Causes of Fluctuating Flowering Time

This article was first published on 09 Aug 2012.

There is nothing like a splash of red in the garden to catch the eye and one of the best reds comes from Rhododendron elliottii. What’s more, it can produce flowers for several months in a row.

In the Rhododendron Dell at Dunedin Botanic Garden, this species has been flowering on the Cynthia Walk since the beginning of April.   Last year it flowered for an even longer time, from mid February to early December.   It achieves this by sending a few florets out at a time rather than full trusses en masse. However, at Pukeiti near New Plymouth, R. elliottii produces full round trusses from early to mid spring. Why so different?

Possibly a clue lies in its native habitat. R. elliottii comes from wet areas of Myanmar, Yunnan and Nagaland where it receives at least 250cm rainfall per annum. Pukeiti’s rainfall is about 450cm, whilst down here in Dunedin we rarely get more than 100cm. So, it seems that R. elliottii requires high rainfall for concentrated flowering.

Less definite seasons have also encouraged prolonged flowering. In the past R. elliottii would flower here in spring and again in autumn, being held in check by summer and winter temperatures. Given a year like 2011 though, when we had a cool, wet summer and mild winter, R. elliottii instead took its cue from available soil moisture and gave us nearly 11 months of flower.

So, every cloud may have a silver lining, but look a bit closer and you will also see a gleam of red.

Doug Thomson is curator of the Rhododendron Dell at Dunedin Botanic Garden.