Touch-me-nots, or jewelweeds, are some of our most common roadside flowers, and few flowers are more delightful. Close relatives of the garden Impatiens plants that seemed to have taken over the nurseries a few years back, they grow in vast colonies along the edge of the woods.
There are two common species in the eastern United States. Impatins pallida, which grows in the north and at higher elevations, has yellow flowers; Impatiens capensis (or Impatiens fulva), which grows in the south and at lower elevations, has bright orange flowers. Pittsburgh is right on the border of their ranges, so we get both, sometimes thoroughly mixed in the same colony. These pictures are all of Impatiens pallida.
The name “touch-me-not” comes from the explosive properties of the seedpods. If you touch a ripe seedpod, it will suddenly explode and send seeds flying in all directions. (The explosion is harmless, of course, but very amusing to children.) The secret is in the tense fibers of the pod, which, when the thin membrane that holds them together is ruptured, curl instantly into little coiled springs. You can tell a pod is ripe when you can see the black seeds through the thin green membrane.
The flowers are perfectly adapted for pollination by bumblebees. Each flower is almost exactly the size of a bumblebee; what the bee wants is far back in the spur of the flower, so that the bee must enter the flower completely and then withdraw, laden with pollento fertilize the next flower.