Flowering of the Rhododendron arboreum, the tree Rhododendron, is said to be extremely temperature sensitive. My own experience also verifies this. Four years ago I found that the second week of March was too early for flowers at Barsey Rhododendron Sanctuary (altitude 2500 m) in Sikkim. But when I was in Yuksom (altitude 1780 m) the previous day, the red Rhododendron had been in full bloom. I verified it this year again. At Kolakham (altitude 1980 m) in the Kalimpong district of West Bengal I could see Rhododendron buds in early March, but in Latpanchar (altitude 1500 m) they were already in full bloom when I went there the next day. When we reached our hotel outside Almora (altitude 1604 m) in early April, there was only one blossom left among all the trees on the grounds. But twenty kilometers away, just inside Binsar National Park, at an altitude of about 2200 m, hillsides were dotted with the red of flowering Rhododendron. These mountain roads are extremely slow, and you may take an hour to travel thirty kilometers. So when we plan trips to view Rhododendron flowers, we focus carefully on details like this.
Altitude causes another strange change. Over most of its range R. arboreum has deep red flowers, sometimes deeper than the red of blood and wine. But as you climb to over 2500 meters in altitude, the same species will produce flowers which can be any shade of pink, and even white. The red flower in the slide show here comes from Binsar National Park, from an altitude of perhaps about 2200 meters. The rest come from the highest point of the road above Munsiyari, perhaps at an altitude of just under 3000 meters. I don’t have words for the variety of pinks that you can see.
It pays off to look closely at the flowers with these lighter colours. Unlike their dark red siblings, they show characteristic spots and stippling. On my first trip to Sikkim I’d been entranced by these details on flowers I had seen in Yumthang valley (altitude 3500 m) late in the season, in early May. Are these a different subspecies? There is a recognized subspecies called the Rhododendron arboreum cinnamomeum, but this is easily identified by the bright cinnamon colour of the underside of the leaf. These variant plants were not of that subspecies. The colour changes were not due to soil conditions, either, since you could see differently coloured flowers on trees in the same slope, sometimes growing so close that their canopies touch. Each tree had flowers of a single colour. Someone would have to do more field work to check whether the colour remains the same from one year to the next, and whether it changes if the seed of a tree is planted at a lower altitude. It is quite possible that this has been studied in the last three hundred years, and a better scholar than me will able to dig out the details.