Welcome back to Ask a Naturalist, your own personal Google for information on all things natural on PEI. Winter is great time to spot things in and on trees, and I was happy to get some great photos from Michael G LeClair (used with permission). Let’s look at galls and burls!
Both galls and burls are abnormal growths on a tree. Although they can look alike and the terms are often used interchangeably, there are some important differences. Galls are not limited to trees, can appear on any part of a plant, and are most commonly caused by insects or disease. I’ve written before about some common PEI examples including Pineapple Spruce Gall and Willow Pinecone Gall, and you’ve likely seen other types around your home or garden.
Some of the most common tree galls you’ll notice this time of year are on the trunks of Spruce (Photo 1) and likely caused by the microorganism Agrobacterium radiobacter. This bacterium is a natural part of our soil and can infect trees wherever there is an opening in the bark. Once infected, the tree develops Crown Gall Disease, with galls that commonly appear on roots and at the base of the trunk but can also be elsewhere on the stem or branches. Crown Gall Disease isn’t limited to Spruce – a range of species including conifers, hardwoods, fruit trees, and flowering plants are susceptible – and while these galls might look ugly, they don’t usually kill the host (although other types of galls can).
While galls can be found on all sorts of plants and are typically caused by pests or disease, burls only occur on trees and why they form is often unknown. Sometimes burls are associated with epicormic buds: spots on a tree that can sprout into a new branch when conditions are right. Genetics, injury, changes in environmental conditions, or reasons mysterious to us can activate the dormant bud and cause it to grow abnormally into a burl, a bit like a tumour.
Other times, no buds are involved. Burls can be associated with past injury, acting as the tree’s way of sealing it up and protecting itself, similar to scar tissue on an animal. More often, we just don’t know why a burl formed, which means we can’t predict or intentionally create them. That’s unfortunate, because the wood grain inside burls can be beautiful and very much desired by woodworkers. A woodlot owner who could create burls in Ash, Birch, Cherry, or Maple, for example, could have some very valuable trees!
From the outside, it can be hard to tell a gall from a burl. Many of the growths on Spruce – especially those near the ground as seen in Photo 1 – are galls, although there are certainly softwood burls. The growths on hardwoods that are covered with bark similar to the tree are more often burls (Photo 2).
Inside, the differences are much more obvious: burls are always woody throughout, while galls may be soft, spongy, hollow, or full of insects. I know one person who cut into what he thought was a burl only to have hundreds of ants pour out!
As a final note, galls and burls are sometimes mistaken for Chaga, a medicinal fungus found most often on Birch (Photo 3). Unlike the smooth or bark-covered exteriors of galls and burls, Chaga is rough and black on the outside, resembling burned wood. Inside is a beautiful golden colour that makes Chaga unmistakeable. You can read more about Chaga – including how to collect and use it – here: https://www.pei-untamed.com/post/chaga.
Whether it’s wildlife tracks in the snow or galls, burls, and fungi that are more visible once the leaves are gone, there’s lots to see while exploring winter on PEI untamed!
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