JARS v60n1 - Let's Talk Hybridizing: Ongoing Work in Denmark

Let's Talk Hybridizing: Ongoing Work in Denmark
Carl Adam Lehmann
Klampenborg, Denmark

We are offering you a few words on what happened in Denmark as far as hybridising is concerned after the Danish Chapter was formed in 1974.

Yes, you are right, "yakushimanum fever" was hitting all hybridisers, also locally. Jens Birck offered two seed lots in one of the first issues of the Danish Newsletter in 1975. Both typical for the period were Rhododendron degronianum ssp. yakushimanum primary hybrids, i.e., with R. rex , and R. strigillosum .

These two crosses are good representatives of the ten first years of the chapter's life, and Jens was lucky, because one of the resulting seedlings eventually was named 'Great Dane'* and is, by any standard, a fine plant and commercially available today in Germany and Holland (see Figs. 1 & 2). At the 1981 ARS meeting in Washington, D.C., I was contacted by East Coast members who also grew the cross, from seed submitted via the ARS Seed Exchange, and they had obtained hardy and pretty plants too. But we were looking towards the next hurdle in finding the "ideal" hybrid, a plant with the good looks of a species, garden value twelve months a year, and the flower power of a hybrid - plentiful and long lasting plus low growing, a category that ruled out 'Great Dane'*.

The next good thing was around the corner, and in 1982 Jens obtained pollen of R. proteoides from Cecil Smith in an envelope marked Rock #147, a clone that he also used to create his now famous cross with R. 'Bambi'.

Jens put the pollen to use on R. degronianum ssp. yakushimanum , but the result was not good enough. Fortunately he obtained more pollen in 1996, this time from the RBGE Rock #151 clone, and had the wisdom to use it on both R. 'Kupferberg', with seedlings only starting to flower a few years ago showing promise (see Fig. 3), and also on Hachmann's R. 'Fantastica', a cross which also produced interesting offspring (see Fig. 4). A clone is tentatively named 'Pretty Dane'*.

Warren Berg luckily gave me a plant of R. proteoides in 1983, pollen from which was put on a pretty orange flowered Tower Court form of R. dichroanthum ssp. scyphocalyx . The resulting plants except for one took a long time to flower, but that plant has been used in hybridizing lately, and when put onto 'Dusty Miller' (a R. degronianum ssp. yakushimanum x 'Fabia' hybrid according to Kenneth Cox) the resulting seedlings started flowering after only two years.
Pollen from Warren's cross R. degronianum ssp. yakushimanum x R. proteoides also crossed the Atlantic Ocean. This latter was used on R. 'Titian Beauty' in 1986, a red R. degronianum ssp. yakushimanum hybrid reported to contain a measure of the orange hybrid 'Fabia'. Unfortunately the result did not produce anything in the orange hue, but a nice plant with a pink/red full truss was raised and selected by Dr. Askjaer (see Fig. 6). It is less than 3 feet tall after nineteen years. We seemed to get closer to dwarf hybrids.

A Few Words on R. proteoides

Since we have not had a severe winter with much below minus 5°F for the last twenty years, we cannot judge the R. proteoides hybrids fully yet, and we do not know for sure if R. proteoides does implant hardiness as well as dwarfness. What Jens found out while collecting in China in 1996 was that R. proteoides lives part of the year under heavy snow cover high up in the mountains and that some plants had branches up to 6 feet long but growing along the ground probably because of the snow. This may indicate a stronger hybrid power than we are looking for, but so far it is too early for us to tell for sure.

When collecting and growing the four old (Rock collections from 1948) clones of R. proteoides , we did encounter difficulties. The clone Rock #147 is very unhappy on its own roots. My plant, obtained from the RSF in 1981, after 25 years having not grown in size, and even as a grafted plant #147, needs a fungicide in the spring to check sudden dieback of branches and is still not really happy. As understock we use mainly 'Cunningham's White', but for yellow and orange flowering plants we use 'Gartendirecktor Rieger'. The clone Berg/Nelson RSF 75/236 is easier to propagate and has few problems, as has a clone obtained from the Cox nursery at Glendoick, identified as from Ascreavie in Scotland. This latter plant, however, also does better as a grafted plant, also because it is difficult to root, and the fourth old clone from RBGE numbered Rock #151 (see Figs. 7 & 8) also needs an understock to perform well. Jens and I are working with the Rock #151 and the Glendoick clones only, giving preference to the #151.

The Future with R. proteoides !

What we see in terms of R. proteoides hybridising is showing much promise. The plants that Jens and I have made so far have contained mostly 50% R. proteoides and are definitely better growing and looking when grafted. Even the 'Bambi' x R. proteoides from Cecil Smith seems to be happier as grafted specimens.

Plants with 25% or less R. proteoides seem to be okay on their own roots too and also bud younger while still some growth retention is evident. As an example, there is a dwarf of only 5 inches in ten years resulting from the cross R. 'Hotei' x ('Berg's Yellow'* x R. proteoides ).

With much help from Jim Barlup and Warren Berg, we have also introduced a number of Cecil Smith's original 'Bambi' x R. proteoides clones into our program, adding local hybrids with yellow/orange tones. The resulting plants are not yet ready for evaluation. We are, however, getting closer to another of the primary goals, plants looking like species but behaving like hybrids.

The Future with R. aureum

The R. aureum hybridising that was mentioned in "Let's Talk Hybridizing" article in the Journal Vol. 54, No. 3 from Summer 2000, showing R. aureum x R. citriniflorum var. horaeum has been continued. When crossed with R. degronianum ssp. yakushimanum x R. citriniflorum var. horaeum a good warm yellow turned up with improved foliage that holds some promise. After fifteen years, these plants are around 6 inches tall.

A group of plants from the cross R. wardii x ( R. aureum x R. wightii ) (see Fig. 9) are hardy but almost too tall-growing for our goal. However, they are seemingly quite hardy, and the dull yellowish foliage associated with R. aureum has definitely gone. The next generation may go down in size.

Patience is the most important tool we use, and armchair crossing is the second. The rest is waiting. We are also trying to locate more pollen from R. pronum , since that plant in the right combination could produce the low, hardy, pretty and floriferous (yes, given the right partner it can) hybrid for the small gardens locally, and perhaps also for the gardens in your area.

In 2002 we decided to try and create a "chapter hybrid" for the Danish Chapter. Given the choice of parentage, I opted for a cross using R. 'Titian Beauty' crossed with R. proteoides Rock #151. Just before leaving for Seattle in April the pollination took place, and the 152 capsules yielded some 600 portions for free distribution to all members enclosed with the spring 2003 newsletter.

How many that were sowed we shall never know, but from photos and articles in the chapter newsletter we can see that the idea was fertile and that the seedlings are good looking. They do look like species but behave like hybrids.

We keep our fingers crossed and end this tale showing a shot of an R. campylogynum hybrid that Jens made twenty-five years ago. Small, flowers everywhere and pretty, yes, but temperamental too. Jens has given her the name 'Tessa Dane'* after his dog (see Fig. 5).

* Name is not registered.

Carl Lehmann is a member of the Danish Chapter, and Jens Birck is a member of the Swedish Chapter.