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Why do mushrooms grow in circles?

Why do mushrooms grow in circles

As an underground fungus grows from its starting point radially, its mycelium forms a circular shape. The fungus’s fruiting bodies called mushrooms form on the edge of this circle. Such circles can increase in diameter every year as the mycelium grows.

These circles are very intriguing and have inspired a lot of art and legends during history. But we will get to that later. Let’s elaborate on the growth of mushroom rings first.

Which mushroom species form fairy rings?

Fairy rings are typical for saprophytic mushrooms, for example, wood blewits, white parasol mushrooms, field mushrooms, or fairy ring mushrooms. Such mushrooms get their nutrients from decaying organic matter, and as they consume the nutrients, they need to grow further.

How saprophytic fairy rings can kill (or nurture) grass

Because the mycelium consumes all nutrients, mushroom rings grow each year. As the nutrients at the center of the fungus are depleted, plants like grass can stop growing in that area. However, then that part of the mycelium often dies, and nutrients will return to the soil, as will grass.

The outer parts of mycelium can release chemicals into the soil in the direction of its growth. Those chemicals break down nutrients in the soil. While the fungus does this for its own purposes, the plants will also benefit for a short time. Grass just outside of a fairy ring is often taller and thicker.

A fairy ring of poisonous false parasols (Chlorophyllum molybdites).
A fairy ring of poisonous false parasols (Chlorophyllum molybdites).

These phenomena are most typical for the growth of Marasmius oreades, commonly known as the fairy ring mushroom or the scotch bonnet. While its effects on our lawn are undesired, we welcome it in our garden. It is a great edible.

Mycorrhizal mushroom rings

Mycorrhizal mushrooms like porcini, which grow in symbiosis with trees, can also form fairy rings. However, such rings can only grow around the roots of their partner tree.

How large can fairy rings grow?

A typical fairy ring can be dozens of feet wide. It will continue to grow as long as there are nutrients available for it. It may stop growing when it reaches a different type of soil or the mycelium of another fungus.

The largest recorded fairy ring is created nearby Belfort, France, by the mycelium of trooping funnels. When described in 1982, it was 2000 feet wide and believed to be 700 hundred years old.

This is a ring of trooping funnels, but quite small, probably about 5 years old.
This is a ring of trooping funnels, but quite small, probably about 5 years old.

Why are mushroom rings called fairy rings?

According to folklore, fairy rings grow in places where fairies danced in rings. Stepping into them is often considered good or bad luck, depending on fairies’ role in the local folklore. Of course, those are just superstitions. Stepping into a fairy ring is neither dangerous nor beneficial.

I often harvest mushrooms that grew in a fairy ring, and I have never noticed any extraordinary luck or unluck afterward.

Rings make foraging easier.

There is, of course, something romantic about the folklore legends. However, for a practical forager like me, the romance of fairy rings is in the amount of edible mushrooms that I can forage from them and come back later for more.

A pasture by woodland near my home has at least a dozen large rings of white parasol mushrooms and two rings of Handkea utriformis puffballs. From late spring to mid-fall, I go there after rain and come back with enough food for a regiment of starved soldiers.

When I forage wood blewits during fall, I also look for fairy rings. In my experience, wood blewits form rings up to 30 feet wide. One such fairy ring is enough to fill my basket with the purple goodness.

With mycorrhizal mushrooms that have tethered fairy rings, the knowledge of the rings is also beneficial. When you find that porcino or chanterelle, look around the nearest tree for more, as that is where the fungus’s mycelium is…

And now, if you excuse me, I am off to dance with some fairies.

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Carlos Warren

Growing up in Texas, I was fascinated by the world of science and invention, thanks in large part to my father's work at Dow Chemical Company. However, my true passion lay in the natural world, and I became an expert in organic gardening and composting at a young age. I spent hours studying the microbiological communities in our family garden, using a microscope to define the quality of the soil. My love for farming and gardening led me to explore new techniques and methods, constantly pushing the boundaries of what was possible.