What are those yellow balls?

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).

The dime-sized fruits of Carolina horsenettle. These were photographed in May – long after they were produced by the plant the previous year, but they were still (mostly) holding their color and shape.

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 holds its pollen tightly but drops it in abundance upon bees that know how to hit the right note during sonication. You can see the flower’s green stigma curving to the left from the center of the anthers.
The backside of Carolina horsenettle flowers are pretty – and pretty fuzzy.

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 think these are male flowers since the stigma (the part that receives pollen and contains the seed-producing ovary) is greatly reduced. You can barely see one in the flower on the right (in between the yellow anthers).

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.

A crab spider waits for any visiting bees that are too anxious to sonicate for pollen to notice the predator lurking there.
This entry was posted in Uncategorized by Chris Helzer. Bookmark the permalink.

About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

7 thoughts on “What are those yellow balls?

  1. Wow, that was interesting! Any idea why, on an evolutionary basis, a plant would burn energy to have sterile pollen production?

  2. As soon as I saw those yellow balls, I guessed the genus, if not the species. We have Solanum elaeagnifolium, the silverleaf nightshade, and it’s almost identical. The flowers are lovely, although the fruits must not be particularly tasty since they often hang around until spring. It’s widespread across the state, and a tough plant. I’ve seen it on our coastal prairies, and I’ve seen it growing up through cracks in the asphalt in the hill country.

  3. Chris, this post was particularly educational and I really enjoyed reading about the bee/flower interaction, nice job!

  4. Pingback: 123 – Yellow – Beach Walk Reflections: Thoughts from thinking while walking


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