Chaga is not a mushroom in the conventional sense. What we harvest and use is the sclerotium, a dense, irregular mass of fungal mycelium and birch wood that forms on the outside of living birch trees (Betula species) infected with the parasitic fungus Inonotus obliquus. This blackened, deeply cracked growth, sometimes called a “conk” or “cinder conk,” can take 5–20 years to develop and typically weighs 2–15 pounds when harvested. The exterior is jet black due to extremely high concentrations of melanin, while the interior is a rich amber-orange color.

The actual fruiting body of I. obliquus (the true “mushroom”) is rarely seen. It appears only after the host tree dies, forming a flat, resupinate structure beneath the bark that releases spores briefly before decaying. By the time most people encounter chaga, the tree is still alive and the sclerotium is the only visible sign of infection.

The Sustainability Problem
Chaga cannot be meaningfully cultivated. While researchers have grown I. obliquus mycelium on grain substrates in laboratories, the resulting product lacks the key compounds (particularly betulinic acid and melanin) that come from the decades-long interaction between fungus and living birch tree. This means all high-quality chaga is wild-harvested, and the explosion in global demand since 2010 has led to serious overharvesting in accessible forests across Russia, Finland, Canada, and the northern United States. Sustainable harvesting requires leaving at least 15–20% of each conk attached to the tree and never stripping all chaga from a stand of birch.