Every spring and summer, I keep an eye out for species whose flowers regularly change color, typically from yellow to red. This is especially evident among the buckeyes and horse-chestnuts (Aesculus) where just-opened flowers will have prominent splotches of yellow (to attract insect pollinators) that change to red (upper image; Aesculus turbinata, Japanese horsechestnut; 219-35*A) over the course of a day or two. My favorite example of color shifting at the Arnold Arboretum can be seen in the yellowhorn shrub/small tree (Xanthoceras sorbifolium; 488-80*B; lower left) which really puts on a show!
The phenomenon of color changes in flowers is actually quite common among flowering plants. In general, yellow to red color shifts, have evolved to help steer insects to newly opened flowers (yellow) that are in need of their pollination services. Ecologists have shown that insects have an innate preference for yellow flowers over red flowers, although insects themselves can reinforce this behavior through learning (yellow flowers have more nectar and pollen, while older red flowers have less—smart insects head to the yellow flowers). An amazing story of pollination innovation that has evolved perhaps more than a hundred times over the course of flowering plant history.
As I pondered this Post, I went back through my thousands of images of plants in the Arboretum to see if I could find other less conspicuous color shifting species. And indeed, I did: Crataegus crus-galli (cockspur hawthorn), Diervilla lonicera (northern bush honeysuckle), Koelreuteria paniculata (golden rain tree), and Ribes aureum (golden currant). Some of these color shifts are subtle. Check out the golden currant flowers in the bottom right image (1119-83*A). The petals (small and less showy than the big yellow sepals) of the youngest flower (blue arrow) are still yellow. The older flowers have petals that have changed to reddish orange (green arrow). Pretty great party trick, if you asked me.