The Pollination Puzzle of American Witch Hazel

December 2020

American witch hazel (Hamamelis virginiana) is a common component of the understory in forests throughout the eastern U. S. with a very uncommon characteristic— it blooms late in the fall (typically Oct/Nov in New England), just as most other plants are going dormant. These delicate blossoms remain in flower for a month or more, defying frosts and the occasional snow storm. This habit brings up many questions of why and how, but here we are going to try to answer just one: who pollinates American witch hazel?

Of course, there is always the possibility that it is primarily wind-pollinated, though that would be unusual for a species with such showy flowers—a trait that generally evolves to attract insects. Witch hazel also produces sticky pollen and nectar with a sucrose ratio typical of bee or fly-pollinated flowers. And, though the data on this is somewhat mixed, Hamamelis virginiana appears to be largely self-incompatible—though it can self-pollinate, little to no viable seed is produced when this occurs. All of these features point strongly toward cross-pollination by insects.

Which insects are involved is much less clear. Some sources list moths, specifically winter-flying members of the genus Eupsilia that have been observed feeding on witch hazel flowers. Intriguingly, these moths can thermoregulate, allowing them to fly amid freezing temperatures that would preclude other insects. Presumably this would enable them to make more frequent visits to Hamamelis flowers, increasing the likelihood of effective pollination. However, the most intensive available study of Hamamelis reproduction points in a different direction. Anderson & Hill (2002) primarily observed flies and small bees visiting witch hazel. Most prevalent were fungal gnats from the genus Bradysia, which did pick up some pollen on their visits, but are too small to be very effective pollinators. Bees, who generally need mild temperatures to fly, visited more rarely but carried much higher percentages of witch hazel pollen on their bodies, increasing the chance of successful pollination.

We are left with a unique array of possibilities—nocturnal moths, swarms of gnats, the elusive but impactful bee—and plenty of exciting potential for new observation and research. Perhaps each of these insects has a role to play; the complex ecological system they are part of definitely has more hidden wonders to uncover.