ChagaInonotus obliquus

Chaga (Inonotus obliquus), also sold as chaga mushroom and known as the clinker polypore or cinder conk, isn't a mushroom with a cap at all. It's a black, cracked growth on the trunk of a living birch. People don't eat it, they brew it as a tea. There's no clinical evidence that chaga benefits human health, but several cases of serious kidney damage have been reported in people who took it daily for months.

How do you identify chaga on a birch? Look for a shapeless lump, usually 10–30 cm across and occasionally up to 40 cm, bursting through the bark where a branch once broke off or the trunk was wounded. Sometimes it runs as a narrow strip along a crack. The outside is black, hard and dry, broken by deep cracks into small blocks, and looks like a piece of charcoal or burnt wood. Break off an edge and you'll see the key feature: inside it's rusty brown, turning yellowish orange towards the wood, often with pale streaks.

That black lump isn't the fruiting body. It's a sterile mass of fungal threads mixed with wood, and it releases no spores. The fungus gets into the birch through wounds and spends years rotting the heartwood; the growth becomes visible roughly three to four years after infection, keeps growing for 10–20 years, and the tree eventually dies. The true fruiting body only forms on the dead trunk, under the bark, and is rarely seen.

Does chaga grow in the UK? Yes, though it's uncommon: most British records come from the birchwoods of Scotland, with fewer from England and only a handful from Wales. It grows right across the northern forests of Europe, Asia and North America, mainly on birch, sometimes on alder and occasionally on beech, maple, rowan or elm. In Ukraine it is at the southern edge of its range and turns up mostly in the forests of Polissia. You can see it all year round, but it's easiest to spot from late autumn to early spring, when the trees are bare.

Chaga vs bracket fungi and burls. Of all the growths on a birch, only chaga has no definite shape and no pores underneath, and only chaga is rusty yellow inside. The tinder fungus is a neat hoof shape with a smooth grey, banded crust and pores underneath. The birch polypore is pale buff to brownish, like a cushion or pancake, with white pores underneath, and grows on dead birches. The false tinder fungus is also a hard, dark hoof with a cracked top, but it has an even layer of brown pores underneath. A burl is an outgrowth of the wood itself: it's covered in bark, and inside it's wood with a swirling grain. King Alfred's cakes form small black balls on dead wood, with black and grey rings when cut.

Is chaga good for you? Honestly, nobody knows. Anti-inflammatory, antioxidant and anti-tumour effects, along with lower blood sugar, have only been seen in cell cultures and animals. Memorial Sloan Kettering Cancer Center's 2023 review says plainly that chaga's safety and effectiveness haven't been tested in clinical studies. Chaga has long been drunk as a tea and taken for gastritis in Russia, where a thick extract called Befungin was produced. It doesn't cure cancer, and using it in place of treatment is dangerous, because the disease keeps progressing in the meantime.

Chaga side effects and your kidneys. Chaga contains very high levels of oxalate, from 2.8 to 11.2 g per 100 g. Oxalate kidney damage has been reported: in Japan, a 72-year-old woman who took 4–5 teaspoons of chaga powder a day for six months ended up on permanent dialysis, and in Korea two men needed dialysis after long-term use. The risk is higher if you have kidney disease or kidney stones, or take large doses of vitamin C at the same time.

Chaga may also boost the effect of blood thinners such as warfarin or aspirin (in mice, chaga extract reduced platelet clumping), and of diabetes medicines that lower blood sugar (shown in the lab). How much this matters in people hasn't been studied, so talk to your doctor before taking chaga regularly. If you notice much less urine, swelling or severe weakness during or after a course, see a doctor straight away, and in an emergency call 112 (999 in the UK).

Harvesting chaga. Traditional guides say to take it only from living birches, between late autumn and early spring: the growth is cut off with an axe, trimmed of loose material, bark and wood, chopped up and dried at no more than 50–60 °C, then ground and brewed. Bear in mind that the kidney cases involved daily use for months. Chaga also takes decades to grow, and almost all chaga on sale is wild-harvested, so the authors of a 2020 review in the journal Sydowia warn that heavy collecting is depleting wild populations.