Rhododendron macrophyllum

Pacific rhododendron

Ericaceae

The Basics

Taxonomy: Kingdom - Plantae (plants). Subkingdom - Tracheobionta (vascular plants). Superdivision - Spermatophyta (seed plants). Division - Magnoliophyta (flowering plants). Class - Magnoliopsida (dicotyledons). Subclass - Dilleniidae. Order - Ericales. Family - Ericaceae (heath). Genus - Rhododendron L. Species - Rhododendron macrophyllum D. Don ex G. Don

Ecology: Pacific rhododendron is found in coastal to low montane conifer forests on soils that are moist but well drained and frequently shallow. In the higher, wetter and cooler mountain hemlock (Tsuga mertensiana) and silver fir (Abies amabilis) zones, Pacific rhododendron is an understory dominant on relatively warm, dry soils at lower elevations within the type.

Identification

Shrubs or trees, to 5 m, sometimes rhizomatous. Stems: bark smooth to vertically furrowed, shredding; twigs with basally branched, crisped/matted, eglandular hairs, very quickly glabrate. Leaves persistent; petiole glabrous; blade elliptic to slightly ovate or obovate, (6-)8.5-14(-20) 2.5-5.5(-7.5) cm, thick, coriaceous, margins entire, plane to revolute, glabrous, apex acute to obtuse or slightly acuminate, surfaces scattered eglandular-hairy (hairs branched basally, crisped, very quickly deciduous), abaxial surface smooth. Floral bud scales multicellular eglandular-hairy (hairs branched basally), and unicellular-hairy (hairs short to elongate) abaxially, margins eglandular-hairy (hairs branched). Inflorescences 10-20-flowered; bracts similar to bud scales. Pedicels 30-60 mm, glabrous. Flowers opening after development of leaves (of flowering shoots), erect to horizontal, fragrant; calyx lobes 1-1.5 mm, glabrous, except margins eglandular- and stipitate-glandular-hairy; corolla white to pink or rose-purple, with yellowish green spots on upper lobe, broadly campanulate, 24-48 mm, outer surface glabrous, petals connate, lobes 14-30 mm, tube gradually expanding into lobes, 10-23 mm; stamens 10, included, unequal, 16-37 mm. Capsules borne on erect pedicels, 13-25 4-7 mm, eglandular-hairy (hairs ferruginous, branched or unbranched) and, often, stipitate-glandular-hairy. Seeds without distinct tails, flattened portion of testa well developed at each end; testa expanded, dorsiventrally flattened, loose.

Threats

In the WFDP: Pacific rhododendron is susceptible to supression-induced mortality in this energy-limited system..

Fire effects: Pacific rhododendron grows in moist Pacific Coast forests where, historically, major fires were infrequent and more frequent surface fires burned small areas. The main fire adaptation of Pacific rhododendron is its ability to resprout from a shallow, tuberlike rootcrown. Pacific rhododendron appears to be top-killed by most fires. The shallow rootcrown could be heat-killed during severe fires, thus killing the entire plant. Low severity fires may allow the survival of basal stem buds, accounting for observations of its increased survival following such fires.

Reproduction

Flowering and Fruiting - The flowers are pollinated by bees and plants begin to bear seed when they are 5 years old. The fruit is a dry, rusty-brown, pubescent capsule that divides into five parts by splitting lengthwise to release the numerous, minute seeds. The seeds, including the wing, are less than 3 mm long. Pacific rhododendron seeds germinate without stratification and are viable for up to 2 years. The seeds require light for germination.

Vegetative Reproduction - If aboveground portions are killed, Pacific rhododendron can regenerate vegetatively by sprouting from stem bases and from the rootcrown.

Species Distribution

Citation

USDA Plants Database
USDA, NRCS. 2016. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.

USFS Plant Database
Habeck, R. J. 1992. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

Flora of North America
Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

Burke Museum of Natural History and Culture
Burke Museum. 2016 [Online]. University of Washington.