Thallus foliose, 3-6 (-20) cm diam., often forming complete or partial rosettes, ± loosely attached. Lobes to 3 mm wide, slightly broadened and truncate at the apices, discrete, or contiguous and overlapping. Upper surface grey-white to grey-green, sometimes tinged brownish towards the lobe ends or if stored for many years, with oval to elongate, ± raised, scattered, white pseudocyphellae which may fuse to form a ± complete, coarse, network. Isidia concolorous or with brown-tinged tips, cylindrical, simple, becoming coralloid, at first arising from pseudocyphellae, later from the rest of upper surface, rather sparse or becoming numerous and obscuring the thallus. Lower surface black, brownish towards the margin. Rhizines dense, simple, occasionally forked.
Anamorph: conidiomata pycnidia. Conidia 6-7 × ca 1 μm, cylindrical or bacilliform.
Teleomorph: ascomata apothecia, occasional, to 1 cm diam, sessile or shortly stalked. Disc red-brown to dark brown. Thalline exciple often isidiate. Ascospores 16-18 × 9-11 μm, ellipsoidal to ovoid, aseptate, thick-walled, smooth.
Chemistry: cortex K+ yellow; medulla C–, K+ orange, KC+ orange, Pd+ orange, UV– (atranorin, salazinic, ± lobaric and ± norstictic acids).
Assessed by Woods & Coppins (2012) as of Least Concern. One of the commonest and most widespread foliose lichens.
Very variable. The morphologically and chemically similar Parmelia sulcata has linear soralia and P. omphalodes is usually darker brown and lacks both soredia and isidia. Possibly several taxa are involved; British material is in need of an analytical study.
Specimens from mostly exposed places with a strongly pruinose upper surface, with pruina covering both the isidia (usually non-pseudocyphellate) as well as the lobes are separated as P. ernstiae. However, more research is needed; there are individual populations that look like P. ernstiae on the exposed side of trees and like normal P. saxatilis on the sheltered side. P. serrana is more or less indistinguishable from P. saxatilis using morphological methods, but is separable using molecular data. It has been reported from GB&I, and more work is needed to understand its distribution.
Widespread and common throughout the British Isles, except in parts of East Anglia & Ireland. BLS map here.
On acid-barked trees and shrubs, Calluna stems, siliceous rocks, walls, memorials, roofing tiles, more rarely on the ground in shady to exposed situations, from the coast to mountain summits; often abundant. Relatively insensitive to SO2 pollution (<70 μg/m3), not found on nutrient-enriched substrata and notably decreasing in nutrient-enriched sites.