Journal ARS Article
Vol. 53: No. 4: Year 1999
The Weston Hybrids
Few commercial growers of rhododendrons in the United States have the capacity, time, and inclination to carry on any kind of breeding program for improving the varieties available to the gardening public. Hybridizing of rhododendrons and azaleas, at least in this country, seems to be a field largely dominated by amateurs.
One notable exception is to be found in a Boston suburb, where the Mezitt family at Weston Nurseries in Hopkinton has been quietly pursuing a number of breeding goals for three generations. Those goals include developing plants that (1) are hardy enough to flourish in the climatic extremes of Zone 5 in New England, (2) possess good, compact habits and superior foliage characteristics, (3) offer improved resistance to insects and diseases, and (4) extend the blooming season at both ends. In undertaking this ambitious program, they have eschewed some of the current fads, e.g., using Rhododendron yakushimanum for compact habit and foliage retention, and striving for plants with gigantic trusses which garner trophies on the show table but which may be miserable failures as landscape subjects. Instead, they have relied on painstaking selection and evaluation of seedling plants for landscape value, generation after generation.
The Mezitts' breeding of rhododendrons falls into four general categories: early-blooming lepidotes, midseason elepidotes, hardy evergreen azaleas, and summer-flowering deciduous azaleas. The lepidote story begins with a cross made in 1939 by Ed Mezitt, son of Peter J. Mezitt who founded the nursery. Peter had received a plant collected by missionaries in the Altai Mountain region of China, later identified as Rhododendron dauricum var. sempervirens (now known as R. dauricum Semper-virens Group). Ed used pollen from this plant on R. minus Carolinianum Group (then known as R. carolinianum) to produce the PJM Group, probably still the most widely planted rhododendron in the country today.
This landmark introduction was subsequently followed by a succession of closely related lepidote hybrids, based on the PJM Group, its parent species, and other related species such as Rhododendron racemosum. Two of the first of these resulted from a 1957 cross of a white form of R. minus Carolinianum Group with the PJM Group: 'Balta' and 'Laurie'. Both form neat, slow growing mounds, with flowers in early May (here in Zone 5) that vary in color, depending on the season, from very pale pink to white. 'Laurie' occasionally produces petaloid flowers, a trait that reappears in a number of more recent hybrids. 'Balta' backcrossed with white R. minus Carolinianum Group carried this line of breeding a step further, with the pure white 'Molly Fordham'.
A 1958 cross of a white form of Rhododendron minus Carolinianum Group with R. mucronulatum 'Cornell Pink', produced a group known collectively as the Shrimp Pink Hybrids. They form upright, semi-deciduous shrubs that cover themselves in late April with a profusion of delicately tinted flowers. Three selections have been named, with slight differences in flower color: 'Caronella', 'Llenroc' and 'Vallya'.
Another 1958 mating, this time with Rhododendron minus Carolinianum Group and an evergreen hybrid, probably (R. mucronulatum x R. dauricum)1, gave us 'Olga Mezitt', a lusty grower that produces a huge root system, substantial evergreen foliage, and mid-pink flowers in early May. A sibling, 'Weston's Aglo', has flowers of a slightly lighter pink.
A continuing goal over the years has been a hardy, red-flowered lepidote. The first introduction in this category, 'Milestone', resulted from a 1969 mating of a compact form of R. minus with R. dauricum Sempervirens Group. This forms a twiggy, compact, semi-deciduous plant with excellent fall color. The flowers open from pink buds, and the color intensifies as they age, to a brilliant magenta-red. Bloom typically coincides with the running of the Boston Marathon on Patriot's Day, April 19. Further breeding in the "red lepidote" category explored other genetic combinations: 'New Patriot' ([(PJM Group x pink R. mucronulatum) selfed] x unknown); 'Red Quest' ('Waltham' x pink R. dauricum Sempervirens Group); and 'Landmark' ('Counterpoint' x Carolina Rose Group). All have flowers in varying shades of deep pink, which appear nearly red from a distance.
In the years following the introduction of the PJM Group, inbreeding and further crosses of the original grex produced a succession of variants, some with deeper colored foliage and flowers, some with an even more compact habit. 'Black Satin' and 'Thunder' (PJM Group F2), and 'Midnight Ruby' ('Olga Mezitt' x deep purple PJM Group ) have foliage that turns deep mahogany-black in winter, and bear deep purple flowers. 'Checkmate' (sport of 'PJM Victor') has foliage and growth habit about two-thirds the size of its parent, while 'Princess Susan' combines the intense deep flower and foliage color with an unusually dwarf, compact habit.
Another line of breeding involved the old hybrid 'Laetevirens'. ('Laetevirens' is commonly listed as a hybrid between Rhododendron minus Carolinianum Group and R. ferrugineum; however, there is evidence that the parentage is actually [R. concinnum x R. ferrugineum].) Dr. Robert Ticknor crossed 'Laetevirens' with R. minus and gave seedlings to Ed Mezitt when he moved from Massachusetts to Oregon. Two of these, 'Waltham' and 'Desmit', were eventually introduced by Weston Nurseries. Both are dense, compact plants with superior, glossy dark green foliage, and both produce pink flowers in mid-May. A 1971 cross by Ed Mezitt combined a sibling of 'Waltham' with an unnamed hybrid of R. minus Carolinianum Group and R. myrtifolium, to produce 'Weston's Mayflower', with even better habit and foliage.
The tendency toward petaloid or double flowers, which you will recall showed up in the early PJM Group hybrid 'Laurie', has been exploited more recently to give us a whole series of double-flowered lepidotes. First to be introduced, in celebration of the nursery's sixtieth anniversary in 1983, was 'Weston's Pink Diamond', a cross of PJM Group and Rhododendron mucronulatum 'Cornell Pink'. As might be expected from this pedigree, it forms a tall (over six feet at maturity), upright plant, semi-deciduous with spectacularly brilliant fall color. The flowers, produced in mid-April here in eastern Massachusetts, are rose pink, with the stamens converted to an inner circle of petals. 'Weston's Pink Diamond' was followed by a bevy of double-flowered lepidotes of complex ancestry, involving PJM Group, pink mucronulatum, white dauricum, white minus Carolinianum Group and 'Gable's Pioneer'. 'April Snow' has star-shaped double flowers of (you guessed it) snowy white; 'April Cloud', of similar hue, has a more formal, camellia-like shape. 'April Song' is probably my favorite; the flowers are clear pink with the inner petals a paler shade. 'Mrs J. A. Withington III' sports flowers of pale mauve that have been likened to a powder puff. All of these more recent introductions make dense plants around 3-4 feet in height in ten years, with about fifty percent foliage retention in winter.
Last but by no means least among Weston's lepidote introductions is 'Blue Baron', from a 1981 cross of 'Gletschernacht' and 'Waltham'. 'Gletschernacht' (often listed as 'Starry Night') is a Hachmann hybrid with stunning deep blue-violet flowers, but unfortunately not hardy enough for Zone 5 gardens. 'Blue Baron' has captured the blossom color of 'Gletschernacht' and the hardiness of 'Waltham' to produce a superb, compact plant with color to rival many West Coast augustinii hybrids.
While the ancestry of the lepidote hybrids was generally well documented, that of the elepidotes was less so, at least in the early years. In addition to raising named varieties of the familiar old catawbiense ironclads, the nursery practice was to raise seedlings of the same, often open-pollinated, selecting every year the best plants from which to harvest seed. There was an occasional infusion of other types, for example, some wardii hybrids received from Dr. Ticknor. Occasionally a seedling would exhibit such desirable qualities that a decision was made to name it and propagate it clonally. The first of these to be so honored was a 1950 selection, 'Noreaster', a low-growing, dense, compact plant with white flowers. 'Years of Peace' was a 1954 hybrid of 'Mrs C. S. Sargent', selfed; it forms an upright, stiffly-branched plant of 5 to 6 feet in ten years, with deep pink flowers. Another, 'Pauline Bralit', a 1958 cross, introduced Rhododendron fortunei genes into the blood lines; and as might be expected, it is also a stiff, upright plant with large, slightly fragrant, creamy-white flowers in early midseason.
Perhaps the best known of all the Weston elepidotes is 'Henry's Red', selected from a field of catawbiense seedlings. Somewhat open and rangy in habit, 'Henry's Red' offers one of the deepest colors to be found in hardy rhododendrons - so deep, in fact, that it tends to recede into the background unless backlit by the sun. Other reds followed: 'Red Frilled' and 'Trojan Warrior', both selected catawbiense seedlings and both fairly low-growing and compact. But the goal of a truly dwarf, hardy, red-flowered hybrid has remained elusive.
As elusive for the Mezitts as it has been for many other eastern hybridizers is another goal: a hardy clear yellow elepidote. Nevertheless, some of the introductions paving the way have turned out to be first-rate plants in their own right. A group of hybrids from 1971 crosses between plants designated simply as "low white"; and "white with yellow center" must surely have some genes from those Ticknor wardii hybrids. 'Arctic Gold' and 'Glacier Queen' form reasonably dense, low-growing plants, 2 or 3 feet in height in seven years, and produce flowers in tints of pale yellow, with a stronger yellow-green flare. 'Victoria's Consort', of similar ancestry, has a more open upright habit. 'Big Deal' (unnamed low, white hybrid x unnamed pale yellow hybrid) has a particularly fine flower composed of large, frilled flowers closely packed in a tight formal truss, which has won trophies at Massachusetts Chapter shows. The most recent introductions in this category are 'Golden Scepter' (unnamed pale yellow hybrid x 'Goldsworth Yellow') whose flowers are the deepest colored in the group, and 'New Century' (pale yellow R. catawbiense hybrid x [unnamed yellow hybrid x 'Bristol Cream'*]), which has the additional attraction of glossy, dark green foliage.
Unlike the rangy habit of many of the old standard purples, 'Tapestry' makes a neat, compact plant 3 feet tall and wide in eight years. The flowers of vivid purple with a near-black flare are close to 'Purple Splendour' quality, and the foliage is sun proof, even on Weston's exposed hillsides.
Among the latest flowering elepidotes is a 1982 hybrid of 'Summer Rose': 'Summer Peppermint', a flashy red-and-white bicolor peaking in late June. With an open habit, the plant reaches 3 feet tall and wide in six years. 'Summer Storm', from the same cross, offers flowers of very pale purple with a prominent deep flare.
Although less extensive than the Mezitts' work in other categories, their introductions of evergreen azaleas make it possible for us Yankees to delight in some of the spectacular masses of color that our Southern neighbors have enjoyed from this group for years. With the evergreen azaleas, the goals have included not only increased hardiness but also compact and floriferous plants. 'Bixby', with dark red flowers, 'Pink Clusters', bright pink, and 'Royal Pillow', purple, satisfy these criteria and offer real Zone 5 hardiness.
From the earliest years the Mezitts understood and appreciated the potential of summer-flowering deciduous azaleas, and recent introductions in this category have extended the blooming season into late July and even early August. In addition to breeding for an unconventional season, the Mezitts have included in their goals superior foliage characteristics, above all resistance to mildew, which can disfigure so many azaleas in our humid eastern summers, and brilliant fall foliage color.
The Weston hybrids have been rigorously tested under conditions that promptly eliminate any weaklings. Fields of rhododendrons are grown in full sun on exposed hillsides and in rocky, clayey soil. Any varieties that flourish despite these adversities are bound to be star performers under less-than-ideal conditions elsewhere.
*Name is unregistered.
1 The parentage of 'Olga Mezitt' and 'Weston's Aglo' has been erroneously registered as (carolinianum x minus). However the correct parentage is (R. mucronulatum x R. dauricum).