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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated August 8, 2022, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), N20.565°, W99.890°, Querétaro state, MÉXICO
BLACK NIGHTSHADE
When the above herb showed up with its small, white, star-shaped flowers bearing slender, yellow "noses," hiding in a tangle of spiny brambles inside a hedgerow of spiny mesquite trees -- explaining why it managed to survive in this land of roving herds of sheep -- it was clear that here was one of several species known as black nightshades. Black nightshades are members of the big, complex, and sometimes confusing genus Solanum. Potato, eggplant and tomato plants are assigned to Solanum, but the genus embraces maybe 1500 mostly tropical and subtropical species, with maybe 150 in Mexico, so to figure out which species this one was, a close look at certain details was needed:
The star-shaped corolla's lobes were curved backwards. Five orange-yellow anthers of varying lengths were connected to one another along their margins, with a hard-to-see pore at each anther tip, from which pollen could be shaken by vibrating bees -- "buzz pollination." All this is typical of the genus Solanum.
From the back it's clear that the vegetative parts and even the corolla are hairy. In figuring out the various Solanum species it's important to notice whether the hairs are simple or branched and, if branched, what kind of branching. These look like simple hairs with no branching. Plus, as seen below, the simple hairs bear no glands atop them:
Fortunately for our identification purposes, in 2021 Geraldine Murillo-Pérez and Aarón Rodríguez published a study entitled "Claves dicotómicas para las especies de Solanum (Solanaceae) en México," providing technical keys to all the known species of Solanum occurring in Mexico."
Using that key, and especially taking into account our plant's form and simple hairs with no glands, we're led to two very similar species, Solanum americanum and Solanum nigrescens.
Older literature -- such as the Flora de Veracruz -- makes clear that S. americanum has the smaller flowers, and "casi siempre," or "almost never" occurs below 1000m in elevation, while Solanum nigrescens with its larger flowers grows at higher elevations. Our plant, with anthers only about 1.1mm long, keys to Solanum americanum, but it occurs well over 1000m. However, many online herbarium collections of Solanum americanum from Mexico were taken at higher elevations. Also, the yellow-green corolla eye is right for Solanum americanum, while the eye of Solanum nigrescens tends to be purplish.
Therefore, SOLANUM AMERICANUM, sometimes in English referred to as American Black Nightshade, to distinguish it from other black nightshade species. The species is native to the Americas, but it's become an invasive in so many tropical and subtropical locations that it's hard to know its original homeland.
Traditionally the plants are used to treat susto, which translates to "fright," but it's more than that, a kind of ongoing, gnawing apprehension, usually caused by someone putting the evil eye on you, or such.
The online Biblioteca Digital de la Medicina Tradicional Mexicana says that our plants are very recommended for dermatological problems, especially for erysipelas, a bacterial infection of the skin's outer layers. Boil the leaves to prepare water for washing the affected area. Also you can mash the leaves, soak in alcohol and apply as a poultice. For mange, mash the leaves and fruits along with regular tomatoes, and smear on the paste. On and on the various cures are listed, from stomach problems and fevers to inadequate milk flow in mothers, and beyond.
On the other hand, a page on the species by North Carolina State University says that all parts of the plant are potentially toxic when eaten, citing gastrointestinal problems, weakness, hallucinations, convulsions and possible death as possible outcomes. The toxin causing the problem is solanine, a glycoalkaloid well described on Wikipedia's Solanine Page.
Despite this, over the years I've learned from local people to eat young greens of both Solanum americanum and Solanum nigrescens. Both require cooking, but as such they're tasty enough, especially when mixed with other potherbs. Both species in Mexican Spanish are called Hierba Mora, and used interchangeably.