The prairie in March – at least here in Nebraska – displays a fairly limited range of color. As a result, a cluster of yellow orbs really stands out against the otherwise brown background of dormant vegetation. While on a hike with some college students yesterday, we walked past some of those fruits and one of the students asked what they were. It’s a common question, and the answer is Carolina horsenettle (Solanum carolinense).
Carolina horsenettle is a member of the nightshade (Solanaceae) family, a large group of plants that includes both very toxic plants as well as some important food crops for humans, including tomatoes, potatoes, bell peppers, and egg plant. At least many members of this family (maybe all?) hold their pollen tightly, requiring visiting bees to use a technique called sonication, aka buzz pollination, to make the flower release its pollen. Many bee species can employ sonication (though not honey bees), but bumble bees are perhaps best known for it.
During sonication, a bee grasps the flower and vibrates its wing muscles at a frequency that causes pollen to be released by the flower. When I’ve watched it in the field, the bee is usually beneath the flower when this happens, so the pollen drops directly onto it. It’s convenient (and probably not accidental) that Solanaceae flowers tend to be oriented slightly or completely downward to help ensure that bees are positioned correctly.
Carolina horsenettle has both male and female flowers. Interestingly, both the male and female flowers contain anthers full of pollen. However, the pollen in female flowers, while apparently nutritious for foraging bees, is nonfunctional in terms of fertilization. When a bee collects pollen from a female flower, it comes into contact with the green stigma that protrudes from the center of the anthers and (hopefully) transfers pollen from other flowers onto the stigma. Male flowers have a non-functional stigma that is much smaller. At least, this is my understanding of how all of it works.
I remember being surprised to learn that Carolina horsenettle is a perennial. I’d always assumed it was an annual because it seems to occur in many places where competition from other vegetation has been reduced. But no, it is a deep-rooted perennial native (to Nebraska) wildflower, and pretty one at that.
It’s considered a weed by some, but I’ve never seen it growing in abundance in our prairies here. If do you come across it in a place you don’t want it, be wary of trying to yank it out of the ground. Besides its long roots, it also defends itself with many spines along its stems and leaves.
If you’ve wondered about those little yellow fruits in the past, now you know what they are. Please don’t eat them, though. While Carolina horsenettle is related to plants that have nutritious and delicous fruits, horsenettle fruits are toxic enough to do you some serious harm.
As long as you don’t stick the fruits in your mouth or grab the plant with your hand, Carolina horsenettle is a nice plant to have in a prairie. Keep an eye out for it in a prairie near you this summer, and when you find one, take a few minutes to watch how visiting bees interact with it. If you sit quietly, you’ll hear how the sound of a bee’s buzzing wings changes frequency when it switches into sonication mode. It’s a pretty great sound, and an important part of the larger prairie symphony.