JARS v39n3 - Forget the Flowers - Breed for Leaves
Forget the Flowers - Breed for Leaves
Dr. M.J. Harvey
Halifax, Nova Scotia
Let me start with an anecdote: In late summer my sister-in-law, no gardener she, was visiting and I was showing her my small collection of rhododendrons in the garden. I was getting enthusiastic about the promise of various young plants and she was looking at them with increasing puzzlement. Finally her puzzlement got the better of her and she just had to ask, "Do they have flowers?" This took me aback but after assuring her they would I got to thinking that my own interest in rhododendrons was blinding me to some obvious faults in the plants I have. That led to a train of thought that resulted in a resolve to entirely change what I am doing in Rhododendron breeding and these ideas I set out below.
The "Do they have flowers?" remark was obviously caused by the fact that many of my plants are small, leggy and weevil bitten. For purely ornamental value as evergreens they compare unfavorably with the average suburban yew, juniper or holly. As my sister-in-law implied the only conceivable excuse for growing such straggly plants would be if they had wonderful flowers. Looking round my neighborhood I can now see why rhododendrons are less often chosen by non-enthusiasts for their gardens than I would expect. The plants are more expensive than other evergreens to purchase and after a few years of neglect, the result is a leggy shrub less satisfying than a well clipped yew.
Except in some climatically blessed areas the elepidote rhododendrons, which are what I am writing about, are difficult to get into absolutely tiptop shape. Gus Mehlquist in the 1979 Proceedings of the Hybridizers Meeting, New York Chapter, points out why. The reasoning goes: the general nursery trade is concerned with making money, time is money in the sense that a plant that can be grown to saleable size in a shorter time makes more money. Hence, the non-specialist trade is concentrated on a few rapid-growing, early budding varieties which can be grown to a superficially attractive state to attract naive buyers. The fact that these may be inherently unsuitable varieties for the use to which they are subsequently put is no concern to the nursery trade, their responsibility ends with the snapping shut of the till, caveat emptor .
Well, what are we enthusiasts doing to counteract this dangerous trade practice? I will tell you what we are doing, we are holding truss shows. We line up flower heads sitting in little jars and the judges walk up and down the rows giving points for bigger flowers, brighter colours, frillier petals, etc. What, you may be thinking, is wrong with that? The annual show is, after all, the most important event in the calendar of any rhododendron society, it is a harmless and entirely admirable exercise which brings everyone together and sets the standards by which we judge our plants. No! I maintain that shows are harmful, that they force us into a mould of our own making, that as a result our scope becomes limited, and that we develop tunnel vision which prevents us from appreciating certain properties of plants because our value-system has not included these features.
The disastrous effect of truss shows is that they have caused growers to neglect the rest of the plant. All our attention has been concentrated on the flowers. Hence, I am campaigning for a better framework and better leaves for our plants. If I had my way shows would be held in autumn when there are no flowers to distract our attention.
What do I want? Well I am writing this at Christmas so let me draw up a list of things I would like to get. In other words a list of the characteristics that my ideal rhododendrons would possess. And by putting it this way I am not joking. This is the way to start a serious breeding program. First draw up a list of features which the final product is required to have, then go out and look for parents with those individual characteristics and finally devise a breeding scheme to bring together the desired character combinations. I am convinced that we have not used a wide enough range of species in our hybridizations. With over 900 species we have so far only tapped a fraction of the gene pool that exists in the genus.
As I have implied already, my ideal suburban elepidotes should look at least as neat in the non-flowering state as well grown yews, hollies or junipers. Most of our current commercial varieties tend to grow too large and open for the average garden so I would like a slower rate of growth. This means shorter inter-nodes so I would favor the more dwarf species as potential parents. Dense leaf cover is also important, I would like to see leaves retained for 4 years. Weevil resistance is important in my garden so unpalatable parents have to be found. In the ideal world I would like a series of forms including prostrate, low mounding, globular and fastigiate growth with a choice of gray-white, deep green, yellow green, purple and variegated foliage. Unfortunately unusual leaf pigmentation and stable chimeras are rare in rhododendrons unlike hollies and maples, so I shall probably have to do without these forms.
Hardiness is a complex characteristic not due to a single or limited number of genes and while, for my own use, I would like plants hardy in Halifax, Nova Scotia (climatically not too different from Boston, Mass, but with cooler summers), the crosses I am going to suggest need to be tried in a range of climates both on the East and West Coasts. There will be a range of heat and cold-hardy forms generated and only by testing them in situ will we know their tolerances.
Finally I have a personal liking, I love densely indumented leaves. I like the appearance and feel of a thick felted layer of hairs and I also note that my plants with indumentum do not get bitten by weevils, so I have two reasons for breeding for a heavy indumentum. Whether it is the hairs that repel the weevils or an unrelated extra concentration of insect-repellent chemicals I do not know.
So that is the list of characteristics that I would like to see combined in a variety of ways to produce beautiful foliage shrubs of various heights. We can now start the search for parental species which will contribute these features. Note that I have not mentioned flowers so far. With rare exceptions all rhododendrons have beautiful flowers and by concentrating on the framework and leaves we are bound to come up with some good flowers but of course not prizewinners in truss shows. As you will see, most of the flowers will turn out to be pale to medium pink. There are, however, various ways of making the flowers more interesting and I shall mention these later.
From my own limited experience and by thumbing through various books I have come up with a preliminary list of species which look as if they have useful genes to contribute to my proposed hybrids. Subsections Taliensia and Pontica would seem to have the most to contribute.
Subsection Taliensia: R. adenogynum , R. balfourianum , R. bureavii , R. clementinae , R. pronum , R. proteoides , R. roxieanum , R. taliense , R. vellereum , R. wasonii , R. wiltonii .
Subsection Pontica: R. adenopodum , R. degronianum , R. hyperythrum , R. makinoi , R. smirnowii , R. yakushimanum .
Subsection Glischra: R. recurvoides (formerly in Taliensia).
Subsection Lanata: R. tsariense .
Subsection Neriiflora: R. beanianum , R. gymnocarpum , R. haematodes , R. mallotum .
The Taliensia themselves are beautiful foliage plants of dwarf or medium height but the better forms are fastidious and impossibly slow growing and hence rare in cultivation. Cecil Smith recently reported on his experience of growing R. proteoides which is, after the prostrate R. pronum , probably the most dwarf species in the group. After 30 years it was 13 inches (33 cm) high and twice as wide (A.R.S. 37:1 1984). Other members are taller and a few, such as R. bureavii and R. roxieanum , have achieved a moderate degree of popularity. Even so it is obvious that the pure species will remain connoisseurs' plants but they contain valuable genes which will contribute dwarfness, beautiful leaves and long leaf retention to their hybrids.
Section Pontica by contrast is extremely popular since propagation is relatively easy and plants are hardy and attractive. The species R. degronianum , R. makinoi and R. yakushimanum are closely related and have been regarded, quite sensibly, as subspecies of R. metternichii adapted to different habitats. I retain the separate species names for my purposes because the plants contain distinct and horticulturally useful genes which merit recognition. R. yakushimanum has had a meteoric rise in popularity since its exhibition and receipt of a First Class Certificate from the Royal Horticultural Society in 1947 and in itself fulfills many of my requirements in that it has attractive foliage, is dwarf and is weevil resistant. I should probably include some other plants in the metternichii group such as var. tsukushianum .
R. recurvoides is so dwarf and slow growing that for many years it was placed with the Taliensia but the bristles on the stems are distinct and its chemical signature fits into Glischra .
Subsection Neriiflora contains some dwarfs such as R. gymnocarpum (which is usually placed in Taliensia because of its short stature) as well as tall, straggly shrubs (at least when grown in shade) like R. mallotum . However, the leaves of R. mallotum are of such high quality with their rust-orange indumentum that hybridization with a very dwarf species is indicated.
The Neriiflora also give hope of introducing reds into the hybrids and might help to spice up the predominantly pale pinks of the Taliensia and Pontica. For yellow flowers there is not a wide prospect, the hybrid R. wasonii x aureum would yield a pale yellow since both parents are available in fairly good yellow varieties. R. aureum (chrysanthum) is an extreme dwarf in Pontica. I did not include it in the list above because it lacks indumentum. The genetics of indumentum inheritance is that generally indumentum is recessive, so any hybrid including a non-indumented species is itself non-indumented.
Plan Of Hybridization
I have listed 23 species and my somewhat weak arithmetic tells me that, taking them two at a time, there are 253 possible F1 hybrids. Obviously, some are going to be more important than others and it is possible to speculate as to which these might be. I must admit though that I would find it extremely educational to have a field with 253 rows in it, each row composed of one of these hybrids.
a) Hybrids within Subsection Taliensia
There is the possibility of hybrid vigour when the various members are intercrossed. Hybrid vigour is not an effect which can be predicted and it turns out in practice that some hybrids lack vigour or are debilitated. The best approach would be to make hybrids between some of the taller species such as R. adenogynum , R. balfourianum , R. bureavii , and R. clementinae , and some of the dwarfs such as R. clementinae , and some of the dwarfs such as R. proteoides , R. pronum and R. taliense . The probability is that the F1 hybrids will be extremely attractive but will still have special requirements which will restrict their use.
b) Hybrids within Subsection Pontica
Some of these hybrids have already been made. There is a R. yakushimanum x smirnowii in Halifax planted by Dick Steele many years ago which closely resembles R. yakushimanum . Both R. smirnowii and R. yakushimanum have been extensively used over the past twenty years but mainly to pollinate ironclads or other complex hybrids. Most of these offspring have been unhappy combinations combining the hairless leaves of one parent with the dilute flower colour of the other. However, used on other indumented Pontica there should be some handsome plants of immediate garden utility without the need for further breeding.
c) Hybrids between Pontica and other subsections
These are possibly the most promising set of hybrids in offering potentially attractive combinations of height and leaf characteristics. R. yakushimanum x R. bureavii is already available commercially from specialist nurseries and is very attractive. While I do not think that R. yakushimanum has been overused, I certainly think that other Japanese species such as R. makinoi , R. hyperythrum and R. degronianum have been neglected.
d) Hybrids between Neriiflora and the other Sections
These, as mentioned earlier, offer the possibility of red-flowered hybrids since the red colour often comes through in their offspring. How the notorious colour-diluting effect of R. yakushimanum will work has to be seen and some backcrossing may be needed in that particular case. I would particularly like to see R. bureavii x mallotum because I think the leaves will be excellent and the plant should be of medium vigour. R. gymnocarpum hybridized with the more dwarf Taliensia should also produce some interesting plants of a low nature.
Now separately and on an unplanned basis some, possibly quite a few, of the F1 hybrids I would like to see must have already been made. For instance, Cecil Smith mentions having distributed seed of R. proteoides x R. yakushimanum through the seed exchange and presumably there are plants of this now grown in a good size and flowering. Could we have a report on these? How fast did they grow? How big are they now? How long do the leaves persist? These and other hybrids are probably sitting in gardens not given the scrutiny they deserve because our flower-oriented evaluation system down rates them. If you have one let me know.
Among the hybrids that have achieved some recognition, the following deserve mention. R. yakushimanum x R. pseudochrysanthum was described by Jim and Betty Caperci in the Fall 1980 A.R.S. Bulletin and the name 'Golfer' proposed for it. The photograph shows a dwarf plant with attractive gray indumentum-covered young leaves not unlike some of the forms of R. yakushimanum itself. They make the point that it is a beautiful plant even for the 11 months of the year that it is not in flower. The heavy indumentum is a slight surprise since the R. pseudochrysanthum parent has only a sprinkling of hairs on the veins of the underside of the leaf.
'Ginny Beale' is a Gable hybrid introduced by Davis in 1979 which I would guess from the parentage ( R. metternichii x R. adenopodum ) is taller growing than 'Golfer'. On the other hand 'Serendipity' ( R. yakushimanum x aureum ) is an extreme dwarf with yellow flowers but I gather is glabrous (lacks hairs) and so is of less interest to me. It might however be useful in further hybridization. Doubtless, there are others I do not know about.
Certain species are going to prove to be more valuable than others in making good-looking F1 hybrids. I would guess that among these will be R. roxieanum , R. proteoides , R. bureavii , R. recurvoides , R. tsariense , R. gymnocarpum , R. smirnowii and R. yakushimanum . I must emphasize, however, that I do not want to produce a bunch of yakushimanum look-alikes. There is quite enough genetic material to produce a range of diverse bush shapes and leaf types.
Some species appear to be virtually unknown as parents of hybrids. The extreme dwarf R. pronum is among these, also the medium height R. clementinae of which I once heard the remark that it must be beautiful because the discoverer (Forrest) named it after his wife. While, as a scientist, I feel there may be a logical gap in the argument, I would still like to see a series of R. clementinae hybrids.
So far, I have discussed the prospects for handsome plants from F1 hybrids and I think that there is indeed a great scope here given a careful selection of pairs of parents. However, the release of genetic variation only takes place in the F2 and subsequent generations so I suggest that the major effort should be concentrated on raising larger numbers of seedlings of various later generations. In this way the more extreme combinations of various characteristics may be obtained. There are several breeding strategies which can be adopted as illustrated by the following hypothetical examples:
) selfed or sibbed
2. ( R. yakushimanum x R. bureavii ) x ( R. yakushimanum x R. proteoides )
3. ( R. yakushimanum x R. bureavii ) x ( R. metternichii x R. adenopodum )
In the first case, which represents an F2 generation in the traditional usage, the seedlings will in some cases show a greater influence of R. yakushimanum , in others more of R. bureavii and there may be a small proportion which will show unusual combinations of characteristics never seen before. The second case, a cross between two different F1 hybrids, has a greater chance of yielding unusual forms but since both F1 parents have a common species, the overall influence of R. yakushimanum will be seen.
The third case, where there is no species in common (except insofar as R. yakushimanum and R. metternichii are closely related), has in theory the greatest chance of producing really interesting offspring and my instinct would be to concentrate on this general type of cross if there are sufficient F1's already existing. I therefore regard it as important to discover the whereabouts of as many already existing and mature F1 's as possible and start a cooperative cross-pollination scheme. Finding these plants is important because starting to raise F1's from scratch may mean a delay of 3-10 years before they start to flower.
A point that geneticists make when dealing with F2 and similar generations is that to pick up unusual combinations of characteristics one should rear large populations, say between 1000 and 10,000 plants. Clearly, this is impossible for all but the extremely affluent amateur, and this whole exercise has to be amateur based since no professional breeder is going to touch a program whose aim is to produce slow-growing specimens.
One way out of the above predicament is to get a number of people interested in foliage rhododendrons and distribute pollen and seed through their appropriate exchange. The numbers problem then decreases since the rearing of plants can be spread over many people. The space problem is also solvable - keep friendly with your neighbors. I find my neighbors are delighted to be given plants. They think they are getting free rhododendrons. I see it as my having free test fields where the mowing, weeding, fertilizing is done for me. I regard the plants as still mine but naturally I keep this a secret. It is the perfect system, everyone is happy. You can always get a cutting back if a particular plant is outstanding.
Now obviously in this article I have been pushing an extreme view. I do not seriously expect anyone to abandon the search for bigger and bigger flowers, nor give up June flower shows. But if there is any sympathy for my views or you have a mature hybrid I don't know about, write and let me know or, better, write a note for the A.R.S. Journal.
I am quite new to Rhododendron breeding and I have been talking about my ideas to others in the group of enthusiasts in our local Atlantic Chapter of the Rhododendron Society of Canada. They seem on the whole to agree and I have picked up many good ideas from them. I particularly want to thank Dick Steele for his knowledge and for plants and John Weagle for his help. John has already made some of the crosses I was only thinking of.
If these ideas are to go anywhere we need to have several groups both on the East and West Coasts and elsewhere participate in making hybrids, exchanging seeds and keeping records and photographs. Since one of the aims is relatively slow growing plants, the project could easily last 20 years or more, so do not expect quick results. However, I look forward to the day when I show my plants to someone and they ask, "Do they have flowers?" and I can reply, "Does it matter?"