Why do Jewelweed seed pods explode?

Jewelweed (wikipedia link) is a common plant in the northeastern US, where its late summer flowers are very popular with bees and hummingbirds. Its most notable feature, however, is its seed pods. When the tiny pods are ripe the slightest touch causes them to snap open, flinging the seeds in all directions (scroll down to see a video). Hence, the alternate name: touch-me-not.

a Rose-breasted Grosbeak eyes a ripe jewelweed pod. Artwork © David Sibley
as soon as the grosbeak touches the seed pod it explodes, flinging seeds away to relative safety. Artwork © David Sibley

The common explanation for this explosive seed-flinging is that it is a way for the plant to disperse its seeds more widely. I’ve heard and repeated that many times myself, but it has always felt a little unsatisfactory. The seeds don’t go very far, and it seems like an overly simplistic explanation for a very complex adaptation.

This summer we have a large stand of jewelweed right outside our back door, and Rose-breasted Grosbeaks have been spending a lot of time in there. A few weeks ago I was able to watch a grosbeak as it foraged in the jewelweed, and I was a little surprised to see it feeding on the seed pods. Watching the grosbeak attempting to eat jewelweed seeds led to a Eureka! moment, an idea that immediately made sense and seemed obvious and logical…

Maybe jewelweed seed pods explode to prevent predators like grosbeaks from eating the seeds!

The grosbeak moved deliberately among the stems, looking for ripe seed pods. When it spotted one it leaned over and, with a quick lunge, took hold of the pod. Instantly the pod exploded, and if the grosbeak had been quick enough, and managed to get a good grip on the middle of the pod, it might still have a seed or two in its bill. Other times the pod exploded as soon as the grosbeak made contact, seeds flew, and the grosbeak was left with nothing.

I don’t know if this is a new idea. I couldn’t find any reference to it, and would be very interested to know if it has been proposed and studied before.

Here’s a photo of the parts after a seed pod explodes: three seeds, the central stalk, and the curled strips of the outer shell

2 thoughts on “Why do Jewelweed seed pods explode?”

  1. Your idea makes a lot of sense. There are lots of examples of plants adapting against predation from animals: thorns, sensitive mimosa plants, and how acacia trees get bitter when giraffes are eating them and even signal nearby trees to get bitter. Brilliant insight, Mr. Sibley. Consider publishing this as a note in a peer-reviewed field journal.

  2. These exploding pods can sometimes hurt birds, I believe. I guess the grosbeak was just trying to look for food. When the pods explode, they scatter. Wildly. This post proved very useful to me and I hope to think about this plant and the bird when I go birdwatching next time.

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