Breeding Azaleas for Minnesota
A. G. Johnson and L. C. Snyder
Respectively, Associate Scientist; and Professor and Head, Department of Horticulture
Reprinted with permission from Minnesota Farm and Home Science Vol. 20, No. 2, 1963
The azalea, although still regarded as an experimental plant in Minnesota gardens, seems destined to become increasingly popular. An earlier report described the culture of some hardier azalea species and hybrids recommended for trial by anyone able to provide the simple minimum requirements of these admired flowering shrubs.
Outstanding at that time for adaptability, vigor, and spectacular bloom was the Mollis azalea. The name "Mollis" unfortunately is ambiguous since it has been applied at times to both the Japanese azalea, Rhododendron japonicum Suringer, and to the Chinese azalea R. molle G. Don; to hybrids between them; and even to certain other species. Our plants appear to be closer to R. japonicum than to the more tender R. molle.
As part of the Department of Horticulture's overall program of breeding ornamental trees and shrubs for Minnesota, crosses were made in 1957 between the Mollis azalea and two Northeastern North American species, the Pinxterflower, R. nudiflorum (L.) Torrey and the Roseshell azalea, R. roseum (Loisel) Rehder.
The Mollis azaleas used in this breeding program now range from 3 to 6 feet in height at 8 years of age. Their large bell-shaped flowers range in color from orange through salmon to a brick red. Their odor, while faint, is considered by some to be unpleasant. The plants are vigorous growers and although hardier than we thought, have suffered loss of some flower buds from desiccation (drying) in open dry winters and from extreme cold. There has been no injury to the wood or vegetative buds.
The Pinxterflower, R. nudiflorum, is a hardy shrub whose very early flowers precede the leafing out of the plant. Its tubular-shaped scentless flowers range in color from pink to white.
The Roseshell azalea, R. roseum, is similar to the Pinxterflower but is readily distinguished by the decidedly pubescent blue-gray undersurface of the leaves. The flowers are usually pink and sweetly scented.
The initial purpose in making the crosses was to attempt to combine some of the floral features of the Asiatic species with the superior hardiness of the North American species. If successful, such crosses could then provide a basis for further breeding work.
The genus Rhododendron, to which the azaleas belong, is notable in many respects including its nearly complete promiscuity under cultivation. There seem to be few genetic barriers to crossings between the species in this large genus. In the wild they evidently maintain their identity largely through geographical or ecological isolation.
The crosses were made, in late May 1957, by using, except in one reciprocal series, selected Mollis plants as female parents. Several flowers on each of the chosen plants were emasculated and pollinated with pollen from the Pinxterflower or the Roseshell azalea respectively. In the case of the reciprocal cross, a Roseshell azalea was similarly treated and pollinated using one of the Mollis plants as the male parent.
The matured fruits, collected in the fall, were stored until January when the seed was extracted and sown under greenhouse conditions. The seedlings were transplanted into flats when large enough to handle and in summer were planted in a lath shade house. In 1960 all the seedlings consisting of the following four populations were moved to their present location in the azalea area of the Minnesota Landscape Arboretum:
57494 Mollis azalea R. roseum 32 57499 Mollis azalea R. nudiflorum 42 57500 Mollis azalea R. roseum 29 57501 R. roseum Mollis azalea 13
The first flowering of the hybrids occurred in the spring of 1961. Bloom was confined to a few of the stronger seedlings in each of the four groups but was extremely gratifying inasmuch as the flowers proved to be of a large size and of good color and substance.
Flowers approach the size of those of the Mollis parent but the shape is intermediate between the bell and tubular forms of the respective parents. The color of the flowers in all of the hybrids is pure pink, uniformly deep pink in the hybrids involving R. roseum and varying from deep pink to pale pink in R. nudiflorum hybrids. The orange and salmon colors, so much a feature of the Mollis azalea, evidently are completely suppressed in these crosses. Similarly, the unpleasant odor of the Mollis azalea is lost in both of these hybrids and replaced in the case of the cross with R. roseum, with the delightful fragrance of that species. There was no significant difference between the reciprocal crosses of the Mollis and the Roseshell azalea.
The plants grew vigorously in 1961 responding well to a routine fertilization program providing a 4-6-8 azalea fertilizer consisting of 28 pounds of cottonseed meal, 10 pounds of ammonium sulfate and 10 pounds of iron sulfate. This was applied once in the early part of the growing season, a small handful around each plant. They received two foliar feedings of 10-8-7 azalea and conifer fertilizer.
The plants have had no protection from winter conditions since being placed in their present location. Examination of flower buds following temperatures below -30° F. in March 1962 disclosed some apparent injury. But to our surprise and pleasure nearly all plants bloomed freely in late May and early June, showing little evidence of the anticipated injury. A few very tall and possibly incompletely matured shoots failed to bloom, probably due to cold injury.
A failure to set seed in 1962, even when the flowers were artificially pollinated, may be attributable to cold injury rather than hybrid sterility, but further testing is needed to properly interpret this observation.
Efforts are now being made to develop methods of propagating the more promising individual seedlings in these hybrid groups so that if further tests show them to be worthy of introduction they can be made available to the nursery trade. The remarkable uniformity of the Mollis X Roseshell azalea populations suggests a possibility of producing this combination by seed if asexual methods of propagation do not prove feasible.