Giant Polypore Identification Guide

First Impression

You do not find the Giant Polypore so much as walk into it. The typical encounter is at the foot of an old beech in late summer or early autumn, where a mass of overlapping brown fans has erupted from the ground against the buttress roots. A single clump commonly spans 30-60 cm and can exceed a metre, with dozens of caps stacked into a rosette, and a large one weighs several kilograms. From a distance it reads as a heap of wet, tan-brown leather.

Two things establish the identity almost immediately. The first is the arrangement: many broad, fan-shaped caps arising from a single short, branched, fleshy base, all of them fused low down. The second is the reaction to touch - draw a finger across the white underside and the line you make darkens to grey and then to near-black within minutes.

The Shape of the Whole Clump

Look at the structure before the details. There is one central stem-like base, usually buried in soil or litter and often difficult to reach; from it, thick branches divide upwards and each terminates in a broad cap. The caps are spathulate to fan-shaped or kidney-shaped, 10-30 cm wide individually, and they overlap in tiers like roof tiles. The clump is broadest at the top and narrows abruptly to that single attachment point.

Sometimes the fungus appears several metres from the trunk, or in apparently open grass, because it is fruiting from a root running out under the turf. If you probe with a knife you will usually find woody root tissue directly beneath.

Reading the Cap Surface

Each cap is flat to slightly funnel-shaped, thin at the wavy, lobed margin and thicker towards the base. The surface is ochre-brown, tan, tobacco-brown or a warm mid-brown, often darker towards the point of attachment and paler at the edge, and it is marked in two ways: with fine radial fibrils or streaks running outward, and with faint concentric zones. Dried or sun-exposed clumps darken and crack; wet ones look glossy.

Feel the surface. It is finely tomentose to nearly smooth, never coarsely scaly like Polyporus squamosus, and never bright orange like Laetiporus. The margin of a fresh cap is pale cream to whitish and slightly inrolled; on an ageing clump that pale rim blackens first.

The Pore Surface - the Decisive Test

Turn a cap over, or bend down and look beneath the tiers. Instead of gills there is a smooth, matt, cream to white field of tiny pores, so small that they are difficult to resolve without a lens - roughly three to five per millimetre, angular in shape. The tube layer is shallow, only a few millimetres deep, and it runs decurrently down onto the stem-like branch below each cap rather than stopping at a clean edge.

Now do the bruising test, which is the single most valuable character in the whole identification. Press a thumb into the pores or scratch a line across them with a fingernail. Within a minute the mark grows grey; within five to ten minutes it is dark grey-brown to black, and the discolouration spreads slightly beyond your finger. An entire clump that has been brushed past by a deer or beaten by rain will be blotched black. Old clumps go uniformly blackish and eventually collapse into dark, sour-smelling masses.

Nothing else of this size and shape blackens so decisively and so quickly.

Flesh, Section and Odour

Cut a cap in half from margin to base. The flesh is white, 5-20 mm thick, and its texture changes with age: soft, watery and fibrous in a young clump, becoming tough, leathery and finally corky. In section you can see the flesh layer sitting above the much thinner tube layer, with no sharp line of demarcation.

The cut flesh also darkens, first to greyish and then blackish, following the same schedule as the pore surface. The odour of a fresh clump is pleasantly fungal and slightly sweet; an old one turns strongly sour.

Taking a Spore Print

Polypores print well if you handle them right. Cut a single cap free at its base, lay it pore-side down on a sheet of dark paper alongside white - the deposit is white, so it needs a contrasting background - and cover it with a bowl for eight to twelve hours. A vigorous clump can drop enough spores to whiten the ground and surrounding vegetation, which is itself a field clue in a wood where several clumps are fruiting.

Microscopically the spores are broadly ellipsoid to nearly spherical, smooth, hyaline and small, about 5-7 x 4-5 um.

Substrate, Host and Season

This is a fungus of tree bases and roots. It behaves as a weak parasite and then a saprotroph, colonising the root system and butt of living and recently dead broadleaf trees and decaying the wood there. Beech is by far its most frequent host across Europe; oak, lime, maple, hornbeam, poplar and occasionally other hardwoods are also used, and it is very common on mature amenity trees in parks and along avenues. It is only rarely reported from conifers.

Fruiting is annual, from late summer through autumn, with the clumps appearing in the same spot year after year for as long as the root system holds out. Fruit bodies are soft and short-lived - a clump goes from fresh to black and collapsing in two or three weeks.

Ecologically it is one of the more consequential root decayers of mature broadleaf trees, hollowing the buttress and major roots while the crown above may still look full; the presence of clumps is a familiar sign to anyone surveying old parkland trees.

Separating the Look-alikes

Hen of the Woods (Grifola frondosa). The closest structural analogue. It also forms a rosette from a branched base, usually at the foot of an oak, but the individual caps are much smaller - 4-10 cm - greyish-brown, spoon- or tongue-shaped, and each sits on a distinct pale stem within the cluster, giving a frilly rather than tiled appearance. Critically, Grifola does not blacken when handled.

Bondarzewia berkeleyi. A large North American rosette at the base of oaks, but pale cream to buff rather than brown, much thicker-fleshed, with distinctly larger pores (roughly one to three per millimetre) that you can see unaided, and it does not blacken.

Meripilus sumstinei. The North American counterpart, extremely similar in form and habit and also blackening; it tends to have paler, greyer caps. Geography does most of the separating.

Laetiporus sulphureus. Bright sulphur-yellow and orange, fruiting in brackets on trunks rather than as a ground-level rosette, with a yellow pore surface.

Polyporus squamosus. Single or few large kidney-shaped caps on a black-based lateral stem, high on trunks and branches, with a coarsely scaly cap and large angular pores 1-2 mm wide.

Sparassis crispa. A cauliflower-like mass at the base of pines, made of flattened ribbon-like lobes with a smooth fertile surface and no pores at all.

Field Checklist

A huge rosette of overlapping tan-brown fan-shaped caps from one branched base; caps radially streaked and faintly zoned; tiny cream pores, three to five per millimetre, running down onto the branches; rapid grey-then-black bruising of pores and flesh; white spore print; at the foot of a living or recently dead broadleaf tree, most often beech, in late summer and autumn.