Giant Cypress Agaricus Identification Guide

The find

Coastal California or southern Oregon, late autumn into winter, under a dark stand of Monterey cypress or in a cypress and pine windbreak near the sea. The ground is a thick springy mat of shed cypress scale-leaves. Something is heaving that duff upwards in a slow dome, and when you brush the litter aside you uncover a mushroom of genuinely startling bulk: a cap that can be broader than a dinner plate, over a stem as thick as a wrist.

Size alone gets your attention, but the colour is what identifies it. The cap ground is whitish to pale buff, and over that ground lie fine appressed fibrils and small flattened scales in a striking purple-brown, vinaceous or lilac-brown shade, densest over the centre and breaking up into scattered fibrils towards the margin. Against pale cypress duff that dusky purple-brown reads from a long way off.

Fruitbodies usually come up in troops, arcs or loose rings around the drip line of the tree, often partly buried, and the biggest ones are frequently found by noticing the cracked, domed litter above them rather than the mushroom itself.

Reading the cap

Caps run 10 to 25 cm across and specimens over 30 cm are recorded. Young buttons are hemispherical to almost square-shouldered, with the margin tucked in tightly against the stem; expansion takes them through broadly convex to nearly flat, often with a slightly flattened disc.

The surface is dry, never viscid. Under a lens the fibrils resolve into fine radiating strands and small appressed, sometimes slightly upturned scales; they do not form coarse shaggy tufts. The margin frequently carries hanging fragments of the partial veil, appearing as pale cottony teeth around the rim after the veil tears, and it may become split or splayed in weathered specimens.

The gill sequence

The gills of this genus run through a distinctive colour sequence and reading it correctly is central to identification. Cut or twist off a young cap and check the underside as it develops.

Gills are free, not touching the stem, leaving a clear gutter between gill ends and stem apex. They are crowded, narrow, and of several lengths. In the youngest buttons they are pale, whitish-grey to pale pinkish-grey. As the fruitbody expands they turn a clear pink, then a dull vinaceous brown, and finally a deep chocolate to blackish-brown at maturity. An old specimen with dark brown gills, a pale fibrillose cap and a large ring is unmistakably in this genus, whatever the species.

Check several fruitbodies of different ages in a troop so you can see the whole sequence at once.

Veil and ring

The partial veil is thick, membranous and substantial in keeping with the size of the mushroom. It stretches from the cap margin to the stem and tears as the cap opens, leaving a large, persistent, pendant ring hanging skirt-like from the upper stem. The upper surface of the ring is smooth and whitish; the underside is often faintly cottony or shows patchy thickened fragments. In older specimens the ring collects falling spores and turns brown on its upper face.

There is no volva, no sac and no free cup at the base of the stem, which is worth verifying explicitly by excavating the whole base.

Stipe, base and flesh

The stem is stout and firm, 8 to 20 cm long and 2 to 5 cm thick, often thickest at the base or bluntly clubbed there, and sometimes tapering downwards where it has grown up through compacted duff. Above the ring the surface is smooth and whitish; below the ring it is smooth to finely fibrillose, occasionally with faint girdles of veil tissue. The interior is solid and white, becoming softer or slightly hollowed in very old specimens.

Slice a fruitbody in half lengthways and watch the cut surfaces for several minutes. The flesh is thick and white and it discolours slowly, drifting towards pinkish, then dull reddish-brown, most noticeably in the stem and around the stem base. This slow pinkish-red drift, without any flash of bright chrome yellow, is a key part of the reading. Handled cap surfaces may dull to a faint ochre or tan tone, but the flesh response is the one to trust.

Odour is pleasant and sweetish, with the almond-like benzaldehyde note characteristic of its section of the genus, best detected by warming a cut fragment in the palm. It is not sharp, inky or phenolic.

Taking a spore print

Remove a cap that has fully expanded but is not yet decayed, set it gills-down on white paper, cover it with a bowl to keep it humid, and leave it several hours. Large caps drop enormous quantities of spores, and with a specimen this size you may need a sheet larger than you expect.

The print is dark chocolate brown, sometimes almost purple-brown in a thick deposit. That colour immediately excludes every white-spored genus, which matters because a large white-capped mushroom with free gills and a ring invites confusion with Amanita until the print is on paper.

Substrate, season and ecology

This species is a litter decayer rather than a wood decayer, working through the deep accumulated duff beneath its host trees. It is closely tied to Monterey cypress and related cypresses, and is also found under Monterey pine and in mixed coastal conifer plantings, including windbreaks, cemeteries, campus plantings and coastal parks well outside the natural range of the cypress.

It is essentially a coastal Californian and Oregonian species, fruiting in the late autumn and winter rainy season, typically November through February, and appearing in flushes a week or two after significant rain. Fruitings recur in the same litter beds year after year, often in expanding arcs.

Separating the look-alikes on structure

Agaricus augustus is the nearest relative in appearance and shares the sweet benzaldehyde odour. It has warmer brown to tawny fibrils on a yellowish ground rather than purple-brown fibrils on white, it usually shows a distinctly shaggy or scaly stem below the ring, and its flesh and surfaces develop a definite yellow response to handling. It occurs in far more varied habitats.

Agaricus subrutilescens also carries purple-brown to vinaceous fibrils, but it is much more slender in proportion, has a conspicuously shaggy, cottony-scaly white stem below the ring, and white flesh that scarcely changes on cutting. It favours conifer duff in mixed forest.

Agaricus bitorquis is smooth and white-capped with no fibrillose pattern, has a distinctive double-edged ring, and pushes up through hard compacted urban ground.

Members of the Agaricus xanthodermus group show grey-brown fibrils and respond to a cut at the very base of the stem with an immediate bright chrome-yellow flush, plus a sharp inky or phenolic smell rather than a sweet one.

Amanita species can also be large, pale and free-gilled with a ring, but they keep white gills at all ages, drop a white spore print, and have a volva, sac or rimmed bulb at the stem base.

Field summary

Cypress or coastal conifer duff, huge stature, purple-brown fibrils on a pale ground, free gills passing pale grey to pink to chocolate brown, a thick pendant ring, a stout base with no volva, slow pinkish-red flesh discolouration, a sweet benzaldehyde smell and a dark brown print.