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Ready the Cannons! - The Cannonball Fungus (Sphaerobolus)

  • Nov 25, 2025
  • 5 min read

Good evening, friends,


Last Friday I went down to New York City to give a presentation to the New York Mycological Society on burn mushrooms of the northeast. On Saturday we all went out to look for mushrooms — not particularly burn mushrooms, but any and all fungi we could find.


There were a lot of great mycologists on the walk and we found a variety of mushrooms that were not only new to me, but also new to New York City (as far as the club’s records go). The most explosive of these was the Cannonball Fungus (Sphaerobolus) which was growing on a large, dead white pine. This particular white pine hosted a variety of other small fungi that captivated the club’s attention for at least a half hour, and they certainly could be the fodder for future Mushroom Mondays, but today we’ll aim our attention at the tiny but mighty Cannonball Fungus.


The cannonball fungus, also known as the artillery fungus, with its brown spore sac loaded and waiting to fire.
The cannonball fungus, also known as the artillery fungus, with its brown spore sac loaded and waiting to fire.

The walk took place in Pelham Bay Park, the largest park in the New York City Parks system and about three times the size of Central Park. The park boasts a variety of features including two golf courses, over a mile-long beach, wooded trails, a boulder arrangement known as “Bronxhenge”, and the largest parking lot I’ve ever seen (so large it can, apparently, be seen from space). The park was created in 1888 and some of the trees where we were appeared to be over 100 years old.


If you like asphalt lots that stretch so far you can almost see the curvature of the earth, this is the place to be.
If you like asphalt lots that stretch so far you can almost see the curvature of the earth, this is the place to be.

Fun Facts


The cannonball fungus is a member of the gasteroid fungi, a group of basidiomycete fungi that develop their spores internally (like puffballs or the dyeball from a month ago). Some gasteroids are passive spore releasers and rely on wind or rain to disperse their spores, but Sphaerobolus takes matters into its own hands and shoots out a spore packet with, purportedly, the most powerful discharge mechanism in all of fungi (Reference 1).


The mushroom is small, about 2 millimeters in diameter, and the spore-laden cannonball (known as the gleba when inside the mushroom, and peridiole after it’s been ejected) is a mere 1 mm. The process of ejecting the gleba is incredibly energy intensive, and a study from Miami University of Ohio found that it apparently takes 80 times more energy for Sphaerobolus to eject its gleba than it does for a gilled mushroom to release its spores (Reference 1 — there are also videos of the spore ejection which are worth watching). That’s likely one reason why these cannons are so tiny.


Sphaerobolus

The fungus creates this cannon-like ejection through water pressure expansion in the cells. When the pressure finally reaches a threshold, the inner membrane of the mushroom snaps outward and propels the gleba four to five meters high, and six to seven meters from the mushroom (all dependent on launch angle). The ejection is so powerful it makes an audible “pop”.


The late University of Wisconsin mycologist Tom Volk calculated the human equivalent of this spore discharge: it equates to a 6 foot tall person throwing a baseball 1.5 miles high, and the ball lands 2 miles away from them. That will get you a minor league contract at the minimum.


The mechanism for ejecting the gleba is similar to these pop top toys that create a strong propulsion with their snap-through inversion.
The mechanism for ejecting the gleba is similar to these pop top toys that create a strong propulsion with their snap-through inversion.

The gleba contains not only spores, but also gemmae — large, oval-like cells (larger than spores, but they’re still measured in micrometers), that act more like clones of the fungus than genetically unique spores. The gemmae germinate quickly, but the spores need a bit more coaxing to turn into fungal hyphae.

Evidently, the spores are more inclined to germinate after being introduced to a little pepsin, a mammalian stomach enzyme, which indicates these fungi might be coprophilic (dung-loving) and prefer to eat and grow on animal dung.


You might hope they just stick to dung, too, because they can wreak havoc around a household if they get a taste for your mulch. They’ll grow prolifically on the mulch around a foundation and then, en masse, they’ll bombard the siding, windows, and even automobiles with their sticky spore packets. Once they land on a substrate, they can be difficult to dislodge and even hold up to pressure washing.


A visual diagram of the cannonball spore ejection. The inner membrane snaps out and shoots the gleba into the air, and the membrane stays inverted for some time after ejection. Photo from W. G. Smith (Reference 7).
A visual diagram of the cannonball spore ejection. The inner membrane snaps out and shoots the gleba into the air, and the membrane stays inverted for some time after ejection. Photo from W. G. Smith (Reference 7).

Etymology


Sphaerobolus comes from the Greek sphaira, which means “sphere”, and bolos, which means “to throw”. This refers to the sphere-like spore packet the mushroom launches into the environment. The species epithet that’s commonly attached to this mushroom is S. stellatus, derived from the Latin stella for “star”. This refers to the star-shaped opening of the mushroom that can split into anywhere from four to nine rays.


A photo of a Sphaerobolus just after it has ejected its spore sac. You can see the translucent inner membrane that acts like a sling shot still flexed outward. This is a photo by Crystal Davidson and was pulled from her Mushroom Observer observation (Reference 4).
A photo of a Sphaerobolus just after it has ejected its spore sac. You can see the translucent inner membrane that acts like a sling shot still flexed outward. This is a photo by Crystal Davidson and was pulled from her Mushroom Observer observation (Reference 4).

Ecology


The fungus is saprobic (it digests dead organic material) and can grow on dead wood, decaying organic material, and as previously noted, dung. These mushrooms were mostly scattered on the conifer log (with at least a few feet between each one), but when they grow on mulch they can grow prolifically and will fruit nearly on top of each other. The mushrooms can pop up spring through fall, but seem to prefer the late fall (iNaturalist observations peak in November).


As previously mentioned, you’ll see this fungus as Sphaerobolus stellatus in textbooks, but there are at least three different species in the northeast (S. stellatus, S. iowensis, and S. ingoldii). Fungi in the genus can be found on all six major continents with Antarctica as the only holdout. Determining the difference in the species comes down to measuring the size of the gleba sacs if you can track them down (or just pluck them out before they get launched).


The cannonball fungus sits on the right end of the little crack in the wood. Not an easy find by any means.
The cannonball fungus sits on the right end of the little crack in the wood. Not an easy find by any means.

Have a great Thanksgiving! If you don’t like turkey or Thanksgiving, you can still get in the spirit by finding some turkey tail mushrooms to make a medicinal tea,

Aubrey


References

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