Cap: (3) 5-12 (25) cm diameter, convex when young then expanded and flattened with slightly depressed centre, more deeply depressed in older specimens, without an umbo (raised area in the middle), margin strongly inrolled when young, later weakly or not inrolled; surface has a faint whitish dusty coating on very young specimens, felted and matt when young, slightly sticky in wet weather, smooth and shiny when dry and with age, often cracking when dry; surface colour grey-brown with an olivaceous shade, soon becoming ochraceous brown, clay-buff or yellowish olivaceous, finally more uniformly coppery brown or reddish brown, rarely with a vinaceous cast. Gills: unequal, narrow, fairly crowded to crowded, decurrent, often forked, branched and joining towards the stem; colour at first pale, yellowish white then rusty brown darkening to rusty reddish or reddish coppery with age, staining red-brown when bruised. Stem: 2.7-5 x 0.5-2.5 cm, stout but sometimes more slender to thin, dry; whitish background colour, more or less pale pink reddish marbled, often with a distinct light yellow zone at the top, staining reddish brown from the base upward later with brown striations. Flesh: yellowish in the stem and in the cap reddish brown to dark-red after a few hours. Chemical reaction: reddish brown to purplish brown with 50% ammonia solution on cap surface. Spore deposit: ochraceous with clearly reddish shade or chocolate brown turning to ochraceous-olivaceous brown over time. Spores: (7.0)7.2-9.6(11.5) x (4.5) 4.8-5.9(6.2) μm with median 8.0-8.6 x 5.2-5.5 μm. Smooth, ellipsoid-ovoid to amygdaliform with a constant to frequent apical constriction.
Description adapted from:
Jargeat, P., et al. (2014). The Paxillus involutus (Boletales, Paxillaceae) complex in Europe: genetic diversity and morphological description of the new species Paxillus cuprinus, typification of P. involutus ss, and synthesis of species boundaries. Fungal biology, 118(1), 12-31.
Henrici, A. (2014). Paxillus – An End to Confusion?. Field Mycology, 15(4), 121-127.
Two other British Paxillus species are often found in open, sunny locations. Paxillus ammoniavirescens can be distinguished by a greenish colour change when a drop of ammonia solution is applied to the cap surface and P. obscurisporus by a darker vinaceous brown spore print. P. involutus is found in more shaded woodland habitats and has spores where only a minority have an apical constriction. P. rubicundulus and closely related species grow in damp leaf litter strictly with their host and have smaller spores (5.5-7.9x3.5-4.5 μm).
Found in groups or sometimes two or three fruitbodies emerging from the same spot, always in bright places such as lawns, parks, river banks, forest edges, and along paths. Growing in association with trees in the family Betulaceae including Betula (birch) Alnus (alder), Corylus (hazel) and Ostrya (hop-hornbeam).