Blue Tree Resin

Welcome back to Ask a Naturalist, your own personal Google for information on all things natural on PEI. Jeff Matheson sent me this wonderful photo of blue resin on the side of a spruce tree and was curious about the unusual colour. Let’s take a look!

First, it’s useful to know the difference between sap and resin (sometimes called pitch). All trees have sap, and it’s analogous to blood in animals: it carries water, sugar, nutrients, minerals, and hormones throughout the tree. Conifers – spruce, pine, fir, cedar, hemlock, and larch – also produce resin, and its main role is to seal wounds and protect the trees from insects and infection. Sap is thin and watery; resin is thick, gummy, and flammable (turpentine is distilled from resin). This looks like resin.

Fresh resin is clear or very pale coloured, and it becomes darker over time due to oxidization. When it first appears as small drops or thin layers, resin scatters rather than absorbs sunlight. Blue light has a shorter wavelength and scatters more, which means our eyes see mainly blue from the resin. This is the same reason the sky, glaciers, lakes, and oceans also look blue.

Blue resin is more common in winter because cold temperatures slow oxidation and the associated colour change. As the weather warms in spring, the resin will darken into amber or brown.

Resin on the side of a tree is usually a sign of stress: mechanical injury, such as wind or pruning damage; biological attack by fungi, bacteria or insects; or environmental challenges such as drought or poor soil. Depending on the tree, its location, and the severity of damage, actions could range from doing nothing, to fertilizing and watering, pruning off infected areas (disinfecting tools afterwards), or removing the tree altogether. In most cases, you’ll see resin (blue or otherwise) on scattered trees in the forest and it’s not a major cause for concern.

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