JARS v47n1 - Dexter Rhododendron Secrets

Dexter Rhododendron Secrets
A Breeder's Perspective
Jonathan Leonard
Sandwich, Massachusetts

In the past, mystery strongly colored our view of Charles Dexter's rhododendrons. Dexter himself kept few notes and named no seedlings. So the ancestries of his plants were uncertain; and thousands upon thousands left his estate in Sandwich, Mass, (now Heritage Plantation) for places as far afield as Seattle.
The experts who named Dexter's plants after he died in 1943 tracked down groups of Dexters, took cuttings, rooted them, grew them on, and evaluated the results. But rooting cuttings was tricky in those days. All the cuttings taken from the Dexter Estate itself apparently failed to root or disappeared. Sometimes the same clone growing in two or three places wound up getting two or three names. Some of the named plants vanished. And lack of a central reference collection added to confusion which, far from being the exception, threatened to become the rule.
While some of this mystery and confusion persists today, things have changed. We still don't know the precise parentage of most Dexters, but we do have a fair grasp of the plants Dexter used to make his crosses. We also have a good reference collection of named Dexters, established at Heritage Plantation by horticulturists Jack Cowles and Heman Howard in the sixties and seventies. We have learned a lot about named Dexter hardiness (see Jon Valigorsky's "Hardy Dexter Rhododendrons," Journal ARS , (Vol. 43, No. 4). And while nothing much can be done about duplicate unregistered names and vanished plants, by now we know pretty well where the duplications are and what has vanished.

Bicolors and White Stars
I first began to realize that this accumulated knowledge could be useful for plant breeding when something odd surfaced in a cross I had made between the leading pink Dexter rhododendron 'Scintillation' AE and 'Melanie Shaw', a deep purple hybridized by Jonathan Shaw with ruffles and a black flare. My aim had been to get good purple color on a 'Scintillation'-type plant, and I was pleased to see that a good many of the seedlings' flowers showed dark color. The odd thing, or at least what seemed odd to me, was that about half the flowers also bore prominent white stars.

Closer examination of a flowering 'Scintillation' in the nursery showed that its flower also contained a white star, though the star was hardly visible because the pink color of the outer petals was so light. This star closely resembled a white star barely visible in the bloom of Rhododendron fortunei , a likely 'Scintillation' forebear; and it even more closely resembled the white star of 'Accomplishment', Charles Dexter's spectacular red and white bicolor. ¹ Indeed, I found that if one mentally subtracted the maroon speckles from the 'Accomplishment' blossom and changed the red outer color to pale pink the imagined flower became virtually identical to that of 'Scintillation'.
Precisely how these common features came about remains a mystery. Perhaps a plant called 'Wellfleet' (a deep pink forebear of 'Accomplishment' with a dense growth habit, white-star flowers, and an R. fortunei or R. decorum heritage) was also a 'Scintillation' parent. But parentage is really not the point. The point is that the white star of 'Scintillation' can apparently be passed along to its offspring. And since the offspring of 'Scintillation' willingly take color from darker parents, it would seem possible to breed a whole host of spectacular bicolors with at least some 'Scintillation' hardiness, density, robustness, large flowers, and glossy leaves.
What's more, if 'Scintillation' can throw white stars, what about other pink Dexters? As a group, these pinks are important. For one thing, they dominate the Dexter clan (of the roughly 130 older named Dexters now extant, some 3% can be classed as whites, 4% as multihued near-whites, 6% as lavender-purples, 7% as beige-apricots, 16% as reds, and 65% as pinks). Beyond that, they are the largest and best single collection of fancy pinks adapted to our Northeast climate. So it seems worth asking how much this transmissible "white star" effect applies to them.
In fact, it could apply quite well. Most of the named Dexter pinks are not pure pinks but bicolors; and while some, like 'Brown Eyes', have darker-colored centers, most have white centers that can be regarded as variations on the R. fortunei white star.
Of course, the collection has many shades of pink and all sorts of white stars. Some of the stars, like those in 'Scintillation', are so understated that we only see them as interesting shading in the bloom; while others, like those in 'Wareham' and 'Janet Blair' ² , take over the whole center of the flower, banishing nearly all pink color to the blossom's outer rim.
Limited experience to date shows that some if not all of these white stars can be transmitted. Tony Consolini, Charles Dexter's head gardener and a hybridizer noted for his interest in red flowers, produced a series of spectacular red bicolors including 'Consolini's Windmill', quite possibly as a result of breeding deep reds with Dexter's white-starred plants. And William Rhein, in breeding 'Janet Blair' with the purple-centered 'Sappho', came up with a seedling he called 'Rhein's Picotee', pictured in the Journal ARS , (Vol. 44, No. 3), that displays the white star of 'Janet Blair' together with a purple rim derived from 'Sappho'.
It seems fair to ask how all these Dexter white star variations came about. We really don't know, because our understanding of Dexter history and flower genetics is murky. But it seems reasonable to suppose that almost any pigmented flower depends on many different genes that direct the creation, distribution, and assembly of chemicals responsible for what we ultimately see as color. ³ So one can imagine the flower's chemistry operating something in the manner of a car manufacturer's parts production and assembly system. And one can also imagine, in the case of a plant like R. fortunei or 'Scintillation', that the system is rather delicate, because the pink color is pale and something has happened to the distribution or on-site assembly apparatus that prevents expression of color in the area we see as white.
If we breed such a plant with a different white-starred pink, we might expect to further disable the system, so that it makes less pink pigment and relegates that pigment to the fringes of the blossom, as in 'Wareham' or 'Janet Blair'. Or, if we breed it with a plant like 'Melanie Shaw' that distributes color throughout its flowers, we could wind up with seedlings showing everything from prominent white stars to fully colored centers, which is in fact the result of the 'Scintillation' x 'Melanie Shaw' cross just described.
Besides accounting for Dexter's constellation of white stars, this theory suggests that the light pink and white Dexters as a class could make useful parents. The pink pigment demonstrates the presence of an intact color system, but the system is malleable. The white star may be transmitted, or else the color system may be taken over wholesale by genes from the other parent. That would explain why Dr. Rhein was able to get a yellow seedling when he crossed 'Scintillation' with the yellow 'Golden Star', and why one of his 'Janet Blair' x 'Sappho' seedlings yielded a near-white flower with the prominent purple flare of 'Sappho' in its heart. ⁴
In recent years, as more people have come to see 'Janet Blair' as a painter's palette, breeders have tended to focus on that plant. But our theory suggests that 'Janet Blair' is not alone, that other Dexter pinks have similar potential, and that breeders stand to gain by taking advantage of the full range of hardiness, density, flower form, and color distribution in Dexter's white-starred throng.

Deep Red Rhododendrons
Compared to his magnificent pinks, one must concede that as a group Dexter's reds offer less to rhododendron lovers. All are pretty, and many have advantages like floriferousness ('Gigi' A.E., 'Glenda Farrell', 'Kelley'), dense foliage ('Gigi', 'Tripoli'), large flowers ('Dorothy Russell', 'Dexter's Giant Red'), and fine flowers with attractive maroon speckles (nearly all). But they are something of a disappointment for color-seekers and breeders, because only two ('Dexter's Red Velvet' and 'Avondale') sport flowers one would call deep red.
Nobody knows just why this is; but Dexter got some of his reds from crosses involving a tender deep red hybrid ('Pygmalion'),a tender deep red species ( R. haematodes ), and a pink white-starred hybrid ('Wellfleet'). So it seems likely that many or even most of his reds came from crossing tender reds with pinks; and that in turn suggests that if one wants really deep red color in a single generation, this is a good approach to avoid.
Conversely, it suggests that a fine way to breed hardy deep red plants is to cross a pair of deep red parents. This idea is so unadventurous that it appeals to those with little or no experience. Indeed, 10 years ago, when I was just starting to breed rhododendrons, it appealed to me. I wanted the classic red 'Mars' color on a hardier plant with better leaves and roots; so I made a back-cross, breeding two deep reds - 'Mars' and a David Leach 'Mars'-derived hybrid named 'Inca Chief that is hardy to -20° F. Nearly all the seedlings, the best of which was introduced this year as 'Alexandra's Ruby', showed up with better leaves and roots than 'Mars'. More to the point, they also had the desired hardiness and color, confirming that this unimaginative method indeed provides a good way of breeding deep red plants.

Fragrance
A similar sort of logic applies to fragrance. Many Dexter pinks have some fragrance, but I know of only three named ones with scent strong enough to bowl you over at a distance of 10 feet. Those three are 'Dexter's Peppermint' (a tender plant with a poor flower), 'Dexter's Spice' (whose spectacular flowers are offset by a weak vascular system and a tendency to look leggy when young), and 'Dexter's Honeydew' (a good all-around plant with nice leaves and large beige-apricot blooms).
None of the three is very hardy. 'Dexter's Spice 'will survive brief brushes with -20° F or so in sheltered spots, but will not bloom below -5° F; and neither 'Dexter's Honeydew' nor 'Dexter's Peppermint' can be expected to survive north or west of Boston. (For those wanting moderately fragrant Dexters that may bloom after exposure to -10°F, 'Wareham' and 'Betty Hume' are good choices.)
This tenderness in the fragrant Dexters is interesting. The most likely reason for it is that the fragrance comes from R. fortunei , and all of the R. fortunei available to Dexter were tender below about -5° F. But there is more to it than that, because superficially the white stars' origins seem similar, and Dexter created lots of plants with strong white stars but few with strong fragrance.
The message could be that fragrance, unlike white stars, resists wholesale transmission. Therefore, those who would breed for top fragrance in a hardy plant might be well-advised to cross two highly fragrant parents. Happily, we now have good R. fortunei selections hardy to -15° F or even -25° F. We know from hybrids like 'Nestucca' ( R. fortunei x R. yakushimanum ) that R. fortunei's open growth habit can be rendered dense in one generation. So there seems no obstacle to breeding a good line of dense, hardy, and highly fragrant plants. ⁵

Beige-apricots and Yellows
Charles Dexter's named varieties also include a small group of beige-apricot varieties whose modest olive-green leaves show a heritage derived from 'Skyglow', a beige-apricot plant Dexter got from the Farquhar Nursery. The group includes 'Dexter's Apricot', 'Dexter's Cream' (a delicate plant), and the well-known 'Dexter's Champagne'. Less well known but equally worthwhile are 'Zanzibar', which bears flowers rivaling those of 'Dexter's Champagne', and the relatively tender 'Ashes of Roses', which covers itself with large and moderately fragrant bloom. (Oddly enough, 'Anna Rose Whitney' has occasionally been sold as 'Ashes of Roses', so one should suspect the identity of any purported 'Ashes of Roses' bearing the large dark green leaves of 'Anna Rose Whitney'.)
There are also a few genuine Dexter beige-apricots with different leaves - including 'Dexter's Brandy Green', 'Dexter's Honeydew', and 'Tan'. Furthermore, Jack Cowles, using Dexter and other varieties, created a large collection of plants with beige-apricot flowers and R. fortunei -type leaves. ⁶ ' This makes one suspect that many of Dexter's light pinks partly derived from R. fortunei may take beige-apricot pigments fairly well.
There is also growing evidence that at least some of these plants will accept yellow. Part of the evidence is indirect and relates to the behavior of R. fortunei . For example, Hardgrove's light yellow 'Golden Star' was produced by crossing the pale yellow R. wardii with R. fortunei , and many of the deep-hued but tender West Coast yellows seem to have complex mixed heritages including R. wardii , R. dichroanthum , and R. fortunei . More direct evidence comes from the fact that 'Scintillation' crossed with both 'Golden Star' and R. wardii has yielded yellows, and that 'Janet Blair' crossed with a yellow plant designated Phipps #32 has produced yellow seedlings with about the same yellow color as their Phipps #32 parent.
We don't know why this is. It may be that most of the genes involved in making yellow color are what the geneticists call "dominant," so that a single collection of such genes from one parent (the yellow one) could give a seedling yellow flowers. ⁷ And it may also be that R. fortunei's malleable color-making system is compatible with the yellow system, so that the genes contributed by the pink parent can actually strengthen the production of yellow color.
It should be stressed, of course, that these ideas are merely speculation. Considerable mystery remains about the yellows, as demonstrated by the fact that despite the repeated efforts of many eager breeders, deep yellow rhododendrons truly hardy for our part of the country are not extant.
More broadly, nothing said here should be taken to mean that we have divined the secrets of the Dexter rhododendrons. Most such secrets, to the extent they exist, reflect our lack of knowledge about how the Dexters originated and how they can be expected to behave. What has really happened on this front is that over time many hardworking people have selected, named, assembled, and grown a large group of Dexter hybrids. For some years anyone so inclined has been able to observe most of them directly. As a result, the ground has been prepared for new theories. What we have presented here is merely a group of such theories - speculative ideas that stand some chance of consolidating what we know and revealing interesting things about the Dexters and other denizens of the rhododendron world.

The assistance of Dick Gustafson and Jay Murray in reviewing the text and table of named Dexters is gratefully acknowledged.

Notes
1. At least one bogus form of 'Accomplishment', an inferior bicolor, has been circulating for some time. The plant referred to in this article was propagated from the 'Accomplishment' at Heritage Plantation, which appears to represent the original selected clone.
2. While the origins of 'Janet Blair' are debated, we know it was either bred by Dexter or has Dexter parentage, and that it is botanically very similar or identical to an unregistered Dexter hybrid named 'John Wister'.
3. At first glance, the best genetic models for pinning down rhododendron inheritance patterns would seem to be the clear cut one or two gene models Gregor Mendel used to describe the inheritance patterns of flowering peas. Unfortunately, such models do not conform to most known rhododendron inheritance patterns very well; and so, except in special cases, we have tended to discard them in favor of vaguer but more effective multiple gene models of the sort applied here.
4. Wise, C.E. "A Unique Garden in the Susquehanna Valley," JARS , Vol. 44, No. 3, pp. 134-37. See photos on p. 136 and the front cover.
5. For those who would like to test this theory, the ARS Seed Exchange has a good quantity of seed from an appropriate cross made with an attractive R. fortunei hardy to -15° F and 'Dexter's Honeydew'.
6. Jack Cowles' hybrids at Heritage and other Dexter-related plants in and around Sandwich are currently being evaluated by an ARS group known as the Sandwich Club.
7. A collection of genes like this, dispersed on several different chromosomes, could produce inheritance patterns very like those we actually see, patterns also fairly compatible with the variations discussed in David Leach's Rhododendrons of the World (pp. 400-402). In contrast, if we were dealing with a single dominant yellow gene in the classic Mendelian mold, then a plant bearing this gene would always show yellow color and would always pass that yellow color along to either all or about half its offspring. No matter how delightful from a hybridizer's standpoint, this "single dominant" model seems unlikely, because it departs radically from what we can observe.

LIST OF DEXTERS NAMED THROUGH THE 1970S
Regarding certain Dexter names not shown below, as best we can determine, Black Cherry = Burgundy Cherry; Cherry Red and Dexter's Red = Wissahickon; Arlequin, Harlequin, and Dexter's Arlequin = Dexter's Harlequin; Red House = Dorothy Russell; and at least some plants circulating under the name Charlestown are identical to Sandwich Appleblossom, the registered name of a Dexter formerly known by the unregistered name of Appleblossom. John Wister is closely akin or identical to Janet Blair (see note 2). Dexter's Salmon (not to be confused with "Dexter, Salmon" sold at the 1980 ARS Convention in Massachusetts), Edwin Beinecke, Emissary, Everitt Miller, Forestdale, and Mrs. H.B. Gardner appear to have vanished. The unregistered "Dexter's Pink" (reportedly identical to a clone known as Vossberg's Pink #1 and SAE #1) should not be confused with an azalea registered under this name. Skyglow and Wellfleet (and also possibly Chatham and Sagamore Bayside) were not Dexter hybrids but rather hybrids acquired by Dexter from the nearby Farquhar Nursery. Non-Dexter hybrids have been registered under the names of Pink Satin and Zanzibar; the Dexters bearing these names may be registered in the future as "Dexter's Pink Satin" and "Dexter's Zanzibar". A few Dexters named before 1980 as well as a good many named since then have not been included on this list, the primary source for which is Hybrids and Hybridizers (P.A. Livingston and F.H. West, eds.; Newtown Square, Pennsylvania; Harrowood Books; 1978).
Acclaim (R)	Dexter's Purple (Pur) a	Mrs. W.R. Coe (Pk)
Accomac (R)	Dexter's Ramona (Pk) a	Nathan Hale (Pk)
Accomplishment 1 (R)	Dexter's Red Velvet (R) a	Newburyport Beauty (Pk)
Adelphia (R) a	Dexter's Spice (W)	Newburyport Belle (Pk)
Alice in Wonderland (Pk)	Dexter's Springtime (Mixed)	Newburyport Charm (W) a
Alice Poore (Pk) a	Dexter's Vanilla (Ap/Bg)	Oh Joy a
Andorra (Pk) a	Dexter's Victoria (Lav)	Parker's Pink (Pk)
Aronimink (R)	Dorothy Russell (R) a, b	Peg Coe a
Ashes of Roses (Ap/Bg)	Dot's Cherry Jubilee (Pk) a	Pink Satin (Pk) a, b
Avondale (R)	Eastham (Pk) a	Pink Sparkler (Pk) a
Barnstable	Elizabeth Poore (Pk) a	Powder Puff (Pk) a
Bass River (Pk)	Fairhaven (Pk) a	Quiet Quality (W/Lav)
Beauty of Halesite a	Festive Feast (Pk)	Red Yard (R) a
Ben Moseley (Pk)	Flaming Snow (W)	Robert Coe a
Betty Arrington (Pk)	Fordham (Pk) a	Rona Pink (Pk) a
Betty Hume (Pk)	Georgette of Quail	Sagamore Bayside (Pk) b
Brown Eyes (Pk)	Hollow (Pk) a	Sagamore Bridge (Pk)
Bryantville (Pk)	Gigi (R)	Sandwich Appleblossom (Pk) b
Burgundy Cherry (Pk) b	Glenda Farrell (R)	Scintillation (Pk)
C.O.D. (Mixed)	Gloxineum (Pk)	Shawme Lake (Pk) a
Charlestown (Pk)	Great Eastern (Pk)	Skerryvore Monarch (Pk)
Chatham (Pk)	Halesite (Pk)	Tan (Ap/Bg)
Clover Coe (Pk) a	Halesite Maiden (R)	Teaticket (Pk) a
Count Vitetti a	Harwich (Pk) a	Todmorden
Delicate Splendor	Hatchville (Pk) a	Tom Everett (Pk)
Dexter's Agatha (R)	Henry Coe (Pk) a	Tripoli (R) a
Dexter's Amethyst (Lav)	Hunting Hill (Pk)	True Treasure (Pk) a
Dexter's Appleblossom (Pk)	Huntington (Pk) a	Up Front (Mixed) a
Dexter's Apricot (Ap/Bg)	John Wister (Janet Blair ?) (Pk) a, b	Wareham (Pk)
Dexter's Brandy Green	Josephine Everitt (Pk)	Warwick (Pk) a
Dexter's Brick Red (R)	Katherine Slater (Pk) a	Westbury (Pk)
Dexter's Champagne (Ap/Bg) a	Kelley (R)	Weston (Pk)
Dexter's Cream (Ap/Bg)	Koster's Choice (Pk)	Whittenton (Ap/Bg)
Dexter's Crown Pink (Pk)	Lady Decora (Pk) a	Wianno
Dexter's Favorite (Pk)	Lady of Belfield (Pk)	Willard (Pk)
Dexter's Giant Red (R)	Lady of June (Pk)	William R. Coe (Pk)
Dexter's Glow (Pk)	Lady of Vernon (W) a	Winneconnet (Pk)
Dexter's Harlequin b	Lady of Wakefield (Pk) a	Winning Ways (R)
Dexter's Honeydew (Ap\Bg)	Lavender Princess (Lav)	Wissahickon (R) a, b
Dexter's Horizon (Pk)	Louisa P. Delano	Wyandanch Pink (Pk) a
Dexter's Orange (Mixed)	Madison Hill (Pk)	Xerox a
Dexter's Orchid (Lav) a	Marshfield a	Zanzibar (Ap/Bg) a, b
Dexter's Peppermint (Pk)	Megansett a	Zest
Dexter's Pink (Pk) a, b	Merley Cream (W) a
Dexter's Pink Glory (Pk)	Mr. W.R. Coe (Pk) a
a = not registered b = see text above

Jonathan Leonard is the proprietor of Briarwood Gardens, a nursery in Sandwich, Massachusetts specializing in the Dexter hybrids.