Grass-green Russula

Russula aeruginea is the commonest of the plain green brittlegills across northern temperate woodland. Its cap is a fairly uniform grass to verdigris green, sometimes with an olive or greyish cast at the centre, and it is smooth rather than cracked into patches — the single most useful character separating it from the other well-known green Russula species.

Green is an unusual colour in gilled fungi, and only a handful of brittlegills produce it. Within that small group, R. aeruginea is defined by its birch association, its slightly greasy cap in wet weather, and its spore print, which is not white but a definite cream to pale ochre. Ageing gills and stipe base commonly develop rust-brown spotting, a small extra clue.

The species belongs to the Russulaceae and shows all the family hallmarks: granular flesh that snaps cleanly, brittle gills, no ring, no volva and no latex. It is a straightforwardly ectomycorrhizal species and a familiar sight on sandy birch heath.

Cap

The cap is 4–10 cm across, convex then flattening and finally shallowly depressed in the centre. It is a fairly even grass-green to verdigris, often dulling to olive, grey-green or yellowish-green with age or in dry weather, and it is characteristically smooth — not broken into a mosaic of patches. The surface is slightly viscid when damp and matt when dry, and the cuticle peels about halfway. Mature margins are faintly furrowed.

Gills

Gills are adnexed, fairly crowded, brittle, and white becoming cream to pale ochre as spores ripen. They frequently develop small rusty brown spots and stains along the edges in older specimens, and they fork occasionally near the stipe.

Stipe

The stipe is 4–9 cm tall and 1–2 cm thick, cylindrical or slightly tapering upward, white and smooth with faint longitudinal wrinkles. It becomes stuffed then hollow and commonly develops brownish or rusty discolouration toward the base. No ring, no volva, no bulb.

Flesh & spore print

Flesh is white and unchanging when cut, brittle and granular. There is no latex. The spore print is cream to pale ochre — a key point, since some green look-alikes give a pure white print. Spores are warty with only partial connecting ridges.

This is a birch specialist, found in birch woods, on birch-invaded heath, along woodland tracks and in mixed stands wherever birch is present. It also occurs less often with aspen and in mixed conifer-birch forest. Soils are typically acid, sandy or peaty, and it is common on the poor ground where birch pioneers.

Its range covers northern and temperate Europe, where it is one of the most frequent green-capped brittlegills, and it extends across northern Asia and into North America. Fruiting bodies appear from midsummer through to late autumn, usually scattered singly or in small groups among mosses and leaf litter.

Russula aeruginea is ectomycorrhizal, forming its principal partnerships with birch. The fungal mantle around fine root tips and the Hartig net inside them create a two-way exchange: nitrogen, phosphorus and water move toward the tree, sugars move toward the fungus. On the impoverished sandy and peaty soils that birch colonises, this relationship is a large part of what makes early woodland succession possible.

Because birch is a pioneer tree, the fungus is often among the first larger fungi to appear as scrub matures into woodland, and it can persist for decades in the same stand. Fruiting bodies are soft and short-lived, quickly broken down by slugs and fly larvae that assist in dispersing the spores locally.

Russula virescens, the Greencracked Brittlegill, has a green cap that breaks into a mosaic of distinct patches or scurfy plates, quite unlike the smooth cap of R. aeruginea, and it grows with beech and oak. Russula heterophylla is greener-grey to olive with a smooth cap but produces a pure white spore print and has notably crowded, forking gills, growing chiefly with beech and oak.

Russula cyanoxantha can show green forms, but its gills are the giveaway: they are flexible and slightly greasy, bending under a finger rather than shattering, and its spore print is white. Green-tinged forms of Russula parazurea have a distinctly bloomed, dull-surfaced cap and grow with oak and lime rather than birch.