Spring of the Wild Petunia
I've been hearing slo-mo popcorn popping from a brown paper bag on top of the fridge...
It’s not popcorn popping. A month ago I stuffed some Wild Petunia stems into a paper sack. The pods were still green, and I wanted to let them finish drying and drop their seeds so I could save them.
Catapulting seeds
Wild Petunia pods don't just drop the seeds! They catapult them. Hard. From anywhere in the house, I can hear the seeds hit the inside of the sack.
Smack!
It's not constant. Maybe two or three pops per hour. Just often enough to startle me pleasantly. Admittedly a mild entertainment.
If the pod were still outdoors, the seeds would go ballistic and land several feet away from the mother plant.
The seed pods are look both streamlined and hairy. They grow close to the stem. It's hard to pull a pod away. Better to pick a whole stem.
How Wild Petunias move
After I started growing Wild Petunias in my prairie flower garden, I found little lavender blossoms in the grass, up to 12 feet from the garden. Too far away to be from seeds dropped by the plant. Too close to come from seeds that birds ate and then pooped out.
I wondered how the seeds got so far from the mother plants. So I looked at a few pods with a microscope. If you carefully cut one along its seams, you find four seeds inside. They're round and flat, like poker chips.
Pods that had dried and opened on their own were split neatly in half, with two separate parts that look like little canoes.
Each "canoe" has two hard white protrusions, like teeth, next to the indentations where the seeds were. A Wikipedia article explains that in the Acanthus family (Acanthaceae), a seed capsule has little hooked stalks, called retinacula, that eject the seeds. I thought the retinacula looked like broken teeth.
All the pods had four slots where the seeds fit, but some the pods had fewer than four retinacula. I pondered whether some of the retinacula broke off in the excitement of the explosion.
Gardener does science
So I taped a green pod partially shut. I let it dry out for a few days, until the two halves parted. When I removed the tape, the four seeds dropped out instantly. But there was the pod, with four undamaged retinacula. They looked like triggers.
Now I could see what was supposed to happen. As the pod splits apart from the base at high speed, each retinaculum flings one seed outward, as if from a catapult.
Show-motion photography of flower pods bursting among other members of the same family show the seeds spinning away like vertical frisbees when the pod bursts open. (It was so cool that must have watched that video 20 times.)
Then I glued the seeds back into the positions where they were before the pod opened. They fit neatly, like four paddlers in a pair of little boats. If you closed the two parts together, everything would fit. So that is how the seeds grew.
I don’t understand the mechanism that causes the pod to open suddenly. If anyone knows how the pod manages to do this or would like to explain what is going on with these wonderful feats of engineering, please put your insight into the comments area below. I'd really like to know!
Wild Petunia in the garden
Wild Petunia is an attractive native wildflower that gardeners are discovering for their gardens, because it does fine in dry soil and doesn't need fertilizing or watering.
It’s useful to the natural ecosystem. Native wild bees pollinate the flowers. Some bees use the flower petals in their nests. (More about wildlife use of the plant at Illinois Wildflowers.)
I hope that, besides the lavender loveliness of the blossoms and the joy of feeding bees, gardeners are also enjoying the popcorn explosions of the seedpods.
Scientific name: Ruellia humilis
Common names: Wild Petunia
Plant family: It is in the Acanthus family (Acanthaceae).
Culture: it is easy to grow and does not need fertilizer or watering, but it needs full sun. It grows more robustly if not in dire competition with grass and weeds. Gets about one foot tall and blooms for a long time in late spring through summer.
Range: It is native in a strip down the center of North America, but not north of Iowa or east of Ohio. In this map from BONAP’s North American Plant Atlas, counties in light green are where the Wild Petunia is native.