#190: Common Mulch Stinkhorns

Stinkhorns love humans. These mushrooms can eke out a living in natural habitats, but they really thrive in artificial ones like mulch beds. Unfortunately, humans don’t feel the same way about stinkhorns. We tend to find them offensive, thanks to their foul odors and phallic shapes. Perhaps no one disliked stinkhorns more than Etty Darwin (Charles Darwin’s granddaughter); legend has it that she would diligently remove stinkhorns from her property and burn them to discourage impure thoughts in her servants.¹ Appropriately (or perhaps inappropriately), the most common mulch stinkhorns belong to two genera named for these mushrooms’ offensive shapes: Phallus and Mutinus.* Of those mushrooms, Phallus rubicundus and Mutinus elegans are the most frequently encountered in North American gardens.

Eggs (Button Stage)

Both of the common mulch stinkhorns, P. rubicundus and M. elegans produce mushrooms that sprout from underground structures known as ‘buttons’. Stinkhorn buttons usually resemble bird eggs, so the buttons are often called ‘eggs’ by mushroom hunters. The eggs are spherical to ovoid structures that are generally pale in color but may have shades of pink, purple, or brown. When sliced open, the eggs reveal a gelatinous layer surrounding a dark spore mass which mostly surrounds the center structure which looks like a shrunken version of a stinkhorn.^2–6 This is the developmental stage that allows stinkhorns to fruit so quickly. Basically, the mushroom is already fully-formed inside the egg; all it has to do is expand (and fully expanded stinkhorns seem to be mostly air anyway). While in the egg stage, there are relatively few physical features visible, so it is difficult to identify the species with any degree of certainty. Once the conditions are right (usually soon after rain), the contents of the egg begin expanding rapidly and burst through the thin external layer.

Description: P. rubicundus

P. rubicundus is for the most part shaped like a long spike. It grows up to 20cm tall but only gets up to 1.5cm thick. The stipe can display various colors from the red, orange, and pink ranges and is usually more vibrant at the tip and paler at the base. If you pick up one of these mushrooms, they are remarkably lightweight; the stipe is hollow and its surface is riddled with long, sponge-like pits, so the mushroom has very little substance. Around the base of the stipe, the remains of the outer layer of the egg form a flimsy membranous cup called a ‘volva’.

The most important part of the mushroom for identification is the tip. P. rubicundus, like all other members of its genus, produces spore slime on a ‘head’ or ‘cap’ that drapes down from the tip of the stipe. The conical head reminds me somewhat of a tiny wizard’s cap sitting on top of an icicle. The spore-containing slime is dark greenish-brown and smells absolutely foul. Since the pungent spore slime coats the outer surface of the head, it never touches the stipe. Underneath the spore slime, the surface of the head is wrinkled.^3,5,6 This is easiest to see after the flies have eaten the slime away. Some specimens of P. rubicundus have a skirt-like veil that drapes down below the head. This veil is composed of the same material as the stipe, so it can be hard to see when dried out.⁵

Description: M. elegans

M. elegans is very similar in appearance to P. rubicundus. Both mushrooms are about the same size, although M. elegans is slightly smaller: that mushroom grows only up to 17cm tall and 1.5cm wide. Another similarity is that M. elegans is also a spike-shaped mushroom with red, orange, and/or pink shades that pale toward the base. Additionally, the outer surface of the egg remains as a volva at the base of both mushrooms. However, the surface of the stipe is slightly different in M. elegans. That mushroom’s stipe is still hollow and sponge-like, but the holes on the surface are tiny and make the surface appear more bumpy than spongy. The most significant difference is in the spore slime. M. elegans does have a foul-smelling olive-brown spore slime, but it is borne directly on the top part of the stipe. It also tends to produce more spore slime; the slime routinely covers well over 3cm of the stipe.^2,4

Ecology

Both mushrooms do exceptionally well in mulch, which they decompose. They are common sights popping up in mulch beds anywhere east of the Rocky Mountains from spring through late summer. Both mushrooms also live on other continents, but their true ranges are hard to pin down because of their preference for gardens.^2–5

M. elegans is supposedly more abundant and has a broader range than P. rubicundus. Despite this, I don’t think I’ve ever seen M. elegans. Every time I inspect a group of stinkhorns growing in mulch, they turn out to have a head and are therefore P. rubicundus. However, Michael Kuo and Gary Emberger both note that the range of P. rubicundus seems to be expanding in the northern part of the United States, probably thanks to the commercial mulch industry. Kuo also suggests that many P. rubicundus specimens are misidentified as M. elegans, so P. rubicundus could be more common than mycologists realize.^3,5

Why do these mushrooms do so well in mulch? In the wild, stinkhorns decompose wood in forest litter.⁴ Mulch provides the wood for food but also provides cover and protection like the leaves and other less woody components of forest litter. On top of that, humans have chopped up the wood into bite-sized pieces for the fungus. This increases the surface area available to the fungus, so nearly all of the nutrients in the mulch are available to the fungus from the outset.

Of course, the most fascinating part of the stinkhorn story is their relationship with insects. Bugs, especially flies, are attracted by the foul smells, walk around in the spores, and then carry the spores to new locations. To read more about the stinkhorn life cycle, see FFF#040.⁷

Edibility

“Can I eat that?” probably isn’t your first question after hearing about “foul-smelling spore slime,” but many stinkhorns actually are considered edible. M. elegans, for example, is edible while it is still in the button/egg stage. P. rubicundus is listed as “edibility unknown,” but many other species of Phallus produce edible eggs. At the very least, I don’t think P. rubicundus is considered poisonous.^4–6

Similar Species

Pretty much every species of stinkhorn will grow in mulch. So, don’t be surprised if your mushroom doesn’t quite match the descriptions above. In addition to the species described below, some stinkhorns found in mulch have multiple stipes or bucky-ball-like features, but those belong to different genera and are easily distinguished.

P. rubicundus resembles ravenelii, P. impudicus and P. hadriani, but they are fairly easy to separate. None of the three look-alikes have orangish colors and they all tend to be stockier than P. rubicundus. There are a couple other species of Phallus that are common in North America, but these have distinctive, net-like skirts that hang down from the head.

M. elegans is very similar to M. caninus and M. ravenelii, but there are subtle differences. M. caninus swells slightly at the tip and the spore slime covers less of the stipe (only 2-3cm). M. ravenelii tends to have larger pits on the stipe, which is pinkish in color (instead of orangish to reddish).²

Getting Rid of Stinkhorns

The biggest problem with stinkhorns fruiting in mulch is that people usually don’t want them there. Given how much time and effort people spend cajoling their gardens into immaculate works of art, I suppose that is understandable. The simplest way to restore the artistic splendor is to follow Etty Darwin’s example and remove the mushrooms from the garden. Of course, this is a Sisyphean task since the fungus – which is still alive and growing in the mulch – will just produce more mushrooms the next time conditions are right.

So, is there a way to kill the fungus? Theoretically, you could douse your entire garden with fungicides, but that would be expensive and impractical and would harm the “good” fungi just as much as the “bad” ones. Another option, which is just as difficult but more practical, is to remove all the mulch in your garden and replace it with an alternative mulch.⁸ Of course, the mulch supplies your plants with nutrients as it decomposes (thanks to fungi like stinkhorns!), which helps keep the plants healthy. So, removing all the mulch also removes the benefits that mulch provides.

Thankfully, stinkhorns wilt just as fast as they fruit. Most stinkhorns collapse within a day and all evidence of the mushrooms can easily disappear within a week. So perhaps the easiest solution is to learn to live with the stinkhorns.

Taxonomy

Both Phallus and Mutinus fall under the family Phallaceae, which contains all the stinkhorns.⁷ There are a few morphological differences between the American and European M. elegans, so don’t be surprised if the American version gets a new species name in the future.² Both P. rubicundus and M. elegans have common names, although I do not hear these very often. P. rubicundus is known as the “Devil’s Stinkhorn” while M. elegans is called the “Elegant Stinkhorn” or “Devil’s Dipstick.”^4,5

* Apparently Mutinus is named after the Roman god Mutinus Titinus (also spelled Mutunus Tutunus), a marriage-related deity symbolized by the phallus.^9,10

Kingdom	Fungi
Division	Basidiomycota
Subdivision	Agaricomycotina
Class	Agaricomycetes
Subclass	Phallomycetidae
Order	Phallales
Family	Phallaceae
Genus	Phallus	Mutinus
Species	Phallus rubicundus (Bosc) Fr.11	Mutinus elegans (Mont.) E. Fisch12