Grooved Bonnet Identification Guide

First Impression and Setting

Grooved Bonnet is a mushroom you notice for its stem rather than its cap. The usual setting is damp, shaded broadleaf woodland in autumn, along a path edge, beside a mossy stump or in the leaf drift around the base of an old oak or hazel. Among the general scatter of small brown fungi, one stands taller and stiffer than the rest, held upright on a wiry, silvery-grey stalk that catches the light with a faint metallic sheen.

They usually grow singly, in pairs, or in small loose groups, not in the crowded tufts that some bonnets form on stumps. The overall look is delicate but not floppy: the cap is small, the stem is long, and the whole fruit body has a stiff, upright poise.

Reading the Cap

The cap is 1 to 3.5 cm across, conical to bell-shaped, sometimes with a small central umbo, and it never quite opens out flat; the best it manages in old age is a broadly conical dome with an upturned margin.

Colour is grey-brown, mouse-grey or occasionally a dull greyish beige, darkest at the centre. The cap is hygrophanous, so a rain-soaked specimen looks distinctly darker and more translucent while a dried one fades to pale grey-buff. The surface is dry to slightly moist, faintly pruinose in very young buttons, and smooth thereafter, with no scales and no slime.

The margin is important. It is conspicuously striate to shallowly grooved, with the lines running inward from the edge and tracing the positions of the gills right through the paper-thin flesh. With age the margin often becomes faintly scalloped or crenate and may split. Hold the cap up against the sky and the striations become obvious.

The Gills

The gills are adnate with a small decurrent tooth: they meet the stem squarely and then run a short way down it as a narrow tongue. Check this in profile after slicing the fruit body vertically, since it is difficult to judge from beneath.

Spacing is rather distant to moderately spaced, and the gills are moderately broad, whitish at first, then greyish, and finally often flushed with a pale pink-grey tone in mature material. The edges stay pale, sometimes noticeably whiter than the faces, and they are even rather than serrated. Importantly, there are no strong cross-veins running between the gills across the underside of the cap; that heavy interveining belongs to a close relative.

The Stem, the Base and the Key Character

Everything hinges here. The stipe is 5 to 12 cm long, sometimes longer, and only 2 to 4 mm thick, cylindrical, hollow, and distinctly rigid and cartilaginous, so it flexes and springs rather than snapping softly. It is silvery grey, sometimes with a bluish or steely cast, and it has a polished look.

Run a lens along it. The surface is finely and evenly grooved along its whole length with many fine longitudinal striations, giving a fluted, wire-drawn effect that is the origin of both the English and the scientific name. On a fresh specimen in raking light these grooves catch highlights like the ribbing on a column. Some bonnets have smooth, glassy stems; this one is unmistakably fluted.

The base is the second half of the identification. It is clothed in coarse white fibrils and strigose hairs, and it tapers into a slightly rooting point. Dig around it gently with a finger. A fruit body that appears to be growing from soil is nearly always attached to a buried twig, a fragment of rotting branch or a dead root just below the litter. Establishing that hidden wood connection turns a probable identification into a confident one.

Flesh and Colour Change

The flesh is extremely thin, greyish and watery in the cap, and tougher and more fibrous in the stem. Cut it and nothing changes: no reddening, no blackening, no blueing. Squeeze the broken stem between finger and thumb and no liquid appears, which distinguishes it from the bonnets that exude a white or coloured juice.

Odour is faint and hard to pin down, occasionally slightly earthy or of raw potato, and is not the strong radish, iodine or nitrous smell of several other Mycena species. That absence is itself useful.

Taking a Spore Print

Small thin caps dry out quickly, so take some care. Cut the stem short, place the cap gills down on black card or a glass slide over dark paper, cover with a small container or an upturned glass to hold humidity, and leave it six to twelve hours in a cool spot. The deposit is white and often thin, so check it by tilting the card against the light rather than expecting a bold patch.

Under the microscope the spores are smooth, broadly ellipsoid, roughly 8 to 10 by 6 to 7.5 micrometres, and amyloid: a drop of Melzer reagent turns them blue-grey to almost black, which is the standard confirmation of the genus. The gill edges carry club-shaped to spindle-shaped cystidia, and the cap cuticle hyphae are covered with short warty outgrowths.

Substrate and Season

Grooved Bonnet is a saprotroph on the fallen and buried deadwood of broadleaves. Oak, beech, hazel, ash and willow are the common hosts, and it will occasionally appear on conifer debris. Because it fruits from buried material, it is often recorded as growing on soil, which sends beginners in the wrong direction.

Search damp, shaded, closed-canopy woodland with a good depth of litter, especially along the edges of paths and ditches where broken branch wood accumulates. The season runs from late summer through autumn and often persists into early winter in mild spells, making it one of the bonnets you can still find after the main flush is over.

Separating the Look-Alikes on Morphology

Mycena galericulata. Larger and tougher, with a 2 to 6 cm radially wrinkled grey-buff cap, gills strongly interveined and flushed pink with age, and a smooth, polished, ungrooved stem. It grows in clusters directly on stumps and logs.

Mycena inclinata. Grows in dense tufts on oak stumps, has a cap margin decorated with distinct brownish scalloped teeth, and a stem that is white at the apex and rich rusty orange-brown towards the base.

Mycena vitilis. Similar habit on buried twigs, but slimmer and smaller-capped, with a smooth, shiny, elastic, ungrooved stem and a long rooting base. The absence of fluting is the separator.

Mycena filopes and Mycena metata. Much more slender with thread-like stems and pinkish or warmer brown tints; several carry a distinct iodine-like smell as they dry.

Mycena pura group. Stouter, with lilac to violaceous tints, a strong radish smell, and an ungrooved stem.

Mycena galopus. Similar grey colouring, but breaking the stem releases a white juice within seconds. Grooved Bonnet never exudes anything.

Field Checklist

Broadleaf woodland, autumn to early winter. Cap 1 to 3.5 cm, conical to bell-shaped, grey-brown, hygrophanous, strongly striate at the margin. Gills adnate with a decurrent tooth, rather distant, whitish greying to pink-grey, not interveined. Stem long, rigid, silvery, finely fluted along its whole length, white-strigose and rooting at the base into buried wood. No juice, no colour change. White amyloid spore deposit.