Few lichens are as instantly recognisable as Xanthoria parietina, often known as the Common Orange Lichen, Yellow Wall Lichen or simply Sunburst Lichen. Its brilliant colour, remarkable resilience and ability to thrive on everything from twigs to metal signs make it an ideal introduction to the ten most common lichens found on our local trees.
Click the image to access the full species description and glossary.
Form, Colour and Growth
Xanthoria parietina is a leafy (foliose) lichen, meaning its body forms broad, leaf‑like lobes that sit loosely on the surface rather than lying flat and crust‑like (crustose). This structure makes it easy to spot and easy to lift gently at the edges—one of the features that distinguishes foliose lichens from crustose species that are firmly glued to their substrate. Unlike fruticose lichens, which are the same colour all around, foliose lichens have two distinct surfaces: an upper surface rich in pigment and an underside that differs in colour and texture.
X.parietina is best known for its orange‑yellow colour, produced by parietin, a pigment concentrated in the outer cortex. Parietin acts as a natural sunscreen, shielding the lichen’s algal partner from intense sunlight and ultraviolet radiation. The thallus shows a stronger yellow colour in well‑lit spots, shifting toward greener tones in shade. (Fig.1)
Fig. 1 Three thalli of Xanthoria parietina showing how light shapes their colour: a vivid golden yellow in full sun, a softer orange in partial shade, and a greenish tone where light is limited. (Image credit: Jagoda Zajac, MKNHS)
The thallus is typically lobed and can reach up to 15 cm across in older specimens. As it ages, the centre may die back, forming rings or arcs that help identify mature individuals. The underside is white, with short rhizines that anchor the lichen to bark, stone, or metal. (Fig.2)
Reproduction is mainly through apothecia, the orange discs scattered across the surface. These are concave when young and become flatter with age. The species lacks soredia and isidia, so it relies on spores and fragmentation. Growth is generally slow—around 2.6 mm per year—though moist, cool habitats can support faster expansion, while dry or exposed sites slow it down. Growth typically peaks in autumn and winter when conditions are wetter.
Fig. 2 White underside of Xanthoria parietina showing its foliose, leaf‑like form, with the circular inset revealing the tiny rhizines that anchor the lichen to its substrate. (Image credit: Jagoda Zajac, MKNHS)
Chemical and UV Tests
Chemical spot tests and UV examination are standard tools in lichen identification and in the video below I demonstrate how Xanthoria parietina responds to them.
The C test uses fresh bleach (such as Milton Sterilising Fluid) to check for lichen substances that react with sodium hypochlorite; if present, the spot shows a quick colour change, but many species—including Xanthoria parietina—remain C–. The K test uses a solution of potassium hydroxide or caustic soda to detect compounds that react with alkali, often producing red or purple colours; in Xanthoria parietina, the reaction is K+, it begins with a red‑orange hue and develops into a vivid crimson‑purple.[2]
Fig. 3 Chemical spot tests on Xanthoria parietina at Caldecotte, MK; no reaction with the C test, but an immediate K+ reaction (Image credit: Jagoda Zajac, MKNHS)
Under a 365 nm UV torch, the thallus shows a bright yellow‑orange glow. This is mainly due to strong reflectance from the parietin pigment, with only weak true fluorescence. Because the glow is obvious, the species is usually recorded as UV+ (orange), even though the effect is not classic fluorescence.
Fig. 4 Under 365 nm UV, the thallus of X.parietina shows a vivid orange glow from its parietin pigment, though this is mostly reflectance rather than strong fluorescence; Linford Wood, MK (Image credit: Jagoda Zajac, MKNHS)
Habitat, Nitrogen Tolerance and Ecology
Xanthoria parietina is strongly associated with nitrogen‑rich environments. Its tolerance of elevated nitrogen allows it to thrive in areas influenced by traffic, agriculture, or general urban activity. It grows on an impressive range of surfaces—including wood, stone, metal and has even been recorded on rabbit bones, a striking reminder of its ability to colonise unusual substrates.[1]
Fig. 5 Three different habitats for Xanthoria parietina: thriving on a twig, spreading across stone, and even colonising a painted metal sign. (Image credit: Jagoda Zajac, MKNHS)
Ecologically, Xanthoria parietina is a miniature ecosystem in its own right. It is one of the most frequently parasitised lichens in Britain, hosting an impressive diversity of lichen‑dwelling fungi.[3] . An example of these fungi is shown in Fig. 1, in the central photograph, where they appear as black spots on the apothecia.
Its thallus also shelters tiny animals such as rotifers and provides food for gastropods.
Extreme Survival: Xanthoria in Space
Xanthoria parietina has attracted scientific attention for its extraordinary resilience. In experiments simulating outer space, it survived exposure to vacuum, cosmic radiation, and dramatic temperature swings. Its ability to endure such extremes has made it a model organism in astrobiology, raising fascinating questions about the limits of life and the potential for survival beyond Earth.[5]
Natural Dyeing
Lichens have a long association with natural dyeing and Xanthoria parietina is no exception. Its vivid parietin pigment produces warm yellow to orange tones when extracted, especially with an alum mordant to help fix the colour to fibres. While some experimental dyers report pink or lavender shades under strongly alkaline or ammonia‑fermented conditions.
Spotting Xanthoria parietina Around Milton Keynes
For anyone exploring the trees, parks, and footpaths of Milton Keynes, Xanthoria parietina is a bright and reliable companion. Widespread across the MK and beyond, it’s one of the easiest lichens to recognise—its vivid colour, abundance, and ecological significance make it an ideal starting point for beginners. And while you certainly don’t need chemical tests or UV lamps to appreciate it, those little experiments simply reveal how much quiet magic even our most familiar lichens can hold.
For more information visit Xanthoria parietina | The British Lichen Society.
Jagoda Zajac, January 2026
References
- British Lichen Society. British Lichen Society Bulletin. 2019;125:41. BLS Bulletin 2019 Winter.pdf
- British Lichen Society. Chemical tests [Internet]. Available from: https://britishlichensociety.org.uk/learning/chemical-tests
- British Lichen Society. Lichenicolous fungi occurring on Xanthoria parietina in the United Kingdom 2024. Available from: Guide – LFs on Xanthoria parietina
- Dobson F, Lichens. An Illustrated Guide to the British and Irish Species. The British Lichen Society. 2018
- Lorenz, C., Bianchi, E., Benesperi, R., Loppi, S., Papini, A., Poggiali, G., & Brucato, J. R. (2022). Survival of Xanthoria parietina in simulated space conditions: vitality assessment and spectroscopic analysis. International Journal of Astrobiology, 21:3 137–153 https://doi.org/10.1017/S1473550422000076
- Whelan P. Lichens of Ireland and Great Britain. A Visual Guide to Their Identification
Volume II. Holm Oak Press; 2024. p.621 Yahr R, Stoakley J, Lichens of Great Britain and Ireland. An introductory Guide. Bloomsbury; 2025. p.139