The majority of spring mushrooms in our area are small. It is in fall that we get our display of the larger, more noticeable species. An exception to our diminutive spring species is Calocybe gambosa, known as the lightning mushroom. They are good edibles and have a long history of being collected for food by Interior First Nations bands.

Lightning mushrooms are large, white fungi that grow in arcs or circles in our grasslands. G. Hunt photo

The ecology of these mushrooms is interesting. They occur in open grasslands, damp meadows, and in sagebrush ecosystems. You would never find them in closed forests. The reason they form circles (“fairy rings”) is that the main body of the fungus lives in the soil as microscopic threads of cells called hyphae. The circular mass of hyphae is called the mycelium or “shiro.” A shiro starts from the germination of spores and spreads in a circle from one starting point. As a saprophytic species, the mycelium consumes organic matter and produces mushrooms on the outer-most, leading edge of the circle

Notice in the picture that the mushrooms form a broad arc. To the right side of the mushrooms you can see that the vegetation is darker green and this zone forms an irregular circle. This dark green zone indicates where the mycelium is present. The plants are greener because the fungus is breaking down organic matter and releasing nutrients into the soil. The diameter of this circle is 8 or 10 m across. The presence of mushrooms on one side of the shiro indicates that the soil is more nutrient rich on that side.

How many years has this shiro been growing? Unless we were to measure how far the arc of mushrooms moves yearly for several years we cannot know for sure, however, we can make a rough guess. Years ago, a mycologist in Williams Lake estimated the outward mushroom migration on shiros in that area was about 10 cm per year. Assuming a constant growth rate (quite a big assumption!) and the current diameter of about 8 m, this shiro may have started about 80 years ago or in 1938.

In our area, we can expect them to appear about when the arrow-leaved balsamroot flowers are out.

Look for lightning mushrooms when the arrow-leaved balsamroot is flowering. G. Hunt photo

If you see lightning mushrooms this spring, you can share your siting on this website.

Revised to fix site errors – Nov. 2025 – D. Smith