Let's Talk Hybridizing: Back Yard Hybridizing - How to Improve Success, Part 2
Anthony D. M. Knights
Longmeadow, Massachusetts

Part 1 of Mr. Knights' article appeared in the winter 2003 issue. It covered the topics: what to hybridize, goals and objectives, knowledge building, plant analysis and color traits (1-6).

7. Progeny Populations and Hybrid Types.

To reiterate, the key to success in back yard hybridizing is to do your homework and then direct the crosses you make towards objectives and goals you set. Hopefully the previous sections of this paper will help you with program structure, knowledge building, analysis and a general understanding of color traits.

Progeny growth lot sizes are important in the back yard situation. They are related to parental complexity and whether there are complementary traits in the parents of a cross. This is a complex subject so I will endeavor to address it based on my own experience together with what I have gleaned from other hybridizers.

7.1 Progeny - Quantities To Grow?

So how many plants should be grown to flower? You need to consider the complexity of the crosses you make, the difficulty and number of the goals you set, and also consider your capacity to accommodate the progeny of your crosses. If you have limited growing capacity but undertake an ambitious program you will probably have to gamble to obtain sufficient good results from population sizes that are less than ideal.

I have limited growing space and do work towards some ambitious goals. I set multiple objectives designed to yield intermediate results from planned genetic mixes along concurrent parallel paths to establish my fundamental breeding lines. I did not want to put all my eggs in only two or three baskets and have to start over when something important did not work out!

Basically I look at the total population of all my crosses and am satisfied when I get good "usable" results from about one out of twenty or less plants. By "usable" I mean suitable for use in further crosses. For "keepers" my yield is closer to one out of forty; these are plants with good garden merit that are kept for long term evaluation.

I touch on quantities in some of the following paragraphs dealing with hybrid types. I must say that the numbers I quote are from my experience based on results versus quantity grown, for the parents I have used and the types of crosses I have made, but not on any scientific basis. Reference 1 gives some quantities based on the number of genes that may affect a given plant feature, e.g., color, fragrance, blotch, hardiness, height, etc. However, the number of genes involved in most features is not yet precisely established.

7.2 Hybrid types.

There are six main categories of primary hybrids described in the following itemized sections. Categories (a), (b) and (c) are the most commonly used in back yard situations. There is also the basic (species 1 x species 2) cross, where progeny lots of about 10 will usually suffice if you want to try them. You can however, find many of the useful species inter-crosses as named hybrids already existing.

(a) (Complex Hybrid 1 x Complex Hybrid 2).

With this type of hybrid you are least likely to get predictable outcomes. A large number (say 100 or many more) of seedlings should be grown to flower to cover the range of possible results from this type of cross when the two hybrids have many different species in their collective heritage. You should definitely avoid the urge to just make "pretty x pretty" crosses; they are usually a waste of time and energy.

When the two hybrids have known common traits or some common recent heritage in their lineage, the predictability improves significantly and the number of seedlings grown out can be reduced. I often aim for 20, end up with 10 or less, and am generally satisfied with my results when I have used carefully selected parents. When I get a poor result I sometimes wonder whether a few more seedlings would have helped!

On occasions such as a "calculated risk shot" (see Section 3 in Part 1), I use a larger population. My cross of ('Rio' x 'Nelda Peach') was one I felt would be important in accelerating my hardy tropical program and about 65 plants were set out. Since 'Nelda Peach' = ('Woody's Peach' ¹ x 'Lem's Cameo') is quite complex and not very hardy the larger quantity was important to me. Note too that both parents exhibit the common trait for the multi-color features I was seeking.

(b) (Hybrid x Species), or vice versa.

This will usually give emphasis to the "species" features. Careful attention should still be paid to the dominant characteristics of the two parents when planning the cross. Here, I would also target a lot size of 20 or so. By way of an extreme example to avoid in a small back-yard; the German hybridizer Hans Hachmann crossed 'Scintillation' with the yellow species R. wardii and named a good yellow result 'Goldbuckett'. He knew what needed to be done and grew well over 200 seedlings to flower. Only two were yellow! This extreme type of hybrid would obviously have a low probability of success when grown out with a small population of seedlings. It is thus more appropriate for professional hybridizers. It also tells us that 'Scintillation' is most likely a poor parent for yellows in back yard hybridizing.

(c) (Hybrid x Species) where the species is also in the hybrid.

This is a "back-cross" and it is used to give prominence to the species' features in the outcome. There is inbreeding implicit here and there may be runts in the progeny. A good technique is to use a different species form, if available, unless the actual form of the species is important. To deal with the runts, grow out more than the number of seedlings needed to a small size where their vigor can be determined, and then discard the runts.

A small population of seedlings can often suffice, about 20 if obviously expressed traits are present. If recessive traits need to be re-enforced then I would use 40 or more.

(d) {(Hybrid 1 x Hybrid 2) x Hybrid 1 or 2}.

This is also a "back-cross" similar in use to (c) and will give emphasis to the characteristics of the back crossed hybrid in the progeny. It is usually better to use a sibling for the back-cross parent to improve the genetic diversity in the cross.

The ARS Journal has three interesting recent articles (Refs. 2, 3, 4) that discuss "back-crossing," "sibling crossing," and "selfing." Readers should peruse these references if they intend to make crosses of these types. I have not made hybrids in any of these last three categories but the literature indicates that large populations may be required (100 plus).

(e) (Hybrid x A Sibling).

A "sibling cross" is a method to recover a factor that did not express well in the primary (original) cross progeny. The two siblings selected should preferably show some expressed difference that would indicate inheritance from species in the primary cross that carry the sought after factor.

(f) (Hybrid or species x self).

Termed "selfing," this cross type is used mostly in scientific work and sometimes by hybridizers to stabilize genetic traits for future breeding (Ref. 1, 4). With species it demonstrates the degree of variation implicit in the species. In hybrids the variation in a large enough population represents the genotypic character of the hybrid. The populations grown are large (100's).

As an example: 'White Peter' was selected from a very small minority of white plants that occurred in a large population (I think 200 or so) grown when Dr. Mehlquist selfed 'Blue Peter', a hybrid with unknown heritage. In fact very few popular hybrids listed in the literature are the direct result of selfing.

8 Selection of Parents.

8.1 Parental Complexity.

It should be noted that when a hybrid's parental DNA are shared and recombined, as it is uniquely and differently in each hybrid seed, the resultant plant does not necessarily show evidence of a sought after characteristic. With recessive characteristics in particular, these may be evident but more often will be visibly absent but potentially recoverable in a second generation of hybridizing.

In a given sized progeny population the probability of recovering a recessive factor (or even a dominant factor) decreases as its origin is located further and further back in a hybrids' lineage. This situation will be worse when the sought after factor is not expressed in a more recent antecedent such as a parent or grandparent. I therefore hesitate to use very complex parents in my hybrids, even if they have an appropriate species content, unless they exhibit the particular features I want. This is why I emphasize careful parental selection in back yard hybridizing. It also avoids having to grow on large progeny batches in many cases.

I feel it is generally a better practice when feasible, to use parental hybrids suitable to meet one's goals with only a few species in them. You can more easily evaluate what each species is likely to contribute to the result. Also, it should be noted that many of the most outstanding hybrids have simple ancestry. They often prove to be excellent parents that continue to promote "hybrid vigor." Hybrid vigor is a horticultural trait where the progeny of a hybrid attain vigor and robustness greater than that of either parent. I also favor occasional re-enforcement of a breeding line by crossing with, or back to, a species rather than creating a large family tree of hybrids.

8.2 Seed Parents and Cross Direction.

When hybridizing in cold climates it is easier to use seed parents that are hardy for your climate. Pollen of less hardy plants can then be conveniently applied outside to plants that flower after the risk of frost has passed. I believe it is still generally thought that the cross direction, i.e., (A x B) or (B x A), does not have any significant bearing on progeny hardiness. Dr. Mehlquist, who was a professor of genetics who later also took up rhododendron hybridizing, concurred with this view when I asked him about it. I do however remember reading somewhere that the old Dutch hybridizers are said to have favored the hardy seed parent direction as more likely to give the hardiest results. If you should acquire some not-so-hardy budded plants, go ahead and use them as seed parents and pollinate them indoors if you need to. Just bear in mind that there may be an advantage from hardy seed parents, and use this as the preferential direction if your goal is primarily for hardiness.

I recently became aware of two instances in which two experienced hybridizers had independently made different crosses which both involved compact and tall parents. Each hybridizer used their same parent plants to make the crosses in both directions. Both hybridizers were convinced that with compact seed parents the progeny tended to be mostly compact but conversely, when the seed parents were tall, then there were more tall progeny. I do not expect that these were rigorous scientific tests but there could be some validity to a degree of seed parent dominance.

8.3 Some Useful Seed Parents.

When you start your program select seed parents with a proven reputation of garden performance and with a good record as parents amongst the hybridizing community. As your program progresses you will, of course, begin to use parents of your own creation.

Some plants make excellent parents in terms of a high proportion of vigorous and hardy progeny, but some others that have good credentials in their heritage are weaker in this respect. Hybridizers tend to focus on a core group after having tried a larger set initially. Here are a few that I like to use:

'Rio' = ('Newburyport Belle' x 'Newburyport Beauty') but species content unknown (-20°F). A nice compact plant with color that is distinctive in the landscape. The salmon pink, yellow centered flowers show very little magenta tone. It produces many hardy and vigorous progeny from most crosses. I use it for yellow/orange/pink/red mixed colors, i.e., my so-called "hardy tropicals." Fig. 2 in Part 1and Fig. 1 in Part 2 show the best results to date from ('Rio' x 'Nelda Peach').

Figure 1. ('Rio' x 'Nelda Peach') #2. Photo by Anthony D. M. Knights

'Janet Blair' = (Heritage unknown) (-15°F). Fragrant and relatively late flowering so probably contains R. fortunei ssp. discolor or maybe R. fortunei ssp. fortunei , or both. Widely considered to be one of the best parents for flower, truss and progeny quality. Has a transparent, or "friendly," character when crossed with a wide variety of color types and particularly with the pale colors including yellows and whites.

'Percy Wiseman' = (ssp. yakushimanum x 'Fabia Tangerine' - selfed) (-10°F). For me this is hardier than the published data. A good parent for pink and yellow bi-colors.

'Mary Belle' = (Atrier Group x Dechaem Group) (-15°F). Looking at its expanded heritage, which can be stated in the parenthetical form useful for hybridizing analysis as: [('Atrosanguineum' x griersonianum ) x ( decorum x haematodes )], it should be noted that it contains both R. griersonianum and R. haematodes . These are two of the species discussed in section 6.2, Part 1, as important when trying to intensify yellow. It has a unique flower that goes through color phases of peach/pink/ yellow and has colored spots on all lobes. Known good parent for mixed colors and yellows, good for truss, flower size and substance improvement. May need to cull seedlings for tendency to leaf spots or for a drooping growth habit. ('Mary Belle' x 'Percy Wiseman') yielded an attractive multi-color flower on a nice looking plant (Fig. 1, Part 1).

('Rio' x 'Mary Belle') . Six of the first nine that flowered had pink edged flowers with buff yellow centers and yellow throats; only one inherited the colored spots from 'Mary Belle'. I have kept two with excellent truss and flower quality. These are now being used for crosses with yellows and exotic western mixed color hybrids.

'Luxor' = (white flowered catawbiense x 'Goldfort') (-25°F). Cream flowers on a good-looking plant that still has some yellow in it after three generations from R. campylocarpum . It thus intrigues me, and I have made several R. wardii based crosses on it.

'Leach's Nepal'* = [white flowered catawbiense x ( wightii x fortunei )] (-25°F). I have used this for blotched whites and yellows ( R. wightii is yellow), but am still awaiting flowers. It does tend to produce more runts than most of my other seed parents, but its quality, genealogy and extreme hardiness are all good. It just needs larger lot sizes and selection of the more vigorous progeny.

'Nile' = (white flowered catawbiense x wardii - 'L.S. & T. 5679') (-20°F). Very hardy and shows R. wardii features which also show in its seedlings. Takes a few years to flower but I have used it for yellow and multi-colors.

'Shaazam' = ('Pink Twins' x 'Leah Yates') (-10°F). Compact, buds well, and is hardier than its published rating. I have just started using this for mixed colors and it produces seedlings with large root systems, which may aid in hardiness, drought tolerance and root disease resistance.

' Wishmoor' = (ssp. yakushimanum x wardii Litiense Group) (-5°F). Although not entirely hardy this has a beautiful bell shaped cream flower, inherited from R. wardii , with superior substance. I cross it with hardy yellows looking for a good quality hardy cream.

'Bravo!' = {white flowered catawbiense x [ fortunei x ( arboreum x griffithianum )]} (-20°F). Pink with very large flowers and excellent bug resistant foliage. It has the red species R. arboreum in its lineage and I have made a few crosses on it for large hardy reds.

Others I have used are:

'Hong Kong' - For yellows, etc.

'Edmond Amateis' - For whites and large leaf crosses with R. macabeanum and R. rex ssp. fictolacteum .

'Hello Dolly' - Fairly hardy here and flowers well every year. Displays a useful yellow/orange trait from the Fabia Group and I have used it in yellow crosses. Produces a lot of runt seedlings though, which make it an inefficient parent.

'Merley Cream' - For blotched whites. Very hardy for me and it has a beautiful upright truss, but my eyes have yet to detect any cream tone!

'Swansdown' , 'Calsap' , and 'Tiana' - For blotched whites.

'Summer Snow' - For late flowering.

Figure 2. ( R. wardii x R. macabeanum ). A Cox hybrid at Glendoick, Scotland. Photo by Anthony D. M. Knights

9. Some Thoughts On Hardy Yellows.

While at the 1996 ARS Convention banquet at Oban's Corran Halls, there was an intense yellow truss in a vase on my table right in front of my seat! It was from a new Cox hybrid of ( wardii x macabeanum ), see Fig. 2. When I also saw two plants of this at the inspiring Cox gardens and nursery at Glendoick, I was further impressed. Later at the Royal Botanic Garden in Edinburgh I saw, R. wardii - L. S & E 15764 (Fig. 3). This is a deep yellow form with trusses of up to 11 flowers. I decided then to use these two deep yellows in my hybridizing endeavors.

Figure 3. R. wardii . L. S. & E. 15764. At the Royal Botanic Garden, Edinburgh, Scotland. Photo by Anthony D. M. Knights

Many hardy eastern yellows are derived from the Litiense Group of R. wardii var. wardii , which is considered to include the hardiest forms. They are typically pale forms, so even if this yellow is fully expressed in resultant hybrids, a strategy for intensifying yellow must also be involved for deeper yellow.

In the deeper eastern yellows R. fortunei is often used as a key to hardiness. If, however, solid (-20°F) hardiness is to be reached, white flowered R. catawbiense , R. brachycarpum (as ssp. tigerstedtii ) or even R. maximum , or their hybrids should probably be involved early in the hybrid development. Rhododendron fortunei , or its hybrids, can be included later in a program to improve flower size, etc.

I have avoided using yakushimanum in my basic breeding lines for yellows because of its dominant fading trait but may introduce its hybrids later. I also hope to introduce R. aureum , and possibly R. pseudochrysanthum , hybrids for compactness later in the program.

After "dabbling" with yellow for a few years, I am now in the midst of a structured yellow hybridizing program with somewhat different starting points. The keys to this are: firstly, new breeding lines using the deepest yellow species and, secondly, in parallel breeding lines, develop hardy yellow "enhancers." At the next generation I will inter-cross these two basic breeding lines and decide on improvements. The structural key to the second generation is that the deep yellow species and the yellow enhancing species are no further back in the lineage than the grandparent level. This allows for more favorable linkages for deeper yellow and should improve the chance of recovering the recessive factors.

There are now four key elements to my hardy yellow program:

1. New hybrids crossing (-20°F) hardy pale yellows with the two deep yellows discussed above. My first budded plant is ['Luxor' x ( wardii x macabeanum )].

2. At the same time crossing a known "transparent" (-15°F) hardy parent with a yellow intensifying species such as ssp. dichroanthum or ssp. scyphocalyx . My ('Janet Blair' x ssp. scyphocalyx ) plants are near budding size and are, at least, plant hardy.

3. Jim Barlup, whose new multicolored hybrids so caught my attention back in the mid-nineties, has been very inspirational and helpful to me in my endeavors by sharing his experiences (and pollen!). He has created a new line of yellow enhancing hybrids (see section 6.2 in Part 1). These are now also being used as pollen parents in parallel paths of my program.

4. Crossing the results of item 1 with the results of items 2 and 3 and moving forward depending on results.

This program for yellow is mostly work in progress so I will of course have to wait and see. Ideally one is only left with the problem of selecting deeper yellows for hardiness, don't I wish!

I expect I will also get some of my own brand of "yellow frustration," but will certainly continue to enjoy hybridizing, as I hope my readers will.

Acknowledgements.

In closing I wish to thank both Ian Donovan of Pembroke, MA, and Jim Barlup of Bellevue, WA, for sharing their expertise and inspiration as my reviewers. Both devoted significant time in reviewing and critiquing this paper and their creative suggestions were very helpful and much appreciated.

Literature Cited
1. Mehlquist, Dr. Gustav A. L. The Breeders Talk (Abstract from Proc. 1973 Breeders Round Table). The Rosebay, ARS Mass Chapter Vol. XXVIII(2): 13. 2. Krebs, Stephen. 2001. Let's Talk Hybridizing, Taboo or Not Taboo: That is the Question. Jour. Amer. Rhod. Soc. 55(3): 127.
3. Halligan, Pat. 2000. Let's Talk Hybridizing, Incest Among Plants. Jour. Amer. Rhod. Soc. 54(1): 33.
4. Konrad, Dr. Mark. 2001. Let's Talk Hybridizing, Hybridizing with a Genetic Mission: Line Breeding. Jour. Amer. Rhod. Soc. 55(4): 225.

* Name is not registered.

Footnotes
¹ 'Woody's Peach' is an unregistered peach colored hybrid of unknown parentage.