JARS v44n2 - Fascinating Bigenerics

Fascinating Bigenerics
Felice Blake
Kallista, Victoria, Australia

One aspect of growing rhododendrons that seems to be generally overlooked is the part that rhododendrons play in the creation of bigeneric hybrids. As most of us know there are a number of bigenerics within the Ericaceae, some of which have been around for many years. In particular we note x Gaulnettya 'Wisley Pearl', a cross which obviously originated at the Royal Horticultural Society's garden at Wisley, and is a hybrid between Gaultheria and Pernettya . This was first noticed in 1929, and received the R.H.S. Award of Merit ten years later in 1939. Hybrids between these two genera are not uncommon, and have been found in New Zealand, also in Central and South America.

Another well known bigeneric is x Phyllothamnus erectus , a cross between Phyllodoce and Rhodothamnus chamaecistus (at one time classified as a rhododendron) which was found in a Scottish nursery well over a hundred years ago.

One bigeneric of particular interest to Americans is x Phylliopsis involving that delightful and choice shrublet from the Umpqua River and Curry County regions of Oregon, Kalmiopsis leachiana (also at one time classified asarhododendron), which teamed up with Phyllodoce in Hillier's nursery in England. A chance seedling was named x Phylliopsis hillieri 'Pinocchio'.

A somewhat similar but deliberate cross has been made since by an English grower and named x Phylliopsis 'Coppelia'. Whilst these two are interesting and quite lovely, to me they lack that indefinable quality possessed by Kalmiopsis leachiana.

Kalmiopsis has been used again and crossed with its near relative Rhodothamnus from the eastern European alps, to form x Kalmiothamnus .

But to get back to rhododendrons. Some years ago when importing rhododendrons from Glendoick Gardens in Scotland, I noticed in the catalogue "Trichostomum x Ledum sp?" and was rather intrigued by it, so it was added to my order. Right from the beginning, this appeared to be a "different" plant, the leaves seemed to be midway between the supposed parents, slightly scaly, but not as densely scaly as R. trichostomum . The flower tends more to resemble a ledum having the typical exserted stamens whereas in the rhododendron they are hidden in the narrowly tubular and lobed corolla. The flowers are held in tight snowball trusses of a brilliant white and are bigger than the usual forms of R. trichostomum . This is a most beautiful plant and well worth growing more extensively, but sometimes it is found masquerading as R. trichostomum , which is unfortunate as it is so distinctive.

It is now thought to be a hybrid with Ledum glandulosum or its variety columbianum as the pollen parent. This plant originally came from H.L. Larson of Tacoma. In accordance with the International Code of Nomenclature for Cultivated Plants which states that bigeneric hybrids must bear a name composed of parts of the names of the parental genera, this hybrid and also other hybrids between rhododendrons and ledums are termed x Ledodendron . The clonal name for this particular hybrid is 'Arctic Tern' an apt name for such a charming plant.

R. 'Arctic Tern'
'Arctic Tern'
Photo by Felice Blake

Another x Ledodendron is 'Brilliant', supposedly a hybrid between Rhododendron 'Elizabeth' and Ledum glandulosum , but it is certainly difficult to see how ledum got into the act! My own plant has never been particularly outstanding, and 'Brilliant' has had the reputation for distorted and split corollas. However, I did see a glorious plant in the Meerkerk Test Garden (the Seattle Rhododendron Society's garden on Whidbey Island) which lived up to its name - the flowers were really a brilliant red on a compact, low-growing bush, with bronzy-red new foliage.

The most fascinating bigeneric hybrid of all that I have seen is Kalmia latifolia x Rhododendron williamsianum . I understand that the cross was made in the 1950's by Halfdan Lem. This is most unusual as Kalmia latifolia has 24 chromosomes and Rhododendron williamsianum has 26 chromosomes, and it had been thought that plants with different chromosome numbers would not hybridize. The leaf shows the influence of R. williamsianum , but the flowers are truly unique. They are in trusses of about six, white (as I saw them) or pinkish white, with quite large saucer-shaped flowers on long pedicels with remarkable long protruding styles gently sweeping upward. This is truly an outstanding plant, the flowers are quite unlike any I have seen. Little wonder that it is colloquially dubbed "Nonsuchianum"!

Of course, it does not follow that all bigeneric hybrids will be world beaters, other hybrids have been made but without momentous results and have been discarded as monstrosities. But the results so far do open the gateway for further experimentation by those dedicated hybridists who will hopefully present us with further keenly sought after and fascinating treasures.

Felice Blake is a keen observer of the genus Rhododendron and its relatives and a faithful contributor to the ARS Journal.