Winter Sex: The Puzzling Case of Witchhazel
This is part of our ongoing series on tree sex.
Witchhazel is one of the most beautiful shrubs or small trees in forests throughout the temperate zone. It is easy to overlook in the summer, mixed in with lots of other shrubs and trees along creek banks and moist lower slopes of forests.
It is in the late fall and early winter that witchhazel stands out from its neighbors, and reveals its strange habits: witchhazel flowers in the winter. American witchhazel, Hamamelis virginiana, grows in eastern North America and flowers in late fall and early winter. Vernal witchhazel, also called Ozark witchhazel, is found from Missouri to Texas and flowers in late winter or early spring (vernal means ‘spring’). There are many ornamental witchazels, including the hybrid shown here, Hamamelis x intermedia ‘Arnold’s Promise’. Whatever their habitat or geographic origin, all of the witchhazels flower when other trees are still snoozing through the winter.
This would not be so odd, except that these are obviously insect-pollinated flowers. The long, bright-yellow petals, the presence of nectar producing a sweet smell and the stamens (pollen-bearing male bits) that are right next to the nectar source are all indicators of an insect pollinated flower.
For a long time, it was a mystery how a winter-flowering plant would get pollinated. A witchhazel that happens to flower on a slightly warm spring when honeybees are making their first flights in desperate search of food might get lucky. That is not a reliable way to reproduce.
It was the renowned naturalist Bernd Heinrich who realized that there was a group of owlet moths (family Noctuidae) called winter moths that are active on cold nights. These moths have a remarkable ability to heat themselves by using energy to shiver, raising their body temperatures by as much as 50 degrees in order to fly in search of food. It is a group of these moths that pollinate witch hazels. The moths that pollinate witchhazel are several species of Eupsilia known as sallows.
It would be easy to conclude that this is a case of coevolution – both organisms having evolved to depend on one another. This is probably not the case – Heinrich observed that these moths mostly feed on bleeding sap from injured trees. So, the tree is dependent on the moth, but the moth is probably not dependent on the tree.
So the next time you see a witchhazel in flower, remember that it is waiting for a shivering moth.
Heinrich, Bernd. 1987a. “Thermoregulation by Winter-Flying Endothermic Moths.” Journal of Experimental Biology 127 (1): 313–32.
———. 1987b. “Thermoregulation in Winter Moths.” Sci. Am 277: 73–83.
———. 1998. The Trees in My Forest. Reprint. Harper Perennial.