Nitrous Waxcap Identification Guide

Where You Are Standing When You Find It

The Nitrous Waxcap is a grassland species, and the grassland matters as much as the mushroom. Picture short, springy turf that has never been ploughed or fertilised: an old churchyard, a sheep-cropped hill pasture, a dune slack, a lawn that has been mown but never fed for fifty years. The sward is thin, full of moss, and studded in autumn with waxcaps in scarlet, gold and pink. Among those bright colours, Neohygrocybe nitrata is the drab one, a dull grey-brown cap that most people walk straight past.

The first useful reaction is therefore not visual at all. Waxcap grassland rewards kneeling down, and once you are down at turf level with a grey-brown waxcap in your hand, the smell reaches you before any other character has been checked. It is sharply chemical, nitrous, like laundry bleach, a swimming pool changing room or the gas escaping from a struck match. Nothing else in short turf smells like that except its own close relatives.

Reading the Cap

Caps are 2-7 cm across, convex at first, then flattening and often becoming shallowly depressed at the centre. Older caps go irregular and undulating, and the margin frequently splits radially, which gives them a slightly battered look.

Feel the surface. It is dry to faintly greasy, matt rather than shiny, and under a lens finely fibrillose or minutely felted, especially towards the centre. It is emphatically not glutinous. Colour runs from grey-brown through olive-brown to a dull mouse-grey, sometimes with a warmer ochre wash over the disc. The flesh is hygrophanous, so a cap drying in the sun pales noticeably from the margin inwards and can look like a different species from the one you photographed an hour earlier.

The Gills

Turn the cap over and check the gills with a fingertip. Waxcap gills are thick, soft and greasy-waxy: rubbed between finger and thumb they smear rather than crumble. On this species they are broadly adnate to slightly decurrent, distant rather than crowded, and often interconnected by low veins where they meet the stem apex. Colour is whitish to pale grey, ageing greyer, with edges usually a shade paler than the faces.

Distant, waxy, faintly decurrent gills plus a white spore deposit puts you firmly in the waxcap group before you have narrowed the species at all.

The Stipe, Base and Flesh

The stem is 3-9 cm long and 0.5-1.5 cm thick, cylindrical or slightly tapering downwards, sometimes compressed or grooved. Critically it is dry, with fine longitudinal fibrils, never slimy or sticky. It is pale grey-brown, whitish at the extreme apex and often whitish again at the very base where it grips the moss. Young stems are stuffed, old ones hollow and easily split.

Now cut the mushroom lengthways with a clean blade and watch for two minutes. The flesh is thin, whitish to pale grey, and fragile. It may dull slowly to a greyish or faintly brownish tone, but it does not flush pink or red. That absence of reddening is the single most important character separating this species from its nearest relatives, so give it real time rather than a glance.

Crush a fragment of the cut flesh in a warm palm and smell again. The nitrous odour is strongest from freshly damaged tissue and persists in a collecting box.

Taking a Spore Print

Waxcap caps are thin and dry out quickly, so a print needs a little care. Cut the stem off flush with the gills, set the cap gill-side down on a card that is half white and half black, and cover it with an upturned cup or a lidded box with a scrap of damp tissue inside to hold humidity. Two to six hours is normally enough, and leaving small caps overnight gives a more reliable deposit.

Expect a pure white deposit. Judge it by scraping the powder into a small heap with a blade rather than reading the thin film, and look at it in daylight against white paper. A white print rules out the pink-spored Entoloma species that share the same turf and can be a similar dull grey-brown.

Substrate, Season and Ecology

This is a species of nutrient-poor, undisturbed grassland with a continuous moss layer and a short sward. It also strays into mossy woodland rides and grazed lawns, but it does not appear in improved pasture, arable margins or recently seeded amenity grass. Fruiting runs from late summer into early winter, peaking from September to November after the first cool, wet spells, with fruit bodies scattered or in loose troops rather than tight clusters.

The ecology is worth knowing because it drives the search image. Waxcaps are not straightforward litter recyclers; current evidence points to a long-lived biotrophic association involving mosses and plant roots, and their mycelium builds up only over decades of undisturbed, unfertilised turf. A site with several waxcap species is a site where this one is worth hunting; a fertilised field is a waste of an afternoon.

Separating It From Look-Alikes

Neohygrocybe ovina. The closest trap. It is larger and much darker, blackish-brown to sooty, and its flesh flushes strongly reddish when cut, with the gills often reddening too. It shares the nitrous smell, so smell alone will not divide them; the cut-flesh reaction will.

Neohygrocybe ingrata. Browner with reddish-brown tints, and reddening readily where handled, bruised or cut. Again the nitrous smell is shared. Handle a cap firmly along the margin and wait: ingrata stains, nitrata does not.

Cuphophyllus species such as C. colemannianus and C. fornicatus grow in the same turf and can be dull grey-brown. Their gills are strongly and obviously decurrent, running well down the stem, and they have no chemical smell. C. fornicatus additionally has a tough, umbonate, almost conical cap.

Gliophorus irrigatus is grey-capped and easy to confuse at a distance, but both cap and stem are covered in slippery glutinous slime and it is odourless. A dry stem versus a slimy one settles it in seconds.

Hygrocybe conica and its allies blacken all over on handling and with age, a change no Neohygrocybe shows.

Entoloma species in grassland can be grey-brown with adnate gills, but the gills are thin and non-waxy, and the spore print is pinkish-brown.

Field Checklist

Dull grey-brown, dry, matt, hygrophanous cap; thick waxy distant whitish-grey gills; dry fibrillose grey-brown stem; flesh that does not redden when cut; a strong nitrous chemical smell; a white spore print; and old, mossy, unimproved turf in autumn. Those seven points together are diagnostic; any one of them alone is not.