Porcini Identification Guide

First Impression and Setting

You are usually on a woodland floor under beech, oak, spruce or pine, often near a track edge, a mossy bank or a grassy clearing, when a solid brown dome pushes through the litter. The first impression is of weight and proportion: a broad, cushion-shaped brown cap sitting on a stipe so thick and swollen that young specimens look like a cork in a bottle. Nothing about it is fragile. Fruit bodies appear singly, in scattered troops and sometimes in arcs, and they may be half-buried under leaves so that the pale bulging stipe base is the first thing visible. This is a mycorrhizal partner of trees, so it always emerges from soil, never from wood.

Reading the Cap

The cap is 8 to 25 cm across, occasionally much more, hemispherical when young with the margin tucked under, then convex and finally broadly cushion-shaped. Colour is variable and depends heavily on the partner tree and on light exposure: pale buff and almost whitish in shaded specimens, through tan, hazel and chestnut, to a deep dark brown. Look at the rim of young caps, where a narrow paler, often whitish band frequently marks the growing edge. The surface is smooth to slightly wrinkled or uneven, faintly greasy and shining when wet, matt when dry, and the cuticle does not peel away easily. Old caps may become slightly pitted or wavy at the margin, but they do not crack into a coarse mosaic.

Reading the Pore Surface

Instead of gills there is a spongy layer of vertical tubes. Press a fingertip into it and note four things.

• Attachment: the tube layer is free to slightly depressed around the stipe apex, and it separates cleanly from the cap flesh when you pull it, a general bolete feature worth confirming.
• Pore size and shape: the mouths are minute and round, so a young pore surface looks smooth and almost solid until you use a lens.
• Colour sequence: white and firm in young buttons, becoming pale cream, then yellow, and finally a dull olive-yellow to olive-brown as the spores mature. Old specimens sag and soften.
• Reaction to pressure: none. A firmly pressed pore surface stays the colour it was, without blueing, greening or reddening. That absence of a reaction is one of the most useful characters in the whole genus.

The Stipe and Base

The stipe is 8 to 20 cm long and 3 to 10 cm thick, markedly bulbous and barrel-shaped in youth, lengthening and becoming club-shaped or nearly cylindrical as the fruit body ages. It is whitish, pale buff or faintly brownish, never red or yellow.

The surface ornament is the key: a fine, raised, white to pale network of ridges, the reticulum, which forms a delicate mesh over the upper part of the stipe and fades out downwards, sometimes covering little more than the top third. Hold the stipe against the light and use a lens; the mesh is fine and pale, not coarse and dark. There is no ring and no volva. Dig the base rather than snapping the stipe, and check that it is rooted in soil and litter, often with white mycelial cords running out through the humus, and that no scaly girdle or coloured zone is present at the very bottom.

Flesh and Colour Change

Cut the whole fruit body vertically. The flesh is white, firm and dense in young material, softening with age, and it stays white on exposure to air: no blue, no pink, no brown flush beyond a slight watery discolouration in very old specimens. Directly beneath the cap cuticle there is often a thin band tinted brownish or wine-coloured, following the surface colour. The stipe flesh is white throughout. Check the base as well as the cap, since some relatives change colour only in the stipe base. Insect tunnels through the stipe are common and their walls may darken, which should not be mistaken for a colour reaction.

Taking a Spore Print

Cut a slice of the tube layer or set a whole mature cap tubes-down on white paper, cover it, and leave it overnight; boletes print more slowly than gilled fungi. The deposit is olive-brown to snuff-brown. Under a microscope the spores are long and narrow, spindle-shaped, smooth and thin-walled, roughly four times as long as broad, which is typical of the group.

Substrate, Partners and Season

This is an ectomycorrhizal fungus, sheathing the fine roots of its partner trees and exchanging soil nutrients for sugars. Beech and oak in broadleaved woodland and spruce and pine in conifer forest are the main partners, with birch also used. It favours well-drained, often acidic soils, and fruits particularly along paths, in mossy clearings and at woodland margins where the soil warms. The season runs from late summer into autumn, typically triggered by heavy rain followed by a drop in night temperature, and a good site can flush repeatedly over several weeks.

Separating It From Look-alikes on Morphology

• Boletus reticulatus fruits earlier in the year, has a duller, drier, matt cap that usually cracks into fine areolae, and carries a strong reticulum over almost the entire length of the stipe rather than just the top.
• Boletus pinophilus has a dark red-brown to copper cap, often bumpy or wrinkled, and a stipe with reddish-brown tones under its reticulum, generally with pines and spruce.
• Boletus aereus has a very dark bronze to almost blackish cap, often with paler blotches, and a browner stipe with a fine brown reticulum, usually with oak in warm situations.
• Tylopilus felleus has a pore surface that turns pink or flesh-coloured with age, a coarse, strongly raised, dark brown reticulum standing out over a paler stipe, and a pinkish-brown spore print.
• Imleria badia has a bay to chestnut cap, yellowish pores that stain blue-green within seconds of being pressed, and a smooth stipe without a net.
• Leccinum species have stipes covered in small dark scabers arranged in vertical rows, not a raised net, and their flesh usually discolours grey, pink or black.
• Neoboletus and Suillellus species show orange to red pore mouths and stain blue instantly.
• Caloboletus and Rubroboletus species show red tones on the stipe or cap and blue-staining flesh.
• Xerocomellus species are smaller, with cracked caps showing paler flesh in the fissures and coarse angular pores.

Field Checklist

Growing from soil under beech, oak, spruce or pine; cap brown, smooth, slightly greasy when wet, often with a pale rim when young; pores minute and round, white then yellow then olive, unchanging when pressed; stipe stout and bulbous, pale, with a fine white net on the upper part; flesh white and unchanging; spore print olive-brown; late summer to autumn.