JARS v64n4 - Rhododendron occidentale - Western Azalea

Rhododendron occidentale - Western Azalea
Don S. Wallace
McKinleyville, California
Reprinted from the Eureka Chapter June 2010 newsletter

This is the only species of azalea that is found west of the Rocky Mountains. It was discovered by the expedition of Capt. Beechy in 1827 and was initially called Azalea calendulacea . In 1855, the plant was recognized as a distinct species, A. californica , but a year later this was changed to A. occidentale . It is now recognized as R. occidentale . The species is found growing in isolated populations from southern California to parts of Oregon, and has been found associated with soils having a high concentration of serpentine.

The species can grow as large as 15 ft [4.6 m] tall, and blooms from late May into July. It is hardy to -12º F [-24º C] and is quite fragrant, with the perfume being detected many yards (metres) away. It is generally unsuccessful in regions of the country with warm humid summers.

One of the largest stands of R. occidentale is in Humboldt County, CA, on Stagecoach Hill just above Big Lagoon. Many collectors from all over the U.S. come every year to photograph this beautiful species and to collect seed. There have been many unique forms found, including 'Pistil Packin Mama', 'Leonard Frisbie' and 'Picotee'.

Editor's note: Soils derived from serpentine are toxic to many plants because of high levels of nickel, chromium, and cobalt; growth of many plants is also inhibited by low levels of potassium and phosphorus and a low ratio of calcium/magnesium. The flora is generally very distinctive, with specialised, slow-growing species. Areas of serpentine-derived soil (derived from ultramafic rocks, in particular serpentinite, formed by the hydration and metamorphic transformation of this volcanic rock from the Earth's mantle) will show as strips of shrubland and open, scattered small trees (often conifers) within otherwise forested areas; these areas are called serpentine barrens.

Don Wallace is a member of the Eureka ARS Chapter and owner of Singing Tree Gardens.