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Journal American Rhododendron Society

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Dr. Glen Jamieson ars.editor@gmail.com


Volume 36, Number 3
Summer 1982

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June Pink - A Hardy Native Azalea for New England
D. G. Routley & G. M. Dunn
Professors of Plant Science
University of New Hampshire, Durham, NH

R. prinophyllum
R. prinophyllum in Peterborough, N.H.
photo by D. G. Routley

 Introduction
One of the most delightful and completely hardy azaleas for New England and other northern areas is Rhododendron prinophyllum (June Pink or Roseshell azalea, formerly known as R. roseum).
       This species is found from northern New England to the Blue Ridge in Virginia and west to Illinois, Missouri, and Oklahoma. June Pink is widely distributed in New Hampshire having been collected in every county except Coos, Belknap and Strafford. It is especially abundant in the southwestern counties, growing most prolifically in open woods near a swamp, river, or pond. It is also found in Vermont, Western New England and New York. The species is not particularly striking as an ornamental when growing in its natural habitat. Competition for nutrients, moisture, and especially light, severely reduces the number of flowers, and the plant is usually thin and spindly when growing in the woods. Its true beauty becomes evident when it is grown in more light or even in full sun. A well-grown specimen will be a mass of flowers which are well exposed because they form before the leaves emerge. Not the least of its virtues is its delightful spicy fragrance. Finally, it is one of the hardiest deciduous azaleas in the United States.
       R. prinophyllum also appears to be resistant to two pests which afflict non-native azaleas. The most serious is the azalea sawfly, a small green caterpillar which defoliates and weakens many deciduous azaleas in June. The other problem is mildew, a fungus which attacks the leaves in late summer and also weakens the plant.
       Although June Pink is a beautiful hardy azalea, very little use has been made of this valuable native plant, partially because it is difficult to propagate by cuttings. However, it can be grown easily from seed, either from natural pollination by bees or from hybrid seed produced by hand crosses.
       The deciduous azaleas are among the most beautiful of all flowering plants, are very long lived, and require a minimum of care. At least 15 species are native to eastern North America. Much variation exists in flower color, fragrance, season of bloom, hardiness, plant size and many other traits in these species.
       Great opportunities exist for amateurs in breeding deciduous azaleas. The technique of hybridizing (described later) can be easily learned in a few minutes. Flower size is large, and the anthers and stigma are easily seen. Many species crosses are easily made within this group of plants. For the interested grower, hybridization can provide an extremely interesting and creative hobby. However, it is important for the amateur breeder to first learn the main characteristics, cultural requirements, etc., of native azaleas in his area, and to develop a clear and simple objective for hybridizing, rather than to simply make a large number of random crosses. Some of the reasons for hybridizing azalea species are the following: a) to increase disease or insect resistance, b) to improve winter hardiness, c) to extend the season of bloom or to provide increased color variation, fragrance, etc. or d) to provide better hybrid vigor, either of seedlings or mature plants. Another potentially valuable improvement would be to produce a strain of deciduous azaleas which could be readily propagated by cuttings.

Seed Collection
Azalea seed is formed in capsules which mature in the fall, usually by early to mid-October in Durham. They can be collected any time after they begin turning from green to brown and become hard. On drying, the capsules open to release the seed, but crushing is usually needed to get all the seed out. The small, brown, papery seeds can be cleaned by using a fine mesh screen or sieve.

Growing Seedlings
Seeds can be planted immediately after collecting because they have no cold requirement for germination. They can be stored for planting months or years later if kept dry and cool. A jar with a tight-fitting lid will work well if kept in the refrigerator.
       At the University of New Hampshire, azalea seeds usually are sown in November or December in flats (containing a soilless mix based on peat which is amended with perlite, vermiculite or sand for good aeration and drainage). Shredded sphagnum moss, either purchased or collected from a bog, is also suitable. Leaf mold will also work. The seeds may be broadcast or sown in rows and either left uncovered or covered very lightly. The flats can be covered with plastic or glass to prevent drying out, but must be kept out of the sun to avoid overheating.
       Germination is hastened with bottom heat provided with a heating cable; a food warming tray will work for a few flats. Where temperature (usually 70° to 80°F) and moisture are adequate, germination will occur in two or three weeks. The seedlings grow better at a somewhat lower temperature of 60° to 70°F. Fertilizing should begin when the first true leaves appear; a very dilute solution (less than 1 teaspoon/gallon) of a soluble, acid-type fertilizer should be used. If the water is hard, rainwater, or water from a dehumidifier might be used.
       Good light is required for seedling growth. A greenhouse is ideal, but in the short days from November to April, extra light is needed. Fluorescent lamps of the cool white or warm white type are most efficient. A day-length of at least 16 hours is needed and continuous light is best. As the days lengthen in spring, the lights can be removed, and shade from the hot sun provided.
       If a greenhouse is unavailable, seed can be germinated and grown entirely under fluorescent lamps. A combination of cool white and warm white tubes is suitable - the special plant growth lamps are not needed. Seedlings should be kept about a foot below the lamps, adjusting the distance if they become stunted from too much light or leggy from not enough. A timer will regulate the "day" length to 16 hours or more, or the lights can be left on continuously.
       If the seed is kept until spring, a hotbed or cold frame can be used for germination and growing. The seedlings need protection from the hot sun and they must never be allowed to dry out. They can remain in the frame until the following spring, thus assuring some protection from winter freezing and thawing.
       Seedlings should be transplanted to give more growing room as soon as they become crowded. The same potting mix as used for germination is adequate. Spacing of about two inches is usually adequate until transplanting in the spring.
       The young plants can be lined out in the garden for their first year outdoors. A generous application of organic matter, preferably peat, is essential for healthy growth. It should be well mixed with soil. A mulch of pine needles or oak leaves will keep the roots cool and moist. Partial shade is helpful for the first year, but full sun after that will favor bud formation. Watering during dry periods is essential to maintain active, vigorous growth of the seedlings.
       Azaleas have shallow, hair-like roots and are easily transplanted whenever the soil is not frozen and moisture is adequate. In their permanent location, flowering is favored by full sun; winter sun will not adversely affect deciduous azaleas because they lack leaves at that season.

Hybridization Techniques
The techniques of crossing rhododendrons or of crossing azaleas are simple. In nature, the plants are largely if not entirely cross-pollinated by various kinds of bees. Rhododendron flowers have 10 stamens, each with an anther or pollen sac, while American azaleas have only five. The long style extends well above the anthers and until the flowers open, the stigma is wrapped up in the ends of the petals, well protected from pollen. In crossing, a flower truss is chosen with mainly unopened flowers and any opened flowers are cut off since insects soon visit an opened flower.
       Emasculation, or removal of anthers, is done by cutting around the base of an unopened flower and pulling both petals and stamens away. Small scissors and tweezers are best for these operations. The stigma is receptive when it is sticky, often before the flower opens, and when at the proper stage, the cross can be made simply by touching the stigma of the emasculated flower with an anther from the male parent. The pollen often pulls out as a sticky thread to cover the surface of the stigma. The stigmatic surface should be well covered with pollen and several anthers may be needed to effect this. Most breeders do not think it necessary to cover the emasculated flowers since they will not attract insects without petals.
       In most cases, the cross can be made in either direction, that is, one can use either parent to provide the pollen. However, if the parents differ widely in date of flowering, it will be necessary to collect and store the pollen from the earlier flowering parent until the female parent blooms. Rhododendron or azalea pollen can be stored for long periods, a year or more, if kept dry and cold. Gelatin capsules from a pharmacy make good containers. These may be stored in a small screw-capped bottle which contains some silica gel or other drying agent. Keep the tightly capped bottle in the refrigerator or, if the pollen is to be kept more than a few weeks, in the freezer. Allow the bottle and contents to warm up before opening or moisture will condense on the inside.

Research at UNH on June Pink
Most Rhododendron cultivars and evergreen azaleas can be propagated readily by rooted cuttings. Rooting is often done in a mist bed or plastic tent. However, percentage of rooting is usually much lower with deciduous azaleas. Most workers consider R. prinophyllum one of the most difficult to propagate successfully. In limited trials at UNH, we have obtained no more than 10% rooting with this species.
       We have grown several species successfully from seed including R. prinophyllum, R. japonicum, R. luteum, and R. arborescens as well as several hybrids among these species. R. japonicum, native to Japan, is a showy, large flowered azalea which is apparently quite hardy in New England. Our plants of this species were grown from seed collected by Dr. Rad Pike on the Japanese island of Hokkaido. They are only two to three feet high and the flowers are bright orange to yellow.
       R. luteum, from eastern Europe, has bright yellow flowers with a sweet fragrance, but is not reliable hardy in New Hampshire. Unfortunately, we now have only one specimen of unknown origin.
       R. arborescens, native to eastern North America, is white with a fragrance like heliotrope. It is one of our best native white azaleas which is hardy in the north. It is considerably later than R. prinophyllum, and our plants are not usually in flower until late June or early July.
       Flowering date for R. prinophyllum, R. japonicum and R. luteum is sufficiently close so that crosses among them can usually be made, but since R. arborescens is distinctly later, pollen must be stored from any of the first three species to cross with it. R. prinophyllum and R. japonicum flower in late May or early June in Durham, R. luteum is a few days later. Flowering date depends on the weather. The peak of flowering in Durham for two plants of June Pink was: 6/2/66, 6/11/67, 5/27/70, 6/4/72, 5/26/75, 5/25/ 76, 5/19/77, 5/29/78 and 5/21/79. The very early flowering in 1977 was due to an unusually hot period near mid-May. Date of flowering in this species can therefore vary by as much as three weeks or more, depending on the season.
       The two plants of R. prinophyllum, which have been used to produce hybrids, were originally obtained by Dr. L.P. Latimer from the woods near Lovell Lake, Vermont. They have flowered every year since we first observed them in 1965. One of them on the northeast side of the house, but with open sky above it, has a spread of eight to nine feet across the crown, is approximately six feet tall, and flowers very well each year. Flowers are more sparse on the other plant which is in a shady area on the edge of the woods.
       Seed of R. prinophyllum has also been kindly provided by an interested citizen in Peterborough and one in Lake Sunapee. Thus far, most plants of June Pink which we have observed in the woods have been near water. We observed natural re-seeding of June Pink in the peat-like soil near Lake Sunapee.
       Four or five nursery flats, of several hundred plants each, have been grown of R. prinophyllum since the fall of 1974. Seedlings have also been grown for two years from the other three species. Seedling vigor is somewhat lower in June Pink than in other species. However, we have been able to obtain some flowering on these seedlings in three to four years. Observations indicate that most, if not all, of June Pink seedlings will be light to medium pink in color with the spicy clove fragrance characteristic of the species.

Pink hybrids of R. japonicum x R. prinophyllum
      Pink hybrids of R. japonicum x R. prinophyllum growing
with orange and yellow R. japonicum in Routley's garden
photo by D. G. Routley

       We have made numerous hybrids of R. prinophyllum with R. japonicum, and this cross appears to have considerable potential. The hybrid plants which closely resemble R. prinophyllum, are more vigorous than it is, hardy with a pink flower intermediate in size between the two parents and an orange blush from the R. japonicum parent. They may not all have the fragrance of June Pink. We have produced second generation seedlings from this cross by inter-pollinating the first generation hybrid plants to see whether useful recombinations of traits from the two parents can be obtained.
       Relatively few hybrids have been obtained from crossing R. prinophyllum with R. luteum or R. japonicum with R. luteum. None of these have yet flowered. This year, in our first attempt, we obtained many hybrid seeds of the cross R. prinophyllum with R. arborescens. This hybrid may provide another period of bloom since the parents differ widely in date of flowering. These hybrid seedlings also have excellent vigor.
       A number of selections of R. prinophyllum with the deeper pink flowers have been made and planted in isolation in a seed orchard.

R. japonicum x R. prinophyllum    R. japonicum x R. prinophyllum
Hybrids of R. japonicum x R. prinophyllum
photos by D. G. Routley

Summary
Based on our experience to date, we believe that June Pink (R. prinophyllum), a beautiful and completely hardy native plant, deserves much more attention and usage by nurserymen and home owners in New England. We feel it is possible to grow seedlings to salable size in about four or five years. Generally, we would encourage greater utilization of native plants such as June Pink, which often have superior hardiness or disease and insect resistance, to plant species shipped into the area.


Volume 36, Number 3
Summer 1982

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