JARS v57n1 - Let's Talk Hybridizing: Back Yard Hybridizing - How to Improve Success

Let's Talk Hybridizing: Back Yard Hybridizing - How to Improve Success - Part 1
Anthony D. M. Knights
Longmeadow, Massachusetts

1. Introduction.

If you have been growing rhododendrons from seed you may want to consider doing some hybridizing and get the added satisfaction of seeing your own creations flower and mature. Perhaps you have considered creating something uniquely different for your collection but think hybridizing is a difficult route to follow. I can assure you that the physical process of hybridizing is very simple; the challenge is more in the process of deciding what you would like to achieve from your efforts and how to get started on a path with reasonable chances for satisfying and successful results.

The various topics covered in this article are designed to provide a beginning framework for aspiring backyard hybridizers. I have tried to translate my own experience in practical terms so that new hybridizers can begin their endeavors with knowledge of all the issues involved and start out with a sense of creative enthusiasm.

The first topic is a brief overview of the main sections of the genus Rhododendron . The next group of topics uses a Typical Scenario to illustrate the key processes of setting Goals and Objectives and then planning your program structure so that you can be productively hybridizing right from the outset.

The next areas covered are Knowledge Building and Plant Analysis, and the purpose here is to help in the analytical assessment of existing hybrids. Using two popular eastern hybrids as examples, I examine how they were influenced by the species and hybrids used in their background. Armed with this sort of knowledge and understanding you can more effectively direct your hybridizing program to achieve results you can be proud of.

A section on Color Traits is included and summarizes the generalities of color inheritance in hybrids in a practical way. As back-yard hybridizers you may, like myself, have limited space in which to grow-on large numbers of progeny. By thoughtfully planning your crosses towards goals you set, you can grow relatively small progeny lot sizes from many of your crosses. This will enable you to use your limited facilities more effectively. The section on Progeny Populations and Hybrid Types provides some guides.

The concluding topics cover Selection of Parents, including some that I use together with some photos of results obtained, and I have closed with some thoughts on my more recent activities on hardy yellows. Since my primary interest is with the elepidotes in the Northeast, the discussion is framed in this area but the methodology and approach can be used with any section of the genus.

2. What to hybridize?

Lepidotes (scaly leaves) and/or Elepidotes (without scales)?

Note: The lepidotes and elepidotes are genetically incompatible and hybridizing between the two groups is basically a fruitless endeavor.

Elepidotes (the large leaf rhododendrons) - easy to do outside, but 5 to 8 years to flower on the East Coast unless greenhouse or other accelerating facilities available, then 3 to 4 years. On the West Coast flowering time is shorter because there are usually two ripened growth cycles per year and flowering time is more related to growth cycles than the number of growing seasons. Many species of diverse types and colors can be used or are present in the heritage of existing hybrids.

Lepidotes (the small leaf rhododendrons) - can be more difficult outside because they typically flower early and frost may abort a pollination. If frosts are likely in your locality then use potted plants as seed parents and pollinate in a frost-free area. Most are compact and can flower at about 3 years. There are trends close to blue in this group and this could be an interesting area to work in.

Deciduous Azaleas - spectacular colors, interesting truss forms, generally very hardy, sun loving, flower young, native species available.

Evergreen Azaleas - early to late season flowering, use potted seed parents if frosty during flowering season. Flower very young, easy to get good looking results, may be harder to get something radically "different."

Vireyas - sub-tropical and often epiphytic. Indoor subjects, interesting colors and forms, a more specialized area.

3. Goals and Objectives.

The key to successful hybridizing is to consider carefully what you want to achieve from your efforts and direct your program accordingly. Consider not only the plant results you hope to get but also what will give you a sustained sense of satisfaction from your endeavors. Set some goals, think of them as your long term targets (what you want to achieve) and then plan a program of hybrids with objectives (results expected) as steps towards your goals. It is also important to consider the physical resources required to raise a reasonable number of plants through to flowering size and beyond. More resources in the area of winter protection of younger seedlings in particular are required in the Northeast.

An analogy may be helpful. You are a fledgling archer whose goal is to shoot an arrow into a target at, or beyond, the extreme range of your the skill, strength and bow. You could shoot all your arrows at the target hoping for a freak wind and almost certainly fail. To assure success you should shoot a smaller set of arrows towards the target but aimed to land short at a way-point under your control. Some arrows will be lost or damaged but some can be picked up and used in further rounds (iterations) of shots as you get closer to the target. This is very much how a hybridizing program should be structured.

3.1 Program Structure.

Unless your goal is simple and easily reachable, plan an iterative program aimed to reach the goal in achievable steps. Each step should have an expected result (objective). It should involve a number of crosses started concurrently to create a set of plants giving some genetic flexibility and to allow for failures. Failures will occur such as aborted pollination (plan on a 50% to 65% pollination success rate), disease, accidents, and results that are not as expected. The most successful progeny from the initial step will ultimately be selected to make new crosses in a further step. The number of steps will depend on the difficulty implied by your chosen goal. A well designed program will give some successful plant results worthy of keeping at each step and thus sustain your continued interest in what is, after all, a long term creative process.

3.2 A Typical Scenario.

Let us assume the goal is a compact pink/yellow bi-color with a large flowered truss and hardiness for Zones 5 or 6. This goal will likely require two generations to achieve. The first objective can be to improve truss size. We start by making (say) six crosses using three Zone 5/6 hardy pink/yellows, such as 'Rio', 'Bali' and 'Bangkok', as seed parents. These are each pollinated from a pair of large flowered pink/yellows with superior flowers and intermediate hardiness (Zone 6/7). Pollen sources could be 'Mary Belle' and one other such as 'Dexter's Champagne' or 'C.O.D.', or 'Bellringer'. A less hardy western hybrid could be included if you want to be more adventurous. There will be some large truss results with good hardiness but probably lacking compactness ('Rio' is actually fairly compact and 'Bali' is of intermediate size).

As a next step objective (compactness/refinement), select the best results and cross them with a reasonably hardy compact pink/yellow bi-color such as 'Percy Wiseman' and maybe one of the compact hardy yellows or bi-colors such as, 'Serendipity', 'Big Deal' or 'Santa Fe', etc. The original goal may be met at this stage or other crosses made to further refine compactness, color contrast, deepness of yellow color, add throat color, or improve truss and foliage quality, etc. This scenario is typical, you will get some interesting plants in the first set of hybrids which should be worth keeping on their own merit. You will have the opportunity to refine objectives and develop future goals based on your results and accumulated experience.

Fig. 1. ('Mary Belle' x 'Percy Wiseman')
Figure 1. ('Mary Belle' x 'Percy Wiseman').
Photo by Anthony D. M. Knights

The late Dr. Gustav Mehlquist was very supportive as I became more seriously interested in hybridizing. He helped me with advice, encouragement (and constructive criticism!) as well as plants and pollen. He advocated also taking a couple of calculated risk shots (he called them "trial populations", Ref. 1) at the target in parallel to the structured program to try and shorten the time line. My structured program is similar to, but not the same as, the above scenario. I crossed ('Mary Belle' x 'Percy Wiseman') as a conservative risk shot, and later ('Rio' x 'Nelda Peach') as an adventurous shot. Each has produced distinctive multi-colored flowers on nice looking plants within the first six that flowered (see Figs. 1 & 2). The test of time has still to be passed, but confirmation of parental choice and a high degree of satisfaction have been achieved.

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

3.3 Set Some Initial Goals.

Setting some tentative goals will help to focus your thoughts on where and how to begin knowledge building so that you can plan your program. Pick three or four, some easy and some harder. For example: whites or pinks with dark blotches are relatively easy and achievable in the first generation. Adding indumented foliage and frilled flowers could be your ultimate goal. Maybe you might use pollen from Rhododendron rex or R. rex ssp. fictolacteum (or a hybrid containing one of these species). This will give blotch, flower and leaf forms not usually seen in the east.

Hardy yellows are, on the other hand, a tougher nut to crack! Don't be afraid to tackle them, just don't expect short-term results and do them together with other easier things to continually fuel your enthusiasm. I have made some suggestions for an approach in section 9 (to be published in Part 2 of this series).

I am getting gratifying results from my work on hardy "tropical" colors, by which I mean flowers like those of 'Lem's Cameo' and its hybrids which have rich warm toned colors combined with yellow, often with three different complimentary colors in their flowers. Once I had thoroughly studied the available hardy parents and possible pollen sources this turned out to be a relatively easy area to work in.

Foliage is important and often neglected in the quest for new flowers. There are many species and hybrids with outstanding foliage, many of which are also of compact form. A good program should have some hybrids being used or created with the goal of bringing overall distinction to your flowered creations.

4. Knowledge Building.

Once goals and tentative objectives have been set, research the prior art (it is largely an art, but a smattering of science helps). Talk to others who hybridize. Read some of the books containing the published heritage of named hybrids down to the species level. Some of the many photos may inspire you to revise your objectives!

Scan the new registrations in the back of ARS Journal; I discovered good sources of knowledge and pollen this way. My own interest in hybridizing for hardy "tropical" colors was stimulated by my researches among the new registrations.

The ARS Seed Exchange list is useful too; you can see the what other hybridizers are doing. You may also find seed that can save time and effort in your program. I recommend doing lots of "arm chair hybridizing"; winter can be a good time for study.

4.1 Hybrids.

Useful books on hybrids:

Greer's Guide to Available Rhododendrons (3rd Ed.) by Harold Greer. Covers Species and Hybrids and is an excellent and affordable starting book.

Rhododendron Hybrids by H. Salley and H. Greer (3rd Ed. - CDROM, the earlier Eds. - hard cover). Has full heritage in family tree form together with summary descriptions of hundreds of hybrids and many photos. Note that the upper branches of the family tree represent the seed parent's heritage. Also included are numerical estimates of each species' contribution to the resultant hybrid.

Rhododendron Portraits by D.M. van Gelderen & J.P.H. van Hoey Smith. Contains hundreds of color portraits, which can be inspiring to new hybridizers. It includes many interesting European plants not usually seen in the USA. Heritage is however limited to immediate parentage.

Notes on nomenclature:

The formal style 1 of a hybrid's parental description is stated thus: A x B, where "A" is the seed parent name and "B" is the pollen parent name. The "x" represents the crossing of the parent plants that created the hybrid. Furthermore, species names are stated in italicized lower case letters and are preceded by the "R." abbreviation denoting the genus name, e.g., R. maximum , R. catawbiense , etc. ( Rhododendron is used instead of the "R." abbreviation when a species name is placed at the beginning of a sentence.) Hybrid names are always stated with an upper case first letter and enclosed in single quotes, e.g., 'Sappho', 'Mary Belle', etc.

It should be noted that the styles used in two of the books on hybrids referenced herein vary slightly from the style described above. The most notable difference is the enclosure of a hybrid's parentage in parentheses, e.g., ('Catalgla' x 'Sappho'). This has the advantage of focusing the reader's attention on the entity under review and is used in this article.


('Hybrid name 1' x 'Hybrid name 2') is a cross between two hybrids, e.g., ('Rio' x 'Mary Belle').

'Hybrid name' x R. species name ) is a cross between a hybrid and a species, e.g., ('Fabia' x R. smirnowii ).

{(A x B) x (C x D)} represents a cross between two hybrids.

{[(A x B) x (C x D)] x (E x F)} represents a cross containing three hybrids. Note how the parentheses are used to convey the order of the ultimate hybrid's creation. (A x B) was pollinated with (C x D) and the resultant hybrid was then pollinated with (E x F).

{(A x B) x B} represents a "backcross" which is typically used to emphasize the characteristics of "B" in the ultimate hybrid. (See section 7.2 in Part 2 for more details on hybrid types and uses.)

4.2 Species.

Understanding of the prior art comes from the realization that the character of a hybrid stems from the species that are in its heritage. This is reflected in two ways, firstly from the individual species present but also, and more importantly in complex hybrids, by the way the species are genetically linked in the hybrid.

Study the various species, particularly those used in hybrids that catch your interest. Start by studying a simple hybrid, i.e., (species x species) or (hybrid x species). Try to understand what characteristics a species has contributed to the resultant hybrid. Discussing your observations with chapter members is very helpful.

A list of species that can be useful in hybridizing elepidotes and their primary contributions to hybrids is appended.

Useful books on species:

The Encyclopedia of Rhododendron Species by Peter and Kenneth Cox. This has readable botanical descriptions and photographs of most listed species and many useful comments. It conforms to the latest (1998) taxonomical revision of the genus.

Greer's Guide (see above). This also contains useful lists of species names of the whole genus under the obsolete Balforian system and the current Edinburgh re-classification. It also has name cross-references, at each species listed, between the two classification systems. (Note: there have been a few changes since this book was published.)

Notes on text in this document:

(i) R. degronianum ssp. yakushimanum var. yakushimanum is abbreviated to yak for brevity in later text.

(ii) Also for brevity in later text, species names, where the full botanical names are not germane, are given at the lowest hierarchical level. For example: R. dichroanthum ssp. scyphocalyx is thus written ssp. scyphocalyx .

Fig. 3. 'Calsap' and 'Tiana' growing side by side
in the author's garden.
Figure 3. 'Calsap' and 'Tiana' growing side by side in the author's garden.
('Silbervolke' in foreground; 'Nepal' behind tree; 'Luxor', distant on right).
Photo by Anthony D. M. Knights

5. Plant Analysis.

To illustrate the foregoing let us analyze a pair of hybrids: (a) 'Calsap' = ('Catalgla' x 'Sappho'), and (b) 'Tiana' = ('Sappho' x yak). Fig. 3 shows them as mature plants in the author's garden.

Both of these hybrids are interesting landscape plants and have show quality white flowers with distinctive dark blotches inherited from 'Sappho'. Both consistently bud up well every year.

'Sappho' (heritage unknown) has one of the best blotched white flowers ever created. The truss is full and rounded and the somewhat frilled flowers have a unique purplish black blotch. It also has one of the worst growth habits, being leggy, open growing and resists branching out even when pinched heavily. It is also not very hardy (-10°F). It is a tantalizing subject and it is no wonder hybridizers have striven for improved growth habit and hardiness while retaining the flower quality.

Fig. 4. 'Calsap'
Figure 4. 'Calsap'.
Photo by Anthony D. M. Knights

(a) 'Calsap' (Fig. 4) is a worthy hybrid and my specimen is almost 7 ft tall and slightly broader. The nice truss is full, tight and upright shaped. It has many flowers, but they are not frilled and are individually smaller than those of 'Sappho'. The bold blotched trusses stand out well from a distance. Plant habit is improved and while it is still a bit open, it does respond to pinching. The catawbiense type foliage is a little sparse, the leaves are retained for two years, and you can "see through" the shrub. This specimen has a slight purplish tone to the white, noticeable when compared to 'Tiana' which stands next to it. It is very hardy (-20°F) and looks good against a dark or shady background.

'Catalgla' (short for R. catawbiense v. album – 'Glass') is a selected F2 seedling clone from the original white form of R. catawbiense discovered by Powell Glass in 1936. Rhododendron catawbiense has been used since the 1800s as the hardy parent of many ironclad (-20°F) hybrids. 'Catalgla' and other known white forms or derivatives of R. catawbiense are important because they do not impart an undesirable purplish tone to light colored hybrids, although they may not be absolutely perfect in this regard. 'Catalgla' was used by Joseph B. Gable (who introduced it). Dr. David Leach and others used it in creating many popular modern hardy hybrids. Other virtues of 'Catalgla' are hardiness, excellent heat tolerance and good truss form. Its growth habit is reasonable but can become rangy with age. Its foliage is typical of the species.

Fig. 5. 'Tiana'
Figure 5. 'Tiana'.
Photo by Anthony D. M. Knights

(b) 'Tiana' (Fig. 5) is also a worthy hybrid and my specimen is 5ft tall and 9ft wide. The truss is shaped rounded slightly flattened, and the pure white frilled edged flowers have a relatively small butterfly shaped dark blotch. The truss is wider but not quite as tight as the truss of 'Calsap'. The growth habit is broad, mounded and fairly dense. Pinching is still required on non-flowering terminals so the rangy habit is an intrinsic factor, albeit diminished. Leaf quality is good, dark green, more leaves per terminal, and with an upright "vee'd" habit that neatly supports the truss. The leaves are retained for three years which improves the shrub's visual density. Hardiness is about (-10°F to -15°F) or colder. 'Tiana' is a nice specimen plant but obviously needs space.

Yak has positively contributed to 'Tiana' in several ways: compactness, foliage quality (form, color, quantity per terminal, leaf retention), flower and truss quality, and hardiness. These, and the indumentum on the backs of its leaves, are the key characteristics of yak and the reason why so many yak hybrids have been made (some say too many!). Also the furry (tomentose) texture and color of the new leaves is a distinctive and attractive feature which lasts for several weeks.

A very dominant trait of yak is its fading effect on the flower color of plants with which it is hybridized. This can be an advantage for white hybrids and in whitening off-color whites but is a nuisance when trying to impart its many other good features when crossing with darker toned flower parents. Dr. Mehlquist (Ref. 1) noted that when yak was crossed with 'Vulcan' = ('Mars' x R. griersonianum ) there was much less fading than with any other red hybrid he tried and he attributed this to a favorable genetic characteristic of R. griersonianum . However with most darker toned flowers, two generations are usually required to obtain desired dark color results.

Fig. 6. ('Janet Blair' x 'Sappho')#2
Figure 6. ('Janet Blair' x 'Sappho')#2.
Photo by Anthony D. M. Knights

5.1 Another Generation from 'Calsap' or 'Tiana'?

They could be crossed together or with another 'Sappho' hybrid and blotched flowers will result, but at the risk of continuing the open plant habit trait. The late Dr. W. L. Rhein created good hybrids of ('Sappho' x 'Janet Blair') with large trusses. I have got excellent flowers and trusses from ('Janet Blair' x 'Sappho'), see Figs. 6 & 7, but am not entirely happy with the plant habit.

Fig. 7. ('Janet Blair' x 'Sappho')#3
Figure 7. ('Janet Blair' x 'Sappho')#3.
Photo by Anthony D. M. Knights

An ideal plan would be to cross them with a hardy compact plant having white flowers with a dark or dark freckled blotch, together with a good branching habit. Hybridizers are now working with the white compact species R. pseudochrysanthum and R. pachysanthum and these may be a good choice too. Dr. James Marchand of the Massachusetts Chapter crossed 'Calsap' with R. pachysanthum but has not yet flowered any of the progeny.
'Golfer' = ( yak x R. pseudochrysanthum ) is also a possible choice. The Hachmann hybrids: 'Charmant', 'Hachmann's Diadem', 'Schneespiegel' and even 'Hachmann's Polaris' should give good results.

It is always good practice to use a plant with many good qualities as at least one parent in a cross. Again I would stress that you plan your crosses to yield objectively useful results even though they may not yield the exact goal you sought. In this way you gain continuing satisfaction from your efforts. You may not get a perfect 'Sappho' truss but you will get some good blotched trusses on good plants and feel a sense progress.

6. Color Traits.

The inheritance of color in rhododendrons is quite complex and still not precisely understood by botanical scientists. There is no single gene that controls the colors and there are no precise rules of dominance or recession that can be applied when planning hybrids. Actual flower color depends on the presence and relative intensities of several chemicals in the surface and underlying tissues of the flower. It is the inheritance of the coloring chemicals and their relationships that still needs to be more fully determined. Who knows, we may be able to use genetic manipulation to create a truly blue rhododendron in the not too distant future!

What we do have are characteristic traits for color and its relationship with hardiness. These have been broadly determined from the collective experience of over 150 years of hybridizing since the first exotically colored species were introduced from the Far East. Many creative hybridizers from all the countries where rhododendrons can be grown have painstakingly learned the characteristic color traits in hybridizing for color with hardiness and continue to do so. We are the fortunate beneficiaries of this wealth of experience.

I have listed some generalities below, but there are exceptions and there will be quirks and surprises that turn up from time to time. For instance: one of my ('Janet Blair' x 'Sappho') seedlings (Fig. 6) is white with a strong green blotch surrounded by gold. This type of blotch can occur in this type of cross; however, there is also a broad band of pale yellow forming an "X" over the axes of the four lower petals! One would not have expected yellow to show up in a hybrid with this parentage. It should be noted that 'Janet Blair' is a plant of unknown parentage and it is known to be somewhat "friendly" when crossed with yellows. Is it possible that it has also some yellow buried in its heritage?

The magenta tinting from R. catawbiense and R. ponticum is a dominant trait. When these species are in a hybrid used as a parent, for example 'Roseum Elegans' or its like, the effect passes to the progeny of crosses. It will often affect the purity of pinks and reds and also tone whites, or suppress yellow.

6.1 The generalities of color traits in crosses are:

Cross type Result
(purplish red x pure red) Majority will be purplish red.
(pure red x pure red) Is more likely to yield pure red.
(reds x pinks) Mostly pinks, and some reds.
(reds x whites) Mostly pinks, some reds and a few whites.
(whites x whites) White, but can be color toned if white hybrids have colored species in them.
(yellow species x yellow species) Yellow.
(yellow hybrid x yellow hybrid) or (yellow hybrid x yellow species) Very dependent on hybrid heritage. Many (yellow x yellow) crosses between hybrids end up lighter. Hardy yellows usually have hardy white parents and the results of their crosses are mostly paler yellow or white! Dr. Mehlquist used to joke that one way to get a good white is to cross two pale yellows!
(yellow species x certain orange/red species in the Neriiflora section) Crosses containing R. wardii var. wardii or R. campylocarpum linked with R. dichroanthum ssp. dichroanthum , or its close relatives, can result in a complimentary effect which gives rise to deeper or orange toned yellows.

Using the above traits as guidelines, hybridizing in the red, pink or white areas is fairly logical. Hardy pure reds are more challenging because hardiness may rely mostly on R. catawbiense , which means coping with the dominant magenta issue. There are, however, several hardy pure red hybrids to use as starting points.

Yellow is the most difficult challenge because it is genetically recessive. The following paragraphs offer some further insight.

6.2 Intensification of Yellow.

As mentioned above, it is the reddish-orange forms of ssp. dichroanthum and some of its relatives that have been most successful in this respect. One might also think that any red species when linked with a yellow species in a hybrid could intensify yellow, but this does not seem to be the general trait. A study of the literature shows that the red species R. haematodes and R. griersonianum have helped to intensify yellow when hybrids containing them, such as 'Fabia', 'Mary Belle', 'May Day' and others, are included in a yellow hybrid's heritage.

Jim Barlup has recently created some exciting new hybrids based on 'Hill's Low Red' (a hybrid of unknown parentage, but possibly containing R. haematodes ) which also help to intensify the yellow when they are included in yellow hybrids.

Rhododendron fortunei ssp. discolor , R. fortunei ssp. fortunei or, less frequently, R. decorum are also present in many yellow hybrids. Although their role may originally have been to influence form, these Fortunea species do seem to accept yellow more easily than the various white Pontica species and, in the case of R. fortunei in particular, still contribute hardiness.

6.3 The Hardy Yellow Problem.

Yellow is the hardest color to hybridize in the colder eastern areas because there are no hardy yellow species 2 . Yellow is very recessive and is usually either diluted by white or suppressed under red and pink. Many eastern hybridizers have worked hard and there are a number of successful hardy pale yellows suitable for -20°F areas and some deeper yellows for -10°F, or colder, areas. There are some fairly hardy large truss yellows too based on ssp. fortunei with var. wardii , but those spectacular warm climate luminous yellows have yet to be emulated in the east.

Appendix: Some Elepidote Species Used In Hybrids Arranged by Effect on Hybrid.
For Hardiness to -20°F For Intensifying Yellow (when combined with yellow species)
R. catawbiense R. dichroanthum ssp. apodectum
R. brachycarpum ssp. brachycarpum Tigerstedtii Group R. dichroanthum ssp. dichroanthum
R. smirnowii (forms) R. griersonianum
R. degronianum ssp. yakushimanum var. yakushimanum (forms) R. haematodes
R. maximum R. dichroanthum ssp. scyphocalyx
For Hardiness to -15°F For Long (Slim) Leaves
R. aureum R. calophytum
R. fortunei ssp. fortunei R. griersonianum
R. hyperythrum R. makinoi
R. smirnowii R. maximum
R. vernicosum R. praevernum
For Hardiness to -10°F R. roxieanum var. oreonastes
R. anwheiense R. strigillosum
R. degronianum (forms) R. sutchuenense
R. makinoi For Large Leaves
R. pseudochrysanthum R. falconeri
For Red R. fictolacteum
R. arboreum (forms) R. grande
R. elliottii R. macabeanum
R. forrestii R. rex
R. griersonianum For Early Flowering
R. haematodes R. argyrophyllum
R. neriiflorum R. calophytum
R. sanguineum ssp. sanguineum R. caucasicum
R. strigillosum R. praevernum
R. thomsonii R. principis
For White R. strigillosum
R. auriculatum R. sutchuenense
R. bureavii R. vernicosum
R. catawbiense (white form) For Late Flowering
R. caucasicum R. auriculatum
R. decorum R. brachycarpum
R. fortunei ssp. fortunei R. dichroanthum ssp. dichroanthum
R. fortunei ssp. discolor R. fortunei ssp. discolor
R. griffithianum R. insigne
R. hyperythrum R. maximum
R. makinoi R. ungernii
R. pachysanthum For Fragrance
R. pseudochrysanthum R. auriculatum
R. degronianum ssp. yakushimanu m var. yakushimanum R. decorum
For Yellow R. fortunei ssp. discolor
R. aureum R. fortunei ssp. fortunei
R. campylocarpum R. griffithianum
R. campylocarpum ssp. caloxanthum For Small Size
R. caucasicum (forms) R. aureum
R. citriniflorum R. pseudochrysanthum (forms)
R. lacteum R. pronum
R. macabeanum R. proteoides
R. wardii var. wardii
R. wightii

Mehlquist, Dr. Gustav A. L. The Breeders Talk (Abstract from Proc. 1973 Breeders Round Table). The Rosebay, ARS Mass Chapter Vol. XXVIII(2): 13.

1 The style used in the International Rhododendron Register.
2 R. aureum is hardy to –15°F but has not been too successful south of the Canadian Maritimes due to its poor heat and drought tolerance. (The Canadian R. aureum hybrids would be worth investigating though.)

Tony Knights has been a Massachusetts Chapter member since the 1970s and a Connecticut Chapter associate member for several years. He started his hybridizing in 1987, mainly interested in elepidote "hardy tropical colors." Born and educated in electrical and mechanical engineering in England, he worked for an aircraft manufacturer and avionics company. After immigrating to the USA in 1968, he worked in various aerospace and industrial capacities. Since retirement he has worked as the horticultural sales specialist in a local retail store. He gained much of his horticultural practical knowledge in S. Devon, England, where his father was the gardener at a small estate. His interest in rhododendrons began when he found his home was in an area with acid soils.