Japanese Honeysuckle: Why There are Two Flower Colors
What Gives with Japanese Honeysuckle Flowers?
Here in the Eastern US there are many different native vines, along with a cadre of introduced or invasive vines as well. Japanese honeysuckle (Lonicrea japonica) is one of them. If you're like me, as a kid I loved plucking the yellow and white flowers, nipping off end and sucking the base of the flower to get the sticky sweet drop of nectar from inside. As an adult I occasionally do the same thing, while cursing the rapid growth of the vine as it takes over my backyard fence. I'm not going to go into the whole life cycle of Japanese honeysuckle here, but if you want to know more check out this great dissertation about its life history and ecology by Anna D. Letherman and her PhD thesis from the University of Tennessee, Knoxville. Mostly what I'm interested in is a very basic question: Why does the Japanese honeysuckle have two different flower colors?
Nature doesn't do anything randomly, there's always a good reason. When I look at the honeysuckle vines, with their golden flowers and white flowers I wonder what's going on. If you look at the pictures below you can make a very quick observation, the golden flowers are farther back on the vine, while the white flowers (and greenish/white unopened flowers) are near the apex or end of the shoot.
What this tells me is that color is playing a role in the plants life cycle or attracting its pollinators. Japanese honeysuckle can spread as a vine, or by producing fruits. However, each flowers has to be pollinated by a genetically distinct flower from itself for it to fruit successfully (mostly this is the case), this is why the vine is pollinated by wasps, flies, hummingbirds, and hawk moths (see picture below) instead of the wind..
The first pollinators I mentioned are all diurnal, or daytime pollinators, which rely on vision and scent to find the vine's flowers. The hawk moths are nighttime pollinators and rely on scent cues. One clue as to the color variation of the flowers is the amount of nectar that each holds. If you get a chance, go outside and try an experiment. If you taste the nectar from the unopened flowers, the white flowers, and the golden flowers, try to distinguish which tastes the sweetest and which flower has the most nectar (pop off the end cap of the flower tube and squeeze a drop out). You'll notice that the golden flowers have the most nectar, the white flowers less, and the unopened flowers almost none.
Many insects don't find white flowers as attractive as yellow, red, or pink flowers. Therefore, yellow has an advantage. The yellow color of the honeysuckle, with the sweetest/richest nectar, is attractive to bees, wasps, humming birds, and flies, butterflies (they like yellow flowers over white most times), and visual pollinators. The white/yellow would also provide at least some light reflection at night for the moths as well. Additionally, the two colors can serve as a "cue" to the pollinators that there is more reward for them if they visit the older flowers first. The flowers last only about 3 days, so the changing colors is a signal of the flowers age. Day 1 is white, Day 2 is gold, Day 3 deep gold and then gone. The flowers coloration indicates the amount of nectar in its coralla (nectar tube), and which flowers most immediately need pollination.
It's no mystery that the Japanese honeysuckle is an invasive vine that takes over native plant habitats; however, it is an endearing part of many summer childhood days, sipping its sweet nectar. Next time you're out in the garden, or along a trail, take time to look at this plant very carefully, and study its interesting blooming pattern. Despite its beauty, remember to remove it where you can, and don't ever plant a vine, tree, or shrub with the name of a country that is not your own (like Japanese honeysuckle or English ivy), because it is an invasive and can harm your local ecosystems.