by Rich Larson, Nursery Manager
North American azaleas number over 15 species, 14 of which grow natively in eastern North America. As a group, they are highly variable in their habit, timing of their flowers, ecology, and genetics. Sweet azalea (Rhododendron arborescens [pictured: right]) for example, may grow to only 3’ or less at high elevations in North Carolina whereas the Coleman azalea (R. colemanii), a newly documented species native from Georgia to west central Alabama, can grow of over 15’ tall and occupies moist, sandy loams along creeks and hillsides covered with oak and pine. The flowers on the majority of species open in late April or early May as their leaves unfold with petals more or less fused and funnel shaped, arranged in loose clusters varying from 3-24 florets. However, a few species bloom much later from early June to mid-June as with the swamp azalea, (R. viscosum) to late July and mid-August as with plumleaf azalea ( R. prunifolium), a rare endemic to southwestern Georgia. Flowers are diverse in color from white to yellow to soft pink to orange and orange-red and are often highly aromatic. I like to characterize this scent as “spicy”. Other variations within this section may include; distinct blotches on insides of petals, the hairiness or non-hairiness (glabrous) surface of twigs and floral bud scales, the tightness or looseness of the seed coat (testae) and seed bearing tails (winged) or no tails (wingless)and their rhizomatous or non-rhizomatous growth habit. Finally most species have two sets of chromosomes (diploidly) but about one third of the species have four sets of chromosomes (tetraploidly).
But though they are divergent in many ways, our native azaleas do share some commonalities. For one thing, they are all deciduous and this can be an unexpected but desirable ornamental asset as many take on excellent fall color. My favorite is this respect is the pink shell azalea (R. vaseyi [pictured: left, below right, and featured image on homepage]), a high elevation species endemic to just a few sites in the Blue Ridge Mountains. This species consistently displays outstanding fall color ranging from orange to red. And yet how many gardeners expand their vision past the spring time and consider fall color a selling point when buying rhododendrons? Then too, all our North American azaleas lack characteristic scales (lepids) on the undersides of their leaves are subsumed within a larger group of taxa called elepidote rhododendrons. This distinguishes them from a large number lepidote species which are mostly small statured, small leafed, evergreen rhododendrons. The often planted Rhododendron ‘PJM’ is noteworthy example of a rhododendron bearing these characteristic glands.
Lastly, I often have suggested that the deciduous nature of native azaleas renders them more amenable to cultivation in central Ohio, a region noted for its open and dry winters. Unlike evergreen rhododendrons, azaleas are better adapted to open, xeric conditions and thrive best in light shade. Consequently, azaleas function beautifully under trees such as oaks, (Quercus) whose canopies allow 25-35% light penetration. But taken as a whole, one can be a bit more liberal in planting azaleas relative to evergreen rhododendrons which by contrast, require more careful siting to protect them from the desiccating effects of winter sun and wind. Remember too, that plants under stress are always subject to greater insect and disease problems and thus intuitively, one might expect azaleas to be relatively free of these issues when juxtaposed to their evergreen cousins. I have rarely seen azaleas ravaged by the chronic effects of phyotphora root rot (Phytophora cinnamomi) and stem cankering and wilting (Botryshaeria dothida) so often evident on evergreen rhododendrons. Next blog: Hunting them down and propagating native azaleas.
Azaleas grow best in moist, slightly acid, sandy loams and in soils enriched with organic matter. As with all rhododendrons, they cannot survive in heavy mineral soils that are poorly drained and structured. But this unfortunately aptly describes the soils (or sub soils) in most urban and residential landscapes. Therefore, extra care must be taken to amend the existing soil conditions through generous addition of organic matter in the form composted oak leaves or pine needles, pine soil conditioner or peat moss.
Join Rich’s program on January 25th, Grafting the Right Way, and learn the art and science of grafting to ensure future success in reproducing your own plants. Visit our calendar to sign up now! View Rich’s Grafting Class.