Description: This is a distinctive and rather attractive, yellow (never orange), crustose lichen species with a rosette-type thallus and lobe-like radiating margins. It forms patches; on average about 2-6cm in extent but has been known to form colonies that cover the entire tops of concrete posts. The lobed edges are flattened and have a white sprinkling of felty pruina. The centre of the thallus is slightly darkened, with granular flakes (isidia) breaking away from the surface. It grows best on man-made substrata such as concrete posts and fencing, wall-top cement, mortar and the tops of gravestones. In Ireland it appears to be commoner in the east [The thallus is K-; orange Caloplaca spp. are always K+ crimson].

Key Identification Features:

• Yellow, rosette-like crusts on man-made substrata. There is no reaction with Potassium hydroxide (K)
• Lobe ends white-crystalline, the centres flecked with granular isidia.

Original text submitted by Vince J. Giavarini

Distribution Map from NBN: Candelariella medians f. medians at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Candelariella medians f. medians at iNaturalist World Species Observations database.