Rhododendron prinophyllum (Small) Millais

Mountain Azalea


CC = 8
CW = 5
MOC = 17

© SRTurner

Family - Ericaceae

Habit - Shrubs or small trees 0.8-3.0 m tall, not colonial.

Stem - Bark dark gray, often becoming scaly with small plates. Twigs densely and often minutely hairy, reddish brown, becoming brown to dark gray, the winter buds ovoid, with several overlapping scales, these sparsely and minutely hairy on the outer surface.

Rhododendron_prinophyllum_stem.jpg Stem and leaf bases.

© SRTurner

Leaves - Alternate but tending to be clustered near the branch tips, short-petiolate. Leaf blades 4-9 cm long, 2-4 cm wide, ovate to obovate, the margins entire and short-hairy, sparsely hairy on both surfaces, the main veins of the undersurface densely hairy.

Rhododendron_prinophyllum_leaves.jpg Leaf cluster near stem tip.

© SRTurner

Rhododendron_prinophyllum_leaf1.jpg Leaf, adaxial.

© SRTurner

Rhododendron_prinophyllum_leaf2.jpg Leaf, abaxial.

© SRTurner

Inflorescences - Short, umbellate racemes of 4-13 flowers, appearing before or with the leaves.

Rhododendron_prinophyllum_inflorescence1.jpg Inflorescence.

© SRTurner

Rhododendron_prinophyllum_inflorescence2.jpg Inflorescence.

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Rhododendron_prinophyllum_inflorescence3.jpg Inflorescence detail.

© SRTurner

Flowers - Slightly zygomorphic, hypogynous, fragrant. Calyces deeply 5-lobed, 1.0-1.5 mm long, the lobes rounded to bluntly pointed at the tip, densely glandular-hairy. Corollas trumpet-shaped, the tube 1.2-1.8 cm long, the flaring to spreading lobes 1.1-1.6 cm long, rosy pink, the outer surface moderately to densely glandular-hairy. Stamens 5, exserted, the filaments 4.0-4.5 cm long, arched upward, hairy toward the base, the anthers tiny, ovate, lacking tubes or awns at the tip, dehiscing by 2 terminal pores. Ovary superior, densely glandular-hairy, with 5 locules, the placentation axile. Style 4-6 cm long, arched upward or slightly S-shaped, exserted to slightly beyond the stamens, mostly not persistent at fruiting, the stigma obconic to disk-shaped, shallowly 5-lobed.

Rhododendron_prinophyllum_flower.jpg Flower.

© SRTurner

Rhododendron_prinophyllum_tube.jpg Corolla tube pubescence.

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Rhododendron_prinophyllum_functional.jpg Stamens and style.

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Rhododendron_prinophyllum_corollas.jpg Corollas.

© SRTurner

Fruits - Capsules 10-14 mm long, 3-5 mm wide, narrowly oblong-ovoid to cylindrical, sparsely to densely glandular-hairy, dehiscing longitudinally from the tip along the sutures. Seeds 2.0-3.5 mm long, 0.6-1.0 mm wide, irregularly ovate to elliptic, strongly flattened and irregularly winged, the wing more or less entire at the tip and more or less fringed or dissected at the base, the surface more or less smooth, yellowish brown to light brown, shiny.

Flowering - April - May.

Habitat - Mesic upland forests and ledges of bluffs, mostly on north-facing exposures and acidic substrates.

Origin - Native to the U.S.

Lookalikes - None when flowering.

Other info. - This attractive shrub occurs in a band through parts of the Missouri Ozarks, extending down into Arkansas and slightly into Oklahoma. A separate disjunct population is found in the northeastern U.S. It has a distinct preference for good quality habitat and acidic soils. The plant is easy to recognize when in bloom by its large pink flowers, which closely resemble nothing else in the state. The vegetative habit is also distinctive, with leaves which are often obovate (widest above the middle) clustered near the stem tips.

Various species of azalea are commonly cultivated. R. prinophyllum would make a fantastic garden subject in the right area and with the proper soil, though it may require regular care. The species has also been known as R. roseum (Loisel.) Rehder and Azalea prinophylla Small.

Photographs taken at Pickle Springs Natural Area, Ste. Genevieve County, MO, 5-11-2020 (SRTurner).