Rhododendrons for Cold Climates: Hybrids of R. dauricum sempervirens
By Dr. J. Breuckner
Mississauga, Ontario
Reprinted from The RSC Bulletin 1981
Vol. 10 No. 1
A well known British personality summed up his opinion of a group of rhododendron hybrids which lacked quality and good flower colours simply with two eight letter words: “Coloured Cabbages”. Odd as it may sound, my first reaction when I came across his remark was that I would be glad to grow any of these hybrids, if only they would survive. This was not long after we have moved from one of the most magnificent rhododendron growing areas of New Zealand and, perhaps, of the world, to New Brunswick, by no means in the coldest part of Canada, though not in the mildest either.
As years went by, and after a more thorough appraisal of the climate and detailed studies of the genus, I came to realize that we can do much better than that, much better than to grow “Coloured Cabbages”. Even amongst the older hybrids several of the better ones could, in my trials, tolerate the cold winters of southern New Brunswick. I am thinking of such fine rhododendrons as ‘Mrs. C.S. Sargent’, ‘Catawbiense Album’, and some others. Then came the new hybrids: ‘Ramapo’, ‘P.J.M.’, ‘Janet Blair’, ‘Evangeline’ to mention a few, gems in any garden anywhere.
It is not surprising that at that stage of experimentation the thought comes to one’s mind: why not push it to its limits? Why not attempt to create rhododendron hybrids of the greatest tolerance to cold possible to achieve at all?
It is well to remember, right here, that crossing the hardiest of species, even with each other, is not necessarily an assurance for obtaining hybrids of similar, maximum, hardiness. Conversely, a cross of two relatively tender rhododendrons may on occasion lead to a surprisingly hardy hybrid.
However, and not withstanding what was said, one can hardly escape the logic that employing species of greatest winter hardiness is still likely to present us with the best chance of breeding plants which will have the greatest tolerance to low winter temperatures.
Hardiness is obviously a very significant goal in breeding rhododendrons for cold climates. Another guideline, equally important to me, is the aesthetic aspect. Only those hybrids should be saved (and propagated) which are as good or better than the non-hardy parent and which are hardier or more attractive than existing hybrids.
It took some time to collect a selection of rhododendrons, almost all species, suitable for a breeding programme of this kind. Some species had to be gathered from the wild because not even a botanical garden had them in their collections. Gradually it was possible to assemble the following:
A. Hymenanthes (Elepidotes)
1. R. catawbiense, including the typical form, var. ‘Catalgla’, var. compactum, var. rubrum and hardy catawbiense hybrids
2. R. aureum (chrysanthum)
3. R. nikomontanum
4. R. brachycarpum, ssp.tigerstedtii
5. R. maximum
6. R. yakusimanum, several clones
7. R. smirnowii
R. aureum as a parent was used in a limited way only, since, with one or two exceptions, it did not perform well for me. The hybrids seemed to lack resilience and durability. As years went by they just simply vanished from the garden. Neither could R. aureum itself (at least forms from the Lake Baikal area and northeastern Siberia) tolerate the climate of South Ontario, though they survived somehow in New Brunswick. I had better experience with R. nikomontanum for breeding hardy, dwarf hybrids.
It is my opinion that of all rhododendrons in Sect. Hymenanthes the Tigerstedt subspecies of R. brachycarpum does have the greatest tolerance to cold winter temperatures, in spite of growing in nature at lower altitudes and in less extremes of cold than R. aureum. The Tigerstedt subspecies is a tall shrub, several meters high, definitely reaching out of the snow cover, something which can not be said of the prostrate growing R. aureum.
B. Rhododendron and Rhodorastrum (Lepidotes) contain species which bear a very realistic promise of having the potential for yielding truly hardy hybrids for even the coldest of locations.
1. R. carolinianum, pink and white clones
2. R. ferrugineum
3. R. kotschyi
4. R. lapponicum
5. R. ledebourei
6. R. dauricum
Perhaps the foremost in my efforts of producing hardy hybrids was the Canadian R. lapponicum from the Great Slave Lake area. In contrast to the prostrate growing clones, this erect growing form must at times reach above the rather scanty snow cover where temperatures can dip to -55C or lower.
R. dauricum, another arctic and subarctic rhododendron was also used extensively, mainly in three forms. One was collected from the wild, north-west of Lake Baikal, by Vladimir Vasak. Another form was grown from seeds, which were an offshoot of Dr. Mehlquist’s genetic studies. He crossed in his studies a white clone of R. dauricum (‘Arctic Pearl’) with a dwarf magenta flowered form. The third is the evergreen R. dauricum sempervirens. The rest of this article will deal only with hybrids obtained by using this clone as one of the parents.
As far as I know, the R. dauricum sempervirens which I used comes, via Moscow, from Siberia. Its foliage, flowers and growth pattern fit perfectly the description given for this shrub by Dr. Leach in his book ‘Rhododendrons of the World’, therefore, the reader can be spared any further taxonomic characterization.
R. dauricum sempervirens is a very hardy, well- behaved plant, attractive in itself all year round, but especially in early spring when covered with flowers. It seemed to me to have all the potentials of being an excellent parent for creating cold- tolerant hybrids. How excellent in reality it proved to be I came to realize only within the last few years. As it appears, R. dauricum sempervirens imparts cold-hardiness to its offspring, but comparatively little of its morphological characteristics.
The size and growth habit of the other parent, the foliage, the size and colour of flowers come through in the hybrids to a marked extent, a trait which I have noticed in other forms of R. dauricum, though not as pronounced as here. Although a coincidence of several factors being responsible for this phenomenon can not be excluded, at least until proof by genetic studies is obtained, the opinion as expressed on the basis of my observations seems to me of having foremost viability.
Because of its early flowering the evergreen R. dauricum was almost exclusively the pollen parent. It will be noted where not.
The crosses which were made with R. dauricum semper-virens and which were successful are discussed below, under the name of the other parent:
1. R.’Augustinii Hybrid’. This cross produced hybrids which have flowers of light to medium violet-blue. So far, of all R. dauricum sempervirens hybrids, I like the light blue form of this cross best. It has widely funnel-shaped flowers, 5 to 6 cm across, of a delicate colour (Violet-Blue Group 91 or 92C, Lobelia Blue). The leaves are oblong-lanceolate, 1.5-2 x 6-7 cm. It is a hardy, robust growing shrub, likely to be tall when mature. Covered with flowers it is a conspicuous sight in the Spring garden, appearing not unlike R. augustinii itself.
2. R. campylogynum, form with claret coloured flowers. Of all hybrids the characteristics of the pollen parent are most noticeable in this cross.
3. R. fastigiatum. Somewhat taller growing with less glaucous to medium green leaves. Flowers are larger than those of the mother plant and of pleasant shades of bluish purple.
4. R.impeditum,the type. Size of plants as well as of flowers are somewhat larger than that of the seed parent. Flower colours are from light to medium dark shades of purplish blue.
5. R. impeditum, a clone of very dwarf growth. These hybrids are rather typical for the pattern of inheritance, as postulated. They are, as the mother plant, dense, compact, small shrubs with comparatively large medium dark purplish violet flowers. Leaves deviate most from those of the seed parent, being I to 1.5 cm long and almost oval.
6. R. patulum, The exact identity of this species (?) is in some doubt, since in my opinion it also has several of the characteristics of R. pemakoense. Much the same can be said for its hybrids as was said for those under 5.
7. R. russatum, a low growing, somewhat twiggy form with deep violet flowers. The hybrids are on the leggy side, growing taller than the mother plant. Flowers are larger, good shades of deep to medium violet, rather more attractive than those of the seed parent. Leaves are broadly elliptic, 2 to 4 cm long.
8. R. moupinense. It was R. dauricum sempervireas which was the seed parent in this cross. All seedlings were lost. Most perished within a year, as they were unfortunately transplanted next to a black walnut tree. The remaining few were devoured by a rabbit; no doubt a gourmet’s delight.
9. Open pollinated seedlings of R. dauricum sempervirens. The seedling plants look like a ‘semi-deciduous R. mucronulatum’, hardly reminiscent of R. dauricum. Since a R. mucronulatum was planted next to the seed parent one must assume that a natural cross occurred. These open pollinated seedlings are the most rampant growing rhododendrons I have ever seen. They developed from seed in less than four years into shrubs 110 to 120 cm tall. The abundant flowers are quite large, over 5 cm across, in shades of light lavender pink to Purple Group 78 B-D.
The climate of southern Ontario is mild in comparison to the rest of Canada except for coastal British Columbia. All hybrids of R. dauricum sempervirens were plant and bud hardy here, in Plant Hardiness Zone 6, where they withstood, without damage, temperatures as low as -25C. This, in spite of being planted in a fairly open location, without any particular shade and Winter protection and with hardly any snow cover during the last two Winters. Although this may be remarkable in itself for a R. augustinii hybrid, since this species is rather on the tender side, all hybrids will have to undergo much harsher testings and further observations in order to ascertain the limits of what they can endure and before appropriate appraisals can be made.
Reprinted from:
The RSC Bulletin 1981, Vol. 10 No. 1
