My Tree is Covered in Green and Orange Crusty Stuff — Is It Diseased?

Across Grange-over-Sands, Cartmel, Kendal and the wider Cumbrian landscape, the trees, stone walls, gate posts and rock outcrops are covered in a variety of crusty, leafy and shrubby growths in shades of grey, green, orange and yellow. If you have not looked at them closely, this guide is an invitation to do so, because they are genuinely fascinating. If you have been worrying that they are a sign of disease in your garden trees, this guide should reassure you.

What Are Lichens?

Lichens are not a single organism. They are a partnership, technically called a symbiosis, between a fungus and either an alga or a cyanobacterium (or sometimes both). The fungus provides the physical structure; the alga or cyanobacterium produces food through photosynthesis. Together they form an organism that is far more resilient and long-lived than either component could be on its own.

Lichens are among the most ancient and most resilient life forms on Earth. They grow on bare rock, on tree bark, on old walls and gravestones, on soil in the most inhospitable environments and in conditions where almost nothing else can survive. Some individual lichens are thousands of years old. They are not plants, not fungi and not algae but something quite distinct.

Are Lichens Harming the Tree?

No. Lichens are not parasites and they do not harm the tree they are growing on. They use the bark as a physical surface to attach to but they do not extract anything from the living wood or interfere with the tree's biological processes in any way. The presence of lichen on a tree tells you nothing negative about the health of the tree itself.

In fact, the presence of a diverse and abundant lichen community on a tree is a positive indicator. Lichens are extremely sensitive to air pollution, particularly sulphur dioxide, and were largely absent from trees across much of Britain during the industrial period when pollution levels were high. The return of lichen diversity to trees across Cumbria and the Lake District in recent decades reflects real improvements in air quality. A tree covered in a diverse array of lichen species is growing in clean air. This is genuinely something to feel good about.

The Lichens of Cumbria and the Lake District

Cumbria and the Lake District are among the most important areas in Britain for lichen diversity. The combination of clean Atlantic air, high rainfall, mild temperatures and the diversity of rock types and tree bark available creates conditions that support an extraordinarily rich lichen flora. Some of the lichen species found on old trees in the Lake District are genuinely rare in a European context, found in good numbers only in the ancient woodland of the north-west Atlantic seaboard.

The crusty orange lichens common on rooftiles and stone walls across Grange-over-Sands and the Cartmel Peninsula are typically species of the genus Xanthoria or Caloplaca. The grey rosette lichens on tree bark are often species of Parmelia or Physcia. The shrubby, branching lichens hanging from old trees in humid Lake District woodlands are beard lichens, species of Usnea, which are particularly sensitive to air quality and their abundance in the Lake District woodlands is a direct measure of how clean the air is in those valleys.

What About Green Algae on Tree Bark?

A related but distinct phenomenon is green algal growth on tree bark, which produces a smoother, slimier green coating rather than the rough or leafy appearance of lichens. Green algae on bark is also harmless to the tree, again simply using the bark surface as a substrate without interfering with the tree's biology. It tends to be more abundant on the shaded, north-facing sides of trunks and in conditions of high humidity.

The combination of high humidity and mild temperatures in the Lake District and South Lakeland means that algal growth on trees, walls and hard surfaces is simply a feature of the environment here, more evident than in drier and more exposed parts of Britain.

When to Be Concerned

If the bark beneath the lichen or algae appears damaged, the bark is peeling or falling away, or the tree is showing signs of overall poor health, there may be a separate issue that warrants investigation. But the lichen or algae itself is not the cause of any such problem. If you are concerned about the general health of a tree and are not sure what you are looking at, we are always happy to come and take a look.

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