Seed Ecology

The seed ecology of rhododendrons is that they produce very large numbers of very small seeds. Why is this? The answer is that they occupy a particular niche in the forest.

Take the horsechestnut. It has a huge seed that weighs around 12 gm. dryweight. This mass represents a lot of starch to provide energy for the growing seedling. By the way, the starch is protected from being eaten by animals by a generous admixture of the intensely bitter saponin aeschlin. This large seed, given a suitable site can quickly produce a deep root and a strong shoot without the need for much initial photosynthesis. The starch represents maternal care. (Who said plants didn’t have maternal care?) These seedlings can then, given a chance, become forest giants. They are one of the emergent, canopy species.

Contrast your rhododendron seed. No, I have not weighed one. Say a single seed weighs 1 mg. for the sake of comparison. Not much maternal investment there – “You’re on your own son.” But by the way of compensation there are huge numbers of seeds that get blown hither and thither in the wind. Why? Well the rhododendron is exploiting the understory niche. The seeds are being sent out to find a gap in the vegetation - some damp patch of substrate where another plant recently died or toppled over. In these very precise habitats rhododendron seeds can germinate, put out their tiny leaves and start to photosynthesise. Their motto is ‘photosynthesise or die’. If the site is too dark they can’t capture light energy, and die. If the site is too exposed, windy or sunny, they also die because without the 12 gm. of starch they can’t get a root deep down in a hurry.

I called rhododendrons ‘Understory Shrubs’. Anyone seeing Peter Wharton’s January slide show from the NW Yunnan border would have been impressed by the 60 foot specimens, growing in some cases horizontally from a cliff.

So folks, please try growing rhododendrons from seed and don’t be disappointed if they die. You might say they are pre-programmed to die unless they find themselves in a very narrow band of conditions.