stress, injury, fungal infection, or bacterial exposure can all set this process in motion, but in many cases the exact cause of a particular burl is unknown. Unlike galls, burls aren’t housing a living organism. They’re the tree’s own tissue growing in a disorganized pattern, sometimes over decades.
How They Look on the Tree
Galls come in a huge variety of shapes and sizes depending on the species that created them. Some look like small round balls on leaves or twigs. Others resemble bumpy cones, spiny pods, or finger-like projections. They tend to appear on newer growth: leaves, small twigs, and branch tips. Many galls are relatively small, from pea-sized to golf ball-sized, though some can grow larger. They often appear in clusters, since the insects that create them may lay many eggs in one area.
Burls look like rounded, bark-covered lumps bulging from the trunk or major branches. They can range from fist-sized to enormous, sometimes spanning several feet across on old trees. Their surface is typically covered in rough, knotted bark. Burls grow slowly over years and are most visible on the main trunk, where they can become quite prominent. Redwood burls in California and Oregon can grow massive enough to attract timber thieves, and sugar maples in New England have been targeted for the same reason, with documented cases of people climbing 30 feet up old trees to cut burls free.
Internal Structure
This is where the two growths diverge most dramatically. Cut open a gall and you’ll typically find relatively ordinary plant tissue surrounding a hollow chamber or soft interior where the insect larva lives. The wood grain inside a gall isn’t particularly interesting. Galls may also contain bark rot, twig knots, or boring-insect damage. They’re functional structures for their inhabitants, not prized for their wood.
Cut open a burl, and you’ll find something entirely different. The wood grain inside is wildly contorted, with fibers growing in many directions at once. This produces wavy, curly, or “bird’s-eye” patterns, where each small circular figure in the grain marks the spot of an aborted adventitious bud. This chaotic grain structure makes burl wood extremely dense and difficult to split. It’s also what makes burls highly valuable to woodworkers and artists. Burl wood from species like redwood, maple, walnut, and elm can sell for significant prices, and the swirling grain takes a beautiful polish.
Which Trees Get Which
Galls are especially common on oaks, where dozens of wasp species create distinct gall types. The oak rough bulletgall wasp produces hard, round growths on twigs, while other species create the papery oak apple galls on leaves. Hackberry trees are prone to their own signature gall from the hackberry nipplegall maker, spruce trees host the cooley spruce gall adelgid, and poplars develop twig galls from specific fly species. Essentially, most deciduous trees can develop galls of one kind or another.
Burls show up across many species but are especially notable on redwoods, sugar maples, oaks, black cherry, walnut, elm, and pine. Redwood burls are famous both for their size and for their ability to sprout new trees. Maple burls are prized in furniture making and turning. The species matters a great deal for woodworking, since the grain patterns and colors vary widely.
Impact on Tree Health
Most galls cause little lasting harm to the tree. They divert some energy and nutrients to feed their inhabitants, and heavy infestations can stress young or weakened trees, but a mature tree with a moderate number of galls will generally continue growing normally. The galls themselves are seasonal. Once the insect completes its life cycle and leaves, the gall stops growing, though it may remain attached to the branch.
Burls are a different consideration. Because they grow from the tree’s own tissue and are extremely dense, they don’t typically weaken the trunk the way a wound or cavity would. A burl’s contorted grain is actually very strong and hard to break. However, burls can sometimes be associated with underlying stress or infection, and removing a burl from a living tree creates a large wound that exposes the tree to decay and disease. This is why cutting burls from living trees, beyond the ethical and often legal problems with burl theft, can seriously damage or kill the tree.
Quick Visual Guide
- Location: Galls favor leaves, twigs, and small branches. Burls grow on trunks and major limbs.
- Size: Galls are usually small, from a few millimeters to a couple of inches. Burls can grow for years and reach several feet across.
- Surface: Galls often have smooth, papery, or spiny surfaces. Burls are covered in rough, knotted bark that matches the rest of the tree.
- Inside: Galls contain chambers for insect larvae. Burls contain dense, swirling wood grain with no hollow interior.
- Cause: Galls are caused by insects, mites, or other organisms. Burls arise from the tree’s own disrupted growth.
- Value: Galls have minimal woodworking value. Burls are highly sought after for their figured wood.