Red Bark Phenomenon

Red Bark Phenomenon is a fairly new occurrence here in North America. Scientists, horticulturalists and the like have been scrambling to learn more about this phenomenon as more and more trees are becoming affected. What they have learned so far is that the reddening of the bark is caused by a microscopic, filamentous green algae (Chlorophyta), that is believed to belong to the genus Trentepohlia. The cytoplasm of these algae contain large quantities of carotenoids and haematochrome (what causes the red colouring in blood) and this masks the green colouring of the chlorophyll. This algae adheres to the bark forming a branching mat of algae that creates a red/orange crust or film over the bark surface. This red film does not appear to penetrate the bark or cambium so it’s occurrence is primarily cosmetic in nature. Researchers have also observed that susceptible trees growing near water were most likely to be affected.

Susceptible Trees

White pine, eastern hemlock, red oak, and American beech appear to be the most susceptible. Red Bark Phenomenon has also been observed on:

  • Apple
  • American Elm
  • Black Cherry
  • Black Oak
  • Chestnut
  • European Larch
  • Green Ash
  • Globe Blue Spruce Standard (a personal observation)
  • London Planetrees
  • Norway
  • Pignut
  • Red Maples
  • Sugar Maples
  • Norway Spruce
  • Red Pine
  • Scarlet Oak
  • Shagbark Hickories
  • Sweet Birches
  • Sycamores
  • White Birch
  • White Oaks
  • Yellow Birch
Red Bark Phenomenon on a Globe Blue Spruce Standard (Picea pungens glauca ‘Globosa’ Standard). This tree is located in Whitby, Ont. It is positioned beside a large pond with fountains. The redness appears on the south to south/east side of the tree…the side facing away from the water. April 2021

Photo credits: photo taken by the author.

References and reading resources:

Boggs J., (2019). Red Bark on Sycamore and London Planetrees. Red Bark on Sycamore and London Planetrees | BYGL (

Cheah C. and Li D.W. (n.d.). The Red Bark Phenomenon. CAES. The Red Bark Phenomenon (

Cheah, Carole. (2016). The Red Bark Phenomenon. (PDF) The Red Bark Phenomenon (

Holland M., (n.d.). Red Bark Phenomenon. Red Bark Phenomenon | Naturally Curious with Mary Holland (

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