Hollow-stemmed Larch Bolete Identification Guide

The Setting: Find the Larch First

There is a shortcut with this species, and it is honest to use it: Suillus cavipes forms mycorrhizas exclusively with larch. If there is no larch overhead, you are not looking at it. So the search begins with the tree, not the mushroom - a stand of European larch, tamarack in a boggy edge, or western larch on a mountain slope, in late summer through autumn.

Under those trees, look at the mossy, damp ground: the margins of tamarack swamps, roadside larch plantings, sphagnum hollows and needle-strewn banks. The fruit bodies push up through moss and needle litter, sometimes with a cushion of moss riding on the cap.

The first impression is of a brown, dry, distinctly fuzzy or shaggy cap - which is already unusual, because most of the boletes you find under larch are shiny and sticky. A dry-capped bolete under larch is worth stopping for.

Reading the Cap

The cap is 3-10 cm across, occasionally to 12 cm. It begins convex, often with a slight point or low broad umbo, and flattens with age, sometimes becoming a little depressed around the centre.

The surface is the character. It is dry and covered in soft, matted, woolly fibrils and small recurved scales, densest over the centre and breaking into a coarser, more separated pattern towards the rim. Colour runs from cinnamon and tawny through rusty-orange-brown to a dark reddish-brown, usually darker at the disc; a distinct yellow-capped form occurs in some populations. In wet weather the cap may become slightly tacky beneath the fibrils, but it never has the varnished, wet-plastic look of a truly viscid bolete.

The margin is the other thing to check. On young specimens it carries a fringe of whitish, cottony veil remnants hanging down like a torn curtain, and these persist as ragged flaps on the rim of mature caps.

The Pore Surface

Turn it over. The tube layer is pale yellow when young, deepening to a stronger yellow and then to a dull greenish-yellow or olive-yellow in age.

Two features stand out and both are important.

The pores are large and angular. They are not the fine, round, pin-prick pores of a typical Boletus; they are wide-mouthed and irregularly angular, often 1-2 mm across, and they are arranged radially, in lines running outward from the stalk, with cross-partitions dividing the radiating channels into compartments. This boletinoid pore arrangement gives the underside a look closer to a stretched honeycomb than to a sponge.

The tubes are strongly decurrent. They do not stop at the stalk; they run a good way down it, sometimes almost to the ring zone, so the junction between cap and stipe is a smooth continuation rather than a clean stop.

Press the pore surface firmly. In most collections nothing happens, or there is at most a very slight brownish dulling after some minutes. There is no strong blueing.

The Stipe - and the Character That Names the Species

The stalk is 3-8 cm long and 0.5-2 cm thick, roughly equal or tapering slightly downward, and it is coloured in two zones: yellow above the ring, where the decurrent tubes run down it, and brown to reddish-brown below, matching the cap, often with a fibrillose or slightly scurfy surface.

Between the two there is a cottony, whitish to pale buff ring zone, the remains of the partial veil that also left the fringe on the cap margin. It is soft and woolly rather than a firm membranous collar, and it can wear away, leaving only a band of contrasting colour.

Now do the one test that settles this identification. Slice the stipe lengthwise from top to bottom with a sharp knife. The interior is not solid. Towards the base - and often through much of the length - it is hollow, or divided into irregular chambers and cavities separated by thin partitions, so the cut face shows an open, cavernous, honeycombed core. In very young buttons the base may only be stuffed with a loose cottony pith that is already pulling away from the walls; in mature specimens the hollow is unmistakable.

No other bolete under larch does this. It is the reason for both the epithet cavipes and the common name.

Flesh and Colour Change

Cut the cap in half as well. The flesh is soft, whitish to pale yellow, more strongly yellow immediately above the tubes, and it is generally unchanging on exposure - at most a faint darkening to buff, or a slight yellowing, over several minutes. There is no blue flash, no red, no black. Note this carefully, because several dry-capped Suillus relatives do change.

Taking a Spore Print

Cut the cap free, remove most of the stalk, and set the cap pore-side down on white paper under a cup for four to eight hours. Boletes print quickly and heavily when fresh.

The deposit is olive-brown to dull olive-ochre - the Suillus signature. It is neither the white of an Amanita or a polypore nor the rusty cinnamon of a Cortinarius, and it is a useful confirmation that you are in the right family before you get to the stalk anatomy. Under a microscope the spores are smooth, colourless to pale yellowish, narrowly ellipsoid to subfusiform, and roughly 7-10 x 3-4 microns.

Substrate, Habitat and Season

The association is ectomycorrhizal and, as far as the field is concerned, absolute: Larix only. The mushroom draws sugars from living larch roots and supplies water and minerals in return, so it fruits on soil among living roots, never on wood and never in a stand with no larch component.

It favours damp, mossy, often acidic ground and is frequently found in sphagnum, on old railway and roadside plantings, in tamarack bogs and along the wetter edges of larch stands. Fruiting runs from late summer to mid or late autumn, often continuing after the first light frosts, and it commonly appears in scattered troops rather than singly. Its range takes in northern Europe, temperate Asia, and northern and montane North America, tracking the natural and planted distribution of larch.

Separating It From Its Look-alikes

Larch bolete (Suillus grevillei) is the species you will most often be comparing with, since it grows in exactly the same place at the same time. Every key character differs: its cap is smooth and distinctly viscid, glossy and sticky in the wet, bright golden-yellow to chestnut or bay; its pore surface is finer and more yellow; and its stipe is solid when sliced, with a firmer ring. Cap texture alone usually settles it, and the lengthwise cut always does.

Painted bolete (Suillus spraguei, formerly S. pictus) is superficially very close - a dry, red-scaly cap with a veil fringe on the margin, radially arranged decurrent yellow pores. Two things separate them: it partners with five-needle white pines, not larch, and its flesh reddens and then browns when cut. Its stipe is solid.

Western painted bolete (Suillus lakei) also has a dry, reddish, fibrillose-scaly cap and radial pores, but it is a Douglas-fir associate, its stipe is solid, and the flesh at the stipe base often stains bluish-green.

Suillus tomentosus has a dry, greyish to yellow-brown finely tomentose cap dotted with small dark fibril tufts, grows with pines, has fine round pores rather than radial ones, and its flesh usually blues on cutting.

Gomphidius and Chroogomphus species also occur under larch and pine and have decurrent structures, but they have true gills, not tubes, and dark grey to blackish spore prints.

Suillus paluster grows in sphagnum near larch with radially arranged reddish decurrent pores, but it is much smaller, pink to purplish-red on the cap, and solid-stemmed.

Field Sequence

1. Confirm larch overhead.
2. Dry, fibrillose-woolly, brown to tawny cap with veil fringe on the rim.
3. Large angular radially arranged yellow pores running down the stalk.
4. Cottony pale ring zone; stalk yellow above, brown below.
5. Split the stipe lengthwise - hollow or chambered at the base.
6. Whitish to pale yellow, essentially unchanging flesh.
7. Olive-brown spore print.