The Blue Rhododendron
An excerpt from the article written by Dr Noel Sullivan in 1996.
Is there a blue Rhododendron? If the word “blue” means a definite blue, an authentic blue (free from any red contamination), then by definition there are no blue rhododendrons. Why is this so?
Flowering plants evolved as a more efficient method of cross pollination, much more reliable than wind to transfer pollen. The flowers signal a reward to pollinators, who in turn rely on flowering plants for at least part of their food, which is in limited supply on any one plant. These pollinators need to cover distance with some degree of speed and are normally able to fly. Generally, the flower signal is detected by sight and should be visible from a distance. So it must be a different colour to stand out from the green foliage and will be therefore in either the red or blue ends of the spectrum. Bird sight is adapted to the red zone and insects to the blue.
Rhododendrons evolved and mostly grow naturally in mountainous terrain with cold winters, and they generally flower in early Spring when conditions are too cold for insect activity. Pollination under these conditions is by birds who with greater mobility, can venture afar in search of food. If the rhododendron flowers were an authentic blue, the birds would not see them, but they can detect and are attracted by the red pigment in the lavender blue. This is the reason why there are some mauve and purple species, but very few approaching a true blue. It could be that the colour mauve or purple was a valuable choice at an earlier time with a milder climate, when the red content could attract birds, and the blue attract insects, thus exercising both options.
Lavender blues are seen in the scaly-leafed rhododendrons in subsection Triflora, and R.augustinii is the better of many shades, those with green spots appear the bluest; also a subsection Lapponia, with R. impeditum and R. russatum among the best. In the elepidotes, R. campanulatum alone has flowers in the near blue, but even here the colour may vary from pale lavender to purple, lilac, plum, rose or white, so with that colour range there is little danger of that species not being seen and pollinated, and thus it survives despite it flowering in April at altitudes of between 2750 and 4250m.
Garden plants do not have to rely on natural pollination to survive. Their future depends on their popularity as a fillip to artificial propagation. Thus we have a number of man-made hybrids which have been derived from their natural progenitors, but only from the selected forms of species which suit our particular tastes - the bluest in this case - but they still retain some of the lavender or violet tints.
