Rhododendron viriosum

Australian Rhododendrons

There are over 900 rhododendron species. The genus is split into several subgenera, one of which is called Vireya. One of Vireya’s characteristics is the presence of scales on the leaves, stems, flowers and fruits. The purpose of these scales is not known but a possible explanation is that they help to protect the plant from the strong solar radiation on the tropical mountains where they are found.

The first mention of an Australian rhododendron  was when Baron von Mueller collected and named Rhododendron lochae  on the Bellenden Ker range, inland from Cairns in Queensland.  Nowadays the specific name, which commemorates Lady Loch, then wife of the Governor of Victoria and a patron of horticulture, is spelt lochiae. The first account of  R. lochiae being cultivated was in the Gardeners’ Chronicle in 1939, where it was reported as growing at Kew. In 1953 Rhododendron lochiae  appeared in the catalogue of Basil Hodgkins, at his nursery in Essendon, Victoria. It quickly became popular and was used as a parent of many hybrids.

In the early 1990s it became apparent that two different plants were being grown and sold as R. lochiae  and the botanist Lyn Craven discovered that the Hodgkin plant was new to science.  To change its name would have caused much confusion in horticulture all round the world, so Craven created a new type species and called the von Mueller plant R. notiale (meaning from the south). Unfortunately this action did not meet with the approval of international botanists so it was renamed R. viriosum, while the von Mueller plant remained as R. lochiae.

Identification

When not in flower the two plants are indistinguishable but in bloom they are easily separated:

R. lochiae has a curved floral tube, with the stamens on one side. This indicates that it is pollinated by birds.

R. viriosum has a straight floral tube, with the stamens arranged in a circle (see photo page 20).

It is possible that further species remain to be discovered in the mountains of North Queensland.

Cultivation

In the wild these plants grow both as epiphytes and on the ground, on steep slopes, and in rich soil with much organic matter. They are subject to frequent heavy downpours of rain and the cool mountains on which they are found are often misty. Although growing in acid conditions, they have a high requirement for calcium.

I have found that they can be grown in leafmould and in a mixture of equal parts of coarse and fine bark. Calcium was added as baking powder (it contains acid calcium phosphate). Temperature is also important. Temperatures below 5°C result in dark spots on the leaves, which also lose their gloss. A winter minimum of 10°C is recommended.  The compost must never be allowed to dry out, yet must be very well drained.

Seedlings bloom in their third year and for me that has been in August. However at the Tatton Park Flower Show R. viriosum has been shown in bloom in late July.

Author : Jeff  Irons

Published: Issue 47 (December 2011)