Guide

Look closerFungi

Chaga: the black, burnt-looking fungus lump on birch trunks

All year

That black, deeply cracked lump bulging from a living birch trunk is chaga, a fungus called Inonotus obliquus, not a burn scar and not a woody burl. It is a sterile mass the fungus pushes out through the bark, and it can sit on a living tree for many years.

Why this time of year

Chaga lumps stay on the trunk all year, so any walk past birches can turn one up. Farther north, studies put it on only about 2 to 4 percent of birches, so spotting one is a real find.

How to notice it

  1. Scan birch trunks, especially yellow and paper birch, for a black, hard, deeply cracked lump that looks like charred wood.
  2. Look at where it comes out: it often sits at an old wound or broken branch stub, and the bark around it may be swollen or cracked.
  3. Look up: the birch it grows on is usually still alive, with buds or leaves overhead.

Try this with kids: Find a birch with a black lump that looks burnt. Look up at the branches: is the tree still alive?

Phrase to know: “Sterile conk”: a hard fungal lump that makes no spores.

Safety

  • A birch with chaga is rotting inside and more likely to break at the lump, so look from a step back and don't climb or hang on it.

Sources

  • www1.maine.govPRIMARY (Maine Forest Service fact sheet, revised May 8, 2023; read as PDF text). Hosts are birches, primarily yellow and paper birch, 'most commonly found growing on yellow birch' (anecdotal). Native, found 'throughout the northern hemisphere where host trees grow'. 'Predicted natural occurrence in the northeastern forest of three to four percent of birch trees' (citing Brydon-Williams et al. 2021). Chaga 'is a sterile conk (it does not produce reproductive spores)'; a basidiocarp 'forms after the host tree's death, usually after the tree falls', 'not often encountered'. Needs a 'fresh infection court (wound)'. Symptoms: 'swelling and/or bark cracking', then 'a conk with a charred appearance'. Cross-section shows 'dark, rough outer surface and the light brown inside'. Caption separates chaga from 'a stem swelling called a burl, showing no sign of associated decay'. Says nothing about Massachusetts.
  • tidcf.nrcan.gc.caPRIMARY (Natural Resources Canada, Canadian Forest Service, Trees, insects and diseases of Canada's forests; distribution given as Canada). 'The fruiting bodies are composed of black, sterile masses. They are rough and remain on the tree for several years.' Main hosts white birch and yellow birch; secondary hosts include cherry birch (black birch) and grey birch. Infection 'through dead branch stubs, trunk wounds, or through pre-existing cankers'. Photo caption: 'The yellow-brown colour of the context of a sterile conk'. Does not cover the United States.
  • mushroomexpert.comSECONDARY, expert (Gary Emberger, Fungi on Wood, hosted on MushroomExpert, page dated 2025). Conks form 'almost exclusively on trunks of living birch'; 'conks found year-round' (basis for months 1-12). 'Resembling charred wood', outer surface 'dark brown to black, hard, and deeply fissured'; inside 'corky and yellow to yellow-brown'. 'They are not fruitbodies'; 'may be present on a tree for many decades without killing the tree'; sexual stage 'beneath the bark on snags or fallen trunks', 'typically appears only after the tree dies'. Core facts are also carried by the two primary sources above, except year-round.
  • scholars.unh.eduPRIMARY (UNH master's thesis abstract, White Mountain National Forest, New Hampshire, 2017-2018 surveys of 2,611 birch trees at 66 sites). 'Only 2% of trees sampled had a visible Chaga conk', 'present in 56% of stands', 'disproportionately associated with yellow birch', favors 'older, large-diameter yellow birches at higher elevations'. Supports 'uncommon'; it is northern New England data, not Massachusetts.
  • api.inaturalist.orgDATASET, community science (iNaturalist research-grade records of Inonotus obliquus, taxon 127021, place 2 = Massachusetts, read 2026-11-06). 171 records statewide; binned by longitude 60 west of -72.6, 63 between -72.6 and -71.8, 48 east of -71.8, including Middlesex Fells, Great Brook Farm State Park, Concord, Acton and Foxborough place names. Shows chaga occurs in eastern Massachusetts. Not on the preferred roster; lead decides whether to cite it or hold it as background only.

Checked against these sources on November 6, 2026.

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