Research Explains Why Mysterious Mushroom Fairy Rings Grow in Circles
- Gauri Khanna
- 33 minutes ago
- 4 min read
A Stockholm University team used DNA extracted directly from soil to confirm, at molecular resolution, that Marasmius oreades grows as a hollow ring underground, not a solid disc.
Transplantation experiments across two fairy rings suggest the fungus advances outward to flee temporarily unfavourable conditions immediately behind its own growing edge, a pattern the researchers call the "transient-escape hypothesis."
The findings are preliminary, drawn from only two rings, but the metagenomic method demonstrated opens a new window onto the hidden lives of fungi in their natural environments.
Fairy rings (mushrooms naturally growing in circles) have troubled and enchanted people for centuries. Medieval folklore attributed them to dancing fairies, witches, and devils. Science, rather more soberly, has known since the early 1800s that they are the work of fungi. Yet a surprisingly basic question has persisted: why does the underground body of the fungus grow as a ring at all, rather than spreading outward as a solid disc?
A new study published in Royal Society Open Science by researchers at Stockholm University, led by mycologist Hanna Johannesson, offers the most molecularly detailed answer yet: though the authors are careful to frame it as preliminary.
Mapping the Invisible
The mushrooms visible above ground are just fruiting bodies, the reproductive structures of a far larger organism. The main body of the fungus is the mycelium: a branching network of thread-like filaments called hyphae, growing silently through the soil. For fairy rings, it had long been assumed that this mycelium adopts a ring-shaped architecture, with the centre dying off as the outer edge advances. But that assumption rested largely on visual inspection of soil, never on high-resolution molecular evidence.
Johannesson's team changed that. Working on two fairy rings formed by Marasmius oreades, also known as the Scotch bonnet mushroom, growing in Berthåga Cemetery in Uppsala, Sweden, they collected soil cores along transects crossing each ring and sequenced all the DNA they contained. By mapping the resulting sequences against the published M. oreades reference genome, they could detect the fungus's DNA with precision.
The results were clear. M. oreades DNA was concentrated at the outer growth front of each ring, peaking at the outermost edge. Inside the ring, DNA levels dropped to background concentrations similar to the soil outside the ring entirely: confirming, for the first time with molecular evidence, that the mycelium genuinely forms a hollow circle rather than a filled disc. The team also confirmed that the mycelia of both rings were dikaryotic, meaning each contained nuclei from two genetically distinct parent organisms coexisting in the same filaments, as expected for this species following a mating event.
The Escape Artist
Demonstrating the ring's shape was one thing. Explaining why it forms that way required a different experiment. The researchers identified five competing hypotheses for why the fungus expands as a ring: that hyphae simply continue growing in whatever direction they already face; that the fungus actively escapes permanently toxic or nutrient-depleted soil behind it; that it escapes a similar but only temporarily hostile zone; that internal signalling coordinates all parts of the ring to grow in concert; or that growth is oriented like a compass, fixed towards particular cardinal directions.
To test these, they dug up rectangular sections of each ring's growing edge, roughly 30 by 70 centimetres, and either rotated them, moved them to the ring's centre, transplanted them to the opposite side of the ring, or placed them entirely outside the ring. Fourteen months later, they sampled the soil again and used the same metagenomic sequencing approach to track where the fungus had grown.
The results were most consistent with what the researchers call the transient-escape hypothesis. When sections were moved to the ring's hollow centre, the fungus was able to grow there, indicating that the centre is not permanently hostile. When a section moved outside the ring continued growing in its original direction rather than expanding in all directions, that too matched the transient-escape prediction. The pattern suggested that the soil immediately behind the advancing growth front becomes temporarily unfavourable, possibly due to nutrient depletion, the release of self-inhibitory compounds, or both, and the fungus effectively keeps moving forward to stay ahead of its own detrimental wake.
Limitations and the Road Ahead
The authors are emphatic about the boundaries of these conclusions. The experiment involved only two rings, several transplantation treatments yielded inconclusive results, and the study cannot yet identify the specific chemical or biological mechanism responsible for the temporary inhibition. As the researchers write, results should be interpreted as qualitative and preliminary rather than as a formal test of alternative mechanisms.
What the study does establish more firmly is methodological. Detecting M. oreades DNA at meaningful resolution within the complex soup of soil metagenomes, where the fungus comprised on average only around 2% of total DNA at the growth front, demonstrates that metagenomic approaches can illuminate cryptic aspects of fungal biology that would otherwise remain invisible. Future work across more rings, combined with laboratory experiments to identify the inhibitory factors, will be needed before the mechanism behind fairy ring growth can be considered understood.
The folklore, for its part, can rest easy. A fungus fleeing its own shadow is, if anything, more mysterious than a fairy.
