Hollow-stemmed Larch Bolete

Long placed in its own genus as Boletinus cavipes, this species is now treated within Suillus, where it sits with the other dry-capped, veiled species such as the Painted Suillus. It is unmistakable once sectioned: the stalk is hollow, with an irregular chambered cavity running through the base, a feature found in no other bolete of its range.

Above ground it is a handsome fungus, with a densely woolly, tawny to rusty-brown cap and a strongly boletinoid pore surface in which the tubes are arranged in radiating rows like the spokes of a wheel. A pale form with yellow rather than brown cap fibrils occurs regularly.

Its fortunes are tied entirely to larch. Wherever larches grow, whether native in northern forests and mountains or planted for forestry, this bolete may appear beneath them, and it is absent from woodland lacking larch.

Cap

3-10 cm across, convex then broadly flattened and often shallowly umbonate. The surface is dry and covered in soft matted fibrils and small woolly scales, coloured tawny-brown, rusty-brown, reddish-brown or, in the pale form, dull yellow. The margin is fringed with whitish veil remnants when young.

Pores

Large, angular and conspicuously radially arranged, becoming decurrent down the stalk. The colour is pale yellow to lemon, dulling to olive-yellow with age, and there is little or no bruising reaction.

Stipe

4-8 cm tall and 0.6-1.5 cm thick, yellow above the ring zone and brown-fibrillose below, with a soft, cottony ring or ring zone left by the partial veil. Sliced from top to bottom, the base is hollow with irregular chambers, the decisive field character.

Flesh & spore print

The flesh is soft, pale yellow to whitish, and does not change colour when cut. The spore print is olive-brown.

Exclusively associated with larch (Larix species), including European, Japanese, Siberian, western and tamarack larch. It grows on soil and litter under larch in mixed and pure stands, along plantation rides, in damp mossy woodland and in mountain forest.

Fruiting runs from late summer through autumn. The species is circumboreal, occurring across northern and montane Europe, northern Asia and northern North America, and it follows planted larch into regions where the tree is not native.

Suillus cavipes forms ectomycorrhizas exclusively with larch, one of several Suillus species that specialise on this deciduous conifer. The mantle it forms over larch root tips greatly extends the tree's access to soil water and mineral nutrients, which is important given larch's habit of colonising thin, cold and often waterlogged soils.

The hollow stalk is an unusual structural adaptation among boletes, producing a tall, well-anchored fruiting body from relatively little tissue. Fruiting bodies decay rapidly once mature and are heavily used by fungus gnats, whose larvae mine the soft tube layer.

Suillus grevillei, the Larch Bolete, shares the same host but has a smooth, brightly glutinous golden to chestnut cap and a solid stalk. Suillus spraguei has a comparably shaggy cap but with brick-red scales, a solid stalk and an association with five-needle white pine.

Suillus lakei is another dry, scaly, veiled species with radial pores, distinguished by its Douglas-fir host and solid stalk. Suillus ochraceoroseus has a rosy to ochre felty cap and also grows with larch, but its stalk is solid and its pores are more yellow than radiating. Sectioning the stalk separates the Hollow-stemmed Larch Bolete from all of them.