QBARS - v32n4 The Hardy True Yellow - A Dream?

The Hardy True Yellow - A Dream?
Dennis Mac Mullan, Greenwich, Conn.

Yellow! A word and a color that brings shivers of joy and wonder to the rhododendron lover. But, a hardy, deep yellow elepidote? Is it really a possibility or just a summer's dream? Not 'Goldsworth's Yellow', or R. aureum 's paled yellow, but a hardy Crest-like yellow - something akin to R. lacteum , but hardy to -20. It is something worth more than just pondering. It may not be here as yet, but it is being worked on, and may arrive any time.
Here are a few of my thoughts on how and why this elusive plant will be bred...in fact it may have already happened - more quietly than expected.
The hardy true yellow will probably be dwarf or semi-dwarf in size if for no other reason than the closer to the ground it grows, the better its chances
for survival in winter. It will benefit from snow cover (when available), and be shaded to a degree by taller plants from winter sun and wind.
There are a number of good yellows available that are hardy to -5. That is not quite good enough for our purposes. But by utilizing what we have, and following what we hope are logical genetic principles, we can make a stab in the right direction. The plants listed are those that I feel present the best current choices from which to proceed. There are many un-named hybrids which would be included here if they were known to me, but a sample list must suffice. Many known hybrids that are hardy have been left off my "yellow list" for a number of reasons which I hope to cover later in the discussion. The plants are listed for their usefulness in further breeding for the hardy true yellow, and that reason only.

Hybrid Parentage Hardy To Comments
'Canary' campylocarpum x caucasicum luteum -10 Deep lemon dwarf to 3'
'Coral Velvet' yakushimanum hybrid (?) -15 Coral fading to cream; dwarf
'Doubloons' 'Carolyn Grace' x 'Moonstone' 0 Medium yellow and dwarf
'Full Moon' 'Hawk Crest' x 'Adrian Koster' -5 Color of 'rest' more compact, great foliage; 4'
'Goldstrike' ( oreotrephes x 'Royal Flush') F2 0 Not too hardy, but what a yellow! Almost buttercup. To 3½ feet
'Hello Dolly' 'Fabia' x smirnowii -10 Yellow with orange-rose shading; Indumentum; 3'
'Holy Moses' smirnowii x 'King of Shrubs' -5 Yellow bi-color. Notice how smirnowii , which is pink, is useful in carrying the yellow pigment in some of its crosses. 4'
'Hotei', P. A. 'Goldsworth Orange' x ( souliei x wardii ) -5 Deep canary with blue-green foliage. Current favorite for hybridizing. To 3'
'Moonstone' campylocarpum x williamsianum -5 Cream yellow, but useful due to dwarf habit and campylocarpum blood. 2'
'Miniwhite' aureum x maximum -25 See notes that follow.
'Serendipity' yakushimanum x aureum -20 See notes that follow.
'Odee Wright' 'Idealist' x 'Mrs. Betty Robertson' -5 Beautiful foliage - waxy green & compact habit. 3'
'Peking' ( catawbiense var album x Hawk g.) x (La Bar's White x 'Crest') -15 An example of the type of start we are looking for.
'Butter' 'Champagne' (Dexter) x campylocarpum -15 See note below*

*This plant well could be the first of the good hardy yellows of the type we are discussing. The breeder, Ed Bobbins, died a few years ago. The original plant is in safe keeping in a "hidden" location, and a New Jersey nurseryman has been given the rights to propagate the plant. To the best of my knowledge, no one has been able to buy a rooted cutting or a small plant. It would seem a shame if 'Butter' became a hidden treasure, and was not made available to others who would like to share in its beauty - or to use it for further hybridizing.

Species Rating Description
R. campylocarpum 'Hooker's form' -5 More compact than var. elatum or unnamed clones. Considered by many to be the best all-around source of yellow pigment and hardiness in species.
R. lacteum +5 Deep yellow; but passes its cranky habit, lack of hardiness and slowness to flower on to progeny. Also taller growing than many other sources.
R. wardii , 'K.W. 4170' 0 The Exbury form used by L. de Rothschild in many of his most successful crosses. Best for purity of color in wardii's . No blotches or spotting in the flower.
R. dichroanthum +5 Genes for orange present. Thought by many to deepen the intensity of yellow in crosses. I prefer using a "purer" source; also take the lack of hardiness into account.
R. brachycarpum var. tigerstedtii -25 A form which seems to carry intensity of yellow pigment in crosses more than other forms of brachycarpum. A cross of this with a wardii by Herr Hobbie produced a very fine, hardy yellow. I do not believe that it is commercially available. Inquiries of H. Hobbie did not result in a reply.
R. aureum also called
R. chrysanthum )
-15 Not generally recommended for its pale color - though there are variable forms from seed c. w. - but for its hardiness and extremely dwarf habit which it imparts in all its crosses.
R. catawbiense var. album , 'Powell Glass', F5 or F6 -30 For its hardiness; purity of white color, and ability to absorb the yellow genes of another plant.

I have only included one R. yakushimanum cross, and omitted yakushimanum itself in the species list because as hybridizers have discovered it passes on its dwarf habit and relative hardiness, but does not generally deviate from its pinkish-white color in crosses. Of the many crosses that have been made with deep red plants, a true, deep red has not evolved in an F1 cross. I have a few plants selected from crosses of yak with caloxanthum , a good yellow species, and they are not what I would call "yellow". Basil Potter's 'Serendipity' is as yellow as I have seen an F1 cross with R. yakushimanum , so perhaps there is hope in this area. Personally, I would use 'Coral Velvet', a supposed yak hybrid, since its coral color fades to cream. This plant, which also roots with ease, might be a better choice.
Utilization of 'Powell Glass', F5, or even R. maximum (with its many faults) in a pale form in a cross with a deep yellow R. lacteum - and then crossing the best in resulting siblings - should result in a fairly good yellow, but it would most likely result in a large plant at maturity. Our friend Basil Potter, living in the "ice-box" region of Port Ewen, N.Y. has an excellent plant from a cross he made of R. aureum x R. maximum . This dwarf, 'Miniwhite' - and it is exactly that - is very hardy, and shows promise for future hybridizing. Some pollen of R. campylocarpum , lacteum or say 'Hello Dolly' placed on it should result in a few plants that would retrieve the yellow genes of R. aureum - and its dwarf habit - and add that deeper yellow that we are seeking. Incidentally, R. campylocarpum and R. wardii were rated the best choices in species for parental stock in breeding for yellows in a study by Messrs. Santamour and Pryor (October 1973 ARS Quarterly).
In working with a tender plant as part of the yellow hybridizing program it is important to note that while it would be logical to assume that the best (deepest color) seedling of a cross of, say, R. lacteum and 'Powell Glass', F5 would also be closest in hardiness to R. lacteum , this is not necessarily the case. The theory of "hybrid hardiness" has been more and more evident. The seedling that is the deepest yellow may well be just as hardy as the seedling that is "pure" white.
I have not included many of my favorite yellow hybrids or species in the preceding list because, for various reasons, I do not consider them to be the best plants for our hardy hybridizing program. Neither did I include many of the orange flowered plants - mainly because of the "muddied" color that results when using these fine plants. Also, most of the more orange colored plants are tender - usually +5 or higher.

Some of the crosses that I would like to see are :
'Serendipity' or 'Miniwhite' x 'Goldstrike'
'Canary' x 'Hotel' 'Butter' x 'Canary' 'Peking' x 'Serendipity'

A few of the crosses that I have made may be of some interest to the reader.
I hope the logic (?) of these crosses is evident.

'Holy Moses' x ( R. chlorops 'Lackamas Cream' x R. aureum )
'Jordan' x ('Hotei' x R. aureum ) ('Hello Dolly' x R. wardii ) x 'Coral Velvet'
('Hotel' x 'Moonstone') x 'Coral Velvet'
['Powell Glass', F5 x ('Fabia' x R. aureum )] x R. aureum

Most of these crosses are hardy to -10, which obviously is not the answer. The 'Powell Glass' cross is hardy to below -20.
No doubt there will be many disagreements concerning my choices. I would expect it...but "that's what makes horse races." There are probably substitutes or additions that would make sense. Chacun a son gout.
My reasons for writing this article are hopefully to stir up some interest on the part of amateurs like myself in the Northeast and Midwest to attempt a few crosses that seem genetically sound, and see what develops. Pollen from the more tender species and hybrids is freely given by most of our friends in the Northwest. The true hardy yellow will eventually be developed - and it will probably come from someone in the Northeast or Midwest - or at least it should.
The reason is simple. It is in our area that such a plant is needed! Washington and Oregon have the R. 'Crest', 'Idealist' and many of the lovely but tender species mentioned in this article. They will continue, as will Exbury and other areas in England to breed for deeper yellows, but not necessarily for hardiness. They will breed plants that are suitable for their climate - and so they should. It is our challenge, and it will be to our benefit when achieved.
If there is any interest in such an effort, perhaps some loosely organized study group might be formed that will exchange ideas, pollen, etc. in order to avoid duplication of effort, and have the knowledge that they are part of a larger effort to breed the elusive hardy yellow.
As a postscript, how about this as a brave attempt...[('Hotei' x 'Canary') x ('Serendipity' x proteoides )] x ('Miniwhite' x lacteum ).