It’s autumn, and that means my wisteria seeds will soon be popping! If you’ve ever seen this happen, you know exactly what I mean, but if you don’t have wisteria in your yard, it’s likely that you’re not aware of the phenomenon.

We all know that plants spread their seeds using different mechanisms; some of the most common methods are distribution by wind, water, and animals. But the most fascinating, at least to me, is explosive dispersal, where a seed pod pops open and flings its seeds away from the plant. The evolutionary explanation for this is to prevent overcrowding of seedlings around the parent plant, giving them the light and space they need to mature.

Examples of plants that exhibit explosive dispersal include impatiens, sweet peas, lupines, California poppies, and pansies. In fact, I love to pick a ready-to-pop impatiens seed pod and casually hand it to an unsuspecting person, then watch them jump with surprise when it goes off in their hand. (Try it with kids and they’ll be searching for every plant in the neighborhood!)

But I digress; let’s get back to the main topic. Wisteria is a member of the Pea family, Fabaceae (as are other exploders including lupine and scotch broom). Most of us know it as a climbing vine, with purple, and sometimes white, flowers in the early spring. Less noticeable, at least from a distance, are the seed pods that form afterwards.

The pea-like pods, unremarkable from an ornamental perspective, are often overlooked in a profusion of vigorous leaf growth, especially because they’re green until they mature into 4-6 inch long hard shells. The pods turn brown as they dry on the vine. It’s after that drying process is complete that the fun begins, that and a hot Indian summer day.

It’s on those hot days that the seed pods are most likely to explode, typically later in the day after they’ve had a chance to absorb the most heat. If you have patience, you can sit and watch it happen. Look for empty husks where there weren’t any before (next to a lawn or walkway is perfect for this), then sit and watch. In fact, it’s more accurate to say “sit and listen”, because it practically impossible to be watching one when the moment occurs. It’s kind of like listening to the first few kernels of popcorn pop, the noise is a sharp crack followed by the sound of seeds landing away from the plant. If the wisteria is near a building, you can catch the sound of seeds bouncing off windows or walls, when it’s near a driveway or street, you can hear the seeds skitter across the surface.

The force of the explosion is truly remarkable; enough so that I don’t look directly at the plant if I’m not wearing glasses. I’ve seen a seed come to rest a full 70 feet from the plant, starting no more than 8’ off the ground and landing in the street (which starts 35’ from the plant), coming to rest against the curb on the far side. It’s exciting to see!

So how does it work? Forces build up inside the pod during the drying process, and when strong enough, the pod pops. The best description I’ve seen states: “as a pod dries, tensions are set up in the wall of the pod eventually causing it to split along two lines of weakness. As the two halves curl back, suddenly released like a tense spring, they flick out the seeds inside in an explosive manner.”1

You don’t need a hot day for the process to happen, it’s just that those days speed the drying process. If you take the pods and set them aside to dry, it’s just a matter of time before they pop; inside or out. Or you can speed up the process by putting the pods into the oven. The photo below shows the result of doing just that. The twisted halves of the pod, demonstrating the forces that had built up, are clearly evident. It’s a bit like running a pair of scissors along one side of a ribbon, where the uneven stretching causes the ribbon to twist and curl. In this case it’s the drying causing it to happen.

Plants serve all types of purposes. And while food and decoration are the most common, it’s kind of nice to find one that also provides entertainment!

1. http://www.countrysideinfo.co.uk/seed_dispersl/explosve.htm

Revised: 26-Oct-2003