Species In Our Midst
Rhododendron calendulaceum is commonly called the "Flame Azalea" due to the resemblance of its expanded but unopened buds to candle flames. The Latin calendulaceum means "like a calendula", a genus of flowering plants with similar flower color. R. calendulaceum is one of 16 species in subgenus Pentanthera, section Pentanthera, subsectionPentanthera, the deciduous azaleas. Pentanthera native to New England include R. viscosum and R. prinophyllum. R. calendulaceum is not found naturally this far north and its range is from southern New York to northern Georgia, generally following the Appalachian Mountains. First identified in 1795 by French botanist Andre Michaux in the Southeastern US, it played an important role in the early development of the Ghent Hybrids, which began with its cross with R. periclymenoides.
Rhododendron calendulaceum forms an upright, spreading shrub or small tree 4 to 15 feet high in the wild but is generally much shorter in cultivation. The inflorescence is 3 to 10 flowered, with flowers appearing before or with the leaves. The corolla is funnel-shaped, about 2 inches long and up to 3 inches wide. A diagnostic feature is the length of the stamens, which protrude from the corolla-tube, being almost 3 times as long. The color range includes yellow, yellow-orange, orange-red and red, usually with an orange blotch on the upper lobe. It is generally not fragrant. Typical bloom time is May through June at which time entire hillsides in its native habitat may be brilliantly colored. Leaves are 1-3 inches long, medium to dark green above, with short hairs below. They sometimes turn a muted yellow in fall. Both leaves and branches often appear in whorls. Its native habitat includes open, dry sites in woods, on cliffs and hillsides, and on balds (open areas on mountaintops), from 600 to 5,000 feet. It is hardy to zone 5 (-25 F or -30 C).
Rhododendron calendulaceum is one of the most spectacular native shrubs of the Appalachian Mountains. E. H. Wilson wrote, "it must be considered one of the most gorgeous of American shrubs." Its flowers are very showy and are larger than most other natives. The color is often termed fiery, and was said to have alarmed early explorers who, upon viewing whole hillsides in bloom, thought they were ablaze. R. calendulaceum has been rated by the Species Study Group as a "good doer" for New England and it is on the Massachusetts Chapter's Proven Performer list.
Rhododendron calendulaceum's closest relative is R. cumberlandense, a smaller plant with a paler flower. There are some putative natural hybrids with R. periclymenoides, R. canescens and R. prinophyllum. The scarcity of natural hybrids may be due to it being a tetraploid, whereas other azaleas in its range are diploid; because of this genetic incompatibility, its hybrids are often sterile.
Rhododendron calendulaceum is said to be difficult to propagate from cuttings, but it is very easy from seed. Seed can usually be obtained from the ARS Seed Exchange, other seed exchanges, or from fellow chapter members. During the past two years, plants I grew from two different seed lots (open pollinated seed from Dick Brooks, and wild collected seed from 5,000 feet on Wayah Bald, NC, from Joe Schild) bloomed for the first time. The color range was quite variable, but within the range described above and very pleasing. Even though natural hybrids are relatively rare, controlled crosses are often made. The 2003 ARS Seed Exchange online catalog lists several controlled crosses between .R. calendulaceum and other deciduous azalea species. I'm growing several such crosses, including R. calendulaceum x R. occidentale from the 2001 ARS Seed Exchange. They are spending their first winter outdoors. In a few months I'll get some idea of whether or not this "best of east meets best of west" cross is hardy in Hopkinton, MA.
Joe Bruso, Zone 5b