Many if not most rhododendron lovers, upon seeing the beautiful truss of a new hybrid, see only its color, size, fullness, and all of the other attributes that make it beautiful, and for them this is all that is important. Those of us who hybridize rhododendrons, however, see more. We see a combination of features that we know are derived from its parentage, from those species which, in combination, make up the genealogy of each hybrid. A tall conical truss most likely derives from catawbiense heritage, a yellow truss probably contains R. wardii or R. lacteum, and a truss with huge flowers may have R. griffithianum somewhere in its past. The same analysis can be applied to foliage and plant form, of course, and I think most of us can identify a hybrid with 'yak' parentage.
For the hybridizer a study of the species and their hybrids which, together, are then combined to produce new hybrids can provide an intelligent framework for the development of strategies that will lead to new and better hybrids. The following article is the first in a series, each of which will focus on one species and the hybrids derived from it. Emphasis in these articles will be placed on understanding the positive and negative features of that particular species and how crosses can be designed to accentuate the former and alleviate the latter.
I have chosen R. griersonianum as the "featured" species for this first effort. There are many reasons for this, all of which are indicated by an interesting statistic; a random tally of Salley and Greer's Rhododendron Hybrids (1992) determined that over 10% of all registered hybrids contain griersonianum in their parentage, a fact which supports the statement by Leach in Rhododendrons of the World (1961) that griersonianum has been used as a parent more than any other species. I suspect that in recent years hybrids with R. yakushimanum parentage may have surpassed the predominance of griersonianum, but I think this reflects the wonderful contributions yakushimanum has made to invigorating the hybrid gene pool and does not indicate that griersonianum has lost its value as a parent.
The species. R. griersonianum was introduced by George Forest in 1917 and was named for his friend and assistant, R.C. Grierson. It grows in thickets and in forest clearings at altitudes ranging from 7,000 to 9,000 feet in the western Yunnan province of China and in Upper Burma. It is a distinct species, as indicated by its assignation to subsection Griersoniana, of which it is the only member. It is characterized by long, pointed flower buds enclosed by a rosette of small, elongated bracts, by long and narrow leaves, and, most importantly, by the uniquely colored, vivid orange-scarlet flowers, often referred to as geranium scarlet. It is the unusual flower color which most accounts for the extensive use of griersonianum as a parent, providing a source of rich, brilliant pigmentation that strongly influences flower color of its hybrids.
The corolla also is a strongly positive feature, funnel shaped, dilating abruptly from a narrow fluted tube to a 3-inch wide trumpet, hairy on the outside. Trusses, unfortunately, are fairly loose, although they contain up to 12 flowers. Leaves are lanceolate, dull green, four to seven inches long, covered on the upper surface with indumentum on new growth, and on the lower surface with whitish to pale brown indumentum, which is retained through maturity. On the negative side, the plant form is relatively poor, open and upright, in contrast with the dense compact form evident in the preferred modern hybrids with yakushimanum. Leach reports that the various forms of the species vary widely, with some exhibiting a thin corolla texture and a dilute color, whereas others, in particular the Exbury form, having especially fine flowers, with a deep, vibrant crimson shade "which seems to glow in the landscape".
As expected from its origin at moderate latitudes, R. griersonianum is not very hardy, only to about 10°F, but, fortunately, it appears to gain hardiness with age, and many of its hybrids are hardier than would be predicted based on hardiness of the combined parentage. It is quite tolerant of heat and a dry siting, but is susceptible to foliage burn with too much exposure. It sets bud quite young, often flowering three to four years from seed. Last, but certainly not least, is the late time of flowering of griersonianum, which is a useful characteristic for extending the flowering time of its hybrids.
The hybrids. There are over two dozen primary species crosses including griersonianum as either the flower or pollen parent, most of which were produced in the first half of this century, mainly in England. Some of the more important of these, either as parents of well known hybrids, or as excellent plants in their own right, include 'Elizabeth' (x forrestii spp. forrestii Repens Group), 'F.C. Puddle' (x neriiflorum), 'Fabia' (x dichroanthum), 'Jock' (williamsianum), 'Matador' (x strigillosum), 'May Day' (x haematodes), 'Azor' (x discolor), 'Tally Ho' (x facetum), and 'Arthur Osborn' (x sanguineum ssp. didymum). 'Jock' is a charming hybrid with dark rosy pink flowers with an orange tint in the throat, displaying the scarlet-orange heritage from griersonianum. Because of the dominant influence of williamsianum, its habit is dense and spreading, and it does well in the sun. 'May Day' is another fine garden plant which fortunately has inherited the broad plant habit of haematodes, and the flower color, a brilliant orange-scarlet, is a combination of both parents. 'Arthur Osborn' also retains the smaller plant form of its parent, didymum, and the flowers are a very dark, almost black red, which bloom very late in the season. Perhaps the best combination for plant form and flower is 'Elizabeth', combining the low, spreading habit of forrestii Repens Group with large trumpet shaped flowers, derived from griersonianum. Flower color is a fine, rich red.
Finally, there is 'Fabia', the most widely used griersonianum primary species cross used in hybridizing. The combination of orange flowers of dichroanthum with the orange scarlet flowers of griersonianum results in a variety of flower colors ranging from orange, to orange with salmon edging, to a nearly pure salmon color, which are displayed by the many members of this large grex. Unfortunately, the lax trusses of both parents are retained, and the open plant habit of griersonianum is only partly tempered by the low spreading form of dichroanthum. As a parent for further hybridizing, however, 'Fabia' is the Diva of rhododendron hybrids, accounting for nearly a fourth of all griersonianum hybrids, and is easily the most common parent listed in Rhododendron Hybrids. To list the hybrids derived from the parent is obviously beyond the scope of this article, as a casual perusal of Rhododendron Hybrids will show. It often provides the orange shades to many of the more complex hybrids, and when combined with yellow crosses, the dichroanthum heritage will synergize to deepen the yellow color. Often, however, hybrids with 'Fabia' parentage will glow with that warm mixture of pink suffused with yellow and orange, a combination resulting from the rich orange-scarlet of griersonianum lightened by other parents, such as seen in the complex yakushimanum cross.
And then there are red and pink crosses resulting from griersonianum crossed with various hybrids. Among the reds the best known is 'Vulcan' ('Mars' x griersonianum) and the opposite cross 'Vulcan's Flame', very similar crosses with bright, fire-red flowers, dark green, pointed leaves, and rounded plant habit. The cross is surprisingly hardy, typically rated at -10° to -15°F, evidence of the relative hardiness of griersonianum hybrids. It also inherits the heat tolerance of griersonianum. Probably the best known pink is 'Anna Rose Whitney' (griersonianum x 'Countess of Derby'), a deep rose pink that seems to glow, certainly a result of the unique griersonianum pigmentation. Unfortunately, it is not particularly hardy (-5°F), although again hardier than might be expected considering that 'Countess of Derby' is only (-5°F) and griersonianum is +10°F.
The Plan. I could continue on in this vein for pages, but in this article I only intend to give examples which represent the range of crosses derived from griersonianum and how its characteristics are passed along. Further, as a hybridizer, I would like to suggest new avenues to pursue in the use of griersonianum, with the following goal. I would like to develop a hybrid with all of the best attributes of griersonianum, i.e., its rich orange-red flower with large trumpet shape, its dark green foliage with indumentum, its heat hardiness (important for New England summers), and its precocious blooming. I would like to remove the least desirable characteristics, i.e., lax truss, open plant habit, sun intolerance, lack of hardiness. In other words, I want a new and improved griersonianum, a concept similar to the artificial species that C.J. and Charlie Patterson are working on in their hybridizing program.
To start on this goal, I would make a griersonianum x yakushimanum cross, if it has not been made, or if I can't find it (I've looked, let me know if you hear of one). This would provide many of the missing ingredients: hardiness, plant habit, sun tolerance. Other species which might supply the same virtues are R. hyperythrum, R. pachysanthum, R. aureum (except for sun tolerance), and R. brachycarpum. Other possibilities which would provide some of the desired characteristics are R. catawbiense, (white forms such as 'Ken's Find', or 'LaBar's White', although frankly I'm not impressed with the plant form on any catawbiense), or one of the members of the Taliensia section, such as R. adenogynum. Of course, all these crosses would be pink, and fade to white, thereby losing the rich orange-scarlet of the species. No problem. Take the best progeny from these crosses and cross them with siblings or with progeny from the other crosses. This is called back crossing, which is the way to get back to the color, flower shape, and heat hardiness of the griersonianum, while retaining the hardiness, foliage, and sun tolerance of the other parent(s). In theory, one fourth of the progeny from these crosses would reacquire the striking color of griersonianum, but a much smaller percentage would display all of the good characteristics from both parents. I figure maybe 200 seedlings from the backcross grown on, tossing those first with poor foliage and plant form and hoping that one of the remaining will be all that your looking for.
How, you may ask, am I going to make crosses using a species that is hardy to only +10°F when I live in New England? Two ways, I answer. One, get some griersonianum pollen from the Rhododendron Species Foundation. Two, buy griersonianum seedlings from the RHS, plant them in two gallon pots, grow them outside during spring, summer, and fall, and put them in your basement (cold) over the winter (C.J. Patterson has promised me an article soon on overwintering tender rhododendrons). I have two griersonianum seedlings which I received from RHS two years ago and both budded up this year. One is only 10 inches tall and has four buds. It has made it through every winter just fine and I can't wait until early June to see the flowers and to start making some of the crosses I just described. But it's a numbers game, this hybridizing, and the more in on the chase, the faster we'll get there, so join me. I think the goal is worth it. Imagine being greeted one fine late May morning by full trusses of rich, glowing, orange scarlet on a dense, compact plant with indumented dark green leaves which grows well in New England in a hot sunny location. To die for.