Green Elf Cup

The Green Elf Cup is famous less for its fruiting bodies than for what its mycelium does to wood. Long before any cups appear, the fungus permeates fallen barkless hardwood with the pigment xylindein, turning the timber a startling blue-green or verdigris colour that is visible from several metres away. The actual apothecia — shallow turquoise discs 2–6 mm across on a short off-centre stalk — appear only in suitably damp conditions and are easily missed.

Chlorociboria belongs to the Helotiales, the inoperculate disc fungi, and is unusual in that the whole genus shares this wood-staining habit. Two species are commonly recognised in Europe and North America, C. aeruginascens and C. aeruginosa, which are essentially inseparable without microscopy.

Green-stained oak has a long craft history: it was prized by cabinetmakers as 'green oak' and used for the inlay work of Tunbridge ware and Italian intarsia panels, so this is one of the very few fungi whose ecological signature became a decorative material.

Fruiting body

Apothecia are 2–6 mm across, at first shallowly cup-shaped and expanding to almost flat, often becoming irregular, wavy-margined or slightly funnel-shaped with age. The upper fertile surface is a clear blue-green, verdigris or turquoise, occasionally with a yellowish cast when old and drying. The outer surface is a similar or slightly duller green and finely downy.

Stipe

Each disc sits on a short slender stalk 1–3 mm long, attached either centrally or, more usually, off-centre so the cup leans sideways. The stalk is the same blue-green colour and arises directly from the stained wood surface, often with many cups in a scattered troop over one patch of timber.

Flesh and spore print

The flesh is very thin, waxy-gelatinous and the same green throughout — cutting a fruiting body reveals no colour change or contrasting layer. A spore deposit is white, the spores themselves colourless, narrowly spindle-shaped and often with a tiny oil droplet at each end. The single best macroscopic character remains the surrounding wood: split a stained log and the discolouration runs deep into the timber rather than sitting on the surface.

The Green Elf Cup colonises fallen hardwood that has lost its bark and reached a soft, damp stage of decay. In Europe oak and ash are the classic hosts, with hazel, beech, birch and willow also frequent; in North America oak, maple and beech are usual. Sticks and branches lying on the woodland floor, particularly those partly buried in leaf litter, are far more productive than standing dead wood.

Stained wood can be found at any time of year and persists for years, but the cups themselves appear mainly from mid-summer through to late autumn, favouring humid spells. The species is widespread across temperate Europe, North America and parts of Asia and Australasia, common in old broadleaved woodland and in shady hedgerow banks.

Chlorociboria aeruginascens is a saprotroph causing a white rot in the sapwood of fallen broadleaves. Its hyphae secrete the quinone pigment xylindein into the wood, which both marks the extent of the colony and appears to act as a territorial boundary, deterring or slowing invasion by competing wood-decay fungi. The strongly stained zone lines visible in split wood mark where colonies meet.

Because the mycelium is long-lived within a log while fruiting is sporadic and weather-dependent, most records of the fungus are made from the stain alone. Spores are released from inoperculate asci, dispersing to fresh barkless wood where the cycle of colonisation and staining begins again.

The near-identical Chlorociboria aeruginosa produces the same green stain and green cups; it is separated by consistently longer spores and a slightly larger, more regularly cup-shaped apothecium, and reliable determination needs a microscope. Nothing else in temperate woodland produces this particular verdigris wood stain.

At a glance, green algal films or the powdery green alga Desmococcus on damp timber can suggest the fungus, but they coat the surface only and wash off, whereas xylindein saturates the wood through its thickness. Small blue-green lichen thalli on wood are dry and crustose rather than fleshy, and lack stalked discs. Other tiny stalked disc fungi such as Bisporella citrina or Hymenoscyphus species share the shape but are yellow or white, never blue-green.