White Rhododendron or Cascade Azalea Rhododendron albiflorum – Native Plant of the Week

Introduction

I first found the White-Flowered Rhododendron, also sometimes called the Cascade Azalea, high in the subalpine forests of the Caren Range on the Sunshine Coast of British Columbia. I was so excited to see a native Rhododendron that was so lovely and so hardy that it grew so high in the mountains. It has gorgeous large white fragrant flowers that capture the eye and lovely shiny light green foliage. Rhododendron albiflorum is native to the Pacific Northwest of the USA and to southern British Columbia and Alberta, Canada. It is a native shrub of the Ericaceae family in the Ericales Order of flowering dicots. If you live in the mountains in this area, I truly recommend going out and seeing it for yourself, or even trying growing it in your yard. Either way, it is a lovely shrub, and you will not be disappointed!

Description of Cascade Azalea Rhododendron albiflorum

Stem & Leaves

Rhododendron albiflorum is a native shrub of the Ericaceae family in the Ericales Order of flowering dicots. It grows from rhizomes and reaches heights of approximately 2.5 m. Its stems may be mostly smooth to furrowed and have unbranched hairs that are not glandular and are usually reddish in color.

The deciduous leaves are narrowly elliptic or ovate to obovate and 2 – 9 cm long by 0.8 – 3 cm wide. They are thin, membranous to chartaceous, and the margins are minutely serrated and ciliate when young. Leaves are hairy when young, and hairs may or may not be stipitate-glandular. Hairs may remain with age, or they may become more or less glabrescent. The apex of the leaf is acute to more or less rounded. Leaves are on hairy petioles (leaf stalks).

Flowers & Fruits

The pleasantly fragrant and somewhat pendulous flowers appear in later spring and summer soon after the leaves open up. They appear in axillary fasciculate inflorescences on 9 – 15 mm pedicels that are hairy and may or may not be stipitate-glandular. Its calyx lobes are 5-17 mm long, eglandular and stipitate-glandular-hairy, and its margins are also glandular-hairy. The partially connate (fused) petals are white and, on rare occasions, may be marked with yellow. They are basally fused into a 6 – 15 mm tube that spreads into five 3 – 9 mm long lobes that may be glabrous or minutely hairy. Flowers are cyathiform (bowl-shaped) and 9 – 22 mm in diameter. There are 9 (-12) stamens that are not exserted and are somewhat unequal in length, varying from 5.5 – 14 mm long.

White-Flowered Rhododendron fruits are capsules that are borne on erect pedicels. Capsules measure 6 – 8 mm long by 5 – 6 mm wide and are hairy. Hairs may be eglandular, stipitate-glandular, or unicellular hairy. The seeds have distinct tails on them, and their testa is closely appressed.

Toxicity

Although no mention of toxicity of this particular species could be found, all parts of plants of the Rhododendron genus are considered toxic, though rarely lethal to humans and occasionally to livestock and pets. Even though native people occasionally ingested it as medicine, it is no longer recommended for internal consumption today.

Similar Species Frequently Confused With

Rhododendron albiflorum is unique from other species of its genus with its white axillary flowers that are nearly actinomorphic and somewhat pendulous. However, the following are the most frequently confused members of its genus that grow in their range.

• Rhododendron menziesii – this is what it is most often confused with due to similar leaves, size, and habitat preferences. However, once in flower, they are easy to distinguish because of the small urn-shaped, somewhat orange or pinkish flowers of Rhododendron menziesii.
• Rhododendron occidentale – this also grows to similar sizes with similar leaves, but its range does not really overlap as it is found in coastal Oregon south to California, while Rhododendron albiflorum is only found in the Cascades in Oregon (somewhat inland). Also, the flowers of Rhododendron occidentale are white and yellow with longer lobes and are not bowl-shaped.
• Rhododendron columbianum – this grows to similar sizes but is usually shorter. It is also found in some of the same range but only grows in wet habitats. Finally, its inflorescences are always terminal, not axillary, and the flowers are not pendulous.

Distribution of Cascade Azalea Rhododendron albiflorum

White-Flowered Rhododendron is found only in Western Canada and the Pacific Northwest of the USA.

In Canada, Cascade Azalea is found in British Columbia and Alberta (in the mountains of Alberta).

In the USA, Rhododendron albiflorum is found in Washington, Oregon, Idaho, and Montana. There is also a disjunct population, sometimes called Rhododendron albiflorum var warrenii found in Colorado that is morphologically unique and considered Critically Imperiled.

Habitat & Growing Conditions of White-Flowered Rhododendron

White-Flowered Rhododendron is found mostly in montane subalpine and alpine habitats from 800 – 3500 m above sea level. They are typically found in coniferous forests, alpine thickets, stream banks, and seeps on rocky outcrops in the mountains.

Cascade Azalea will grow in full sun, part shade, or shade and grows in moist, wet, or dry soil types. However, it prefers full sun to partial shade and moist or wet soil.

Growing Rhododendron albiflorum in Your Garden

Rhododendron albiflorum is very cold-hardy, down to -30° C when dormant. However, flowers and new growth can be damaged by late frosts in the spring. They are best grown in light woodland shade in peaty or well-drained sandy loam soil with an acidic pH between 4.5 and 5.5. They will not grow well in dry arid soils, alkaline soils, or heavy clay soils.

The rootstock of Rhododendron species is surface-rooting, so do not allow other plants to grow on their roots. They form a root ball, though, which makes them relatively easy to transplant as long as the root ball is kept intact. If collecting a plant from the wild, always follow ethical wildcrafting principles and take only one small plant from a large healthy patch. Alternatively, you could air-layer several plants in the summer and then harvest them 14-24 months later. This is the easiest method of propagation. Cuttings of half-ripe wood are difficult to get to sprout roots.

If you ethically wildcraft some seeds, these are best harvested in the autumn and then sown that same autumn in a greenhouse with artificial light. Or, you could sow them in a cold greenhouse in mid-spring. Surface-sow the seeds and do not allow the soil to dry out. Pot the young plants and grow them in a greenhouse the first winter.

Wildlife Values of White-Flowered Rhododendron

Bees of all kinds will visit the flowers when in bloom. Otherwise, the toxicity of the genus prevents most wildlife from utilizing the plants. However, small birds are sometimes seen roosting on them, and small ground-dwelling rodents and lizards will use the thickets for cover.

Status of Rhododendron albiflorum

White-Flowered Rhododendron is considered Globally Secure, G5. It is currently not listed at all on the IUCN Red List.

In Canada, Rhododendron albiflorum is Locally Secure S5 in both provinces where it is found, British Columbia and Alberta.

In the USA, Rhododendron albiflorum is considered Vulnerable S3 in Montana and Critically Imperiled S2 in Colorado. The disjunct Colorado population has a smaller calyx and shorter stamens and is sometimes considered a separate variety of Rhododendron albiflorum var warrenii. In Washington, Oregon, and Idaho, it has not yet been ranked.

Cascade Azalea is not found outside of the western USA and western Canada.

Traditional or Other Uses of Cascade Azalea

Cascade Azalea as Food

The Okanagan-Colville people used to make tea with the leaves of Cascade Azalea. The Thompson used the branches in the bottoms and tops of their berry-picking baskets to keep their berries fresh.

Rhododendron albiflorum Medicinal Uses

The Okanagan and the Thompson both used a poultice of powdered burned wood mixed with grease for swellings and a decoction of the bark for stomach ailments. The Skokomish used a decoction of the buds for colds and sore throats. They also chewed the buds for an ulcerated stomach and applied the chewed buds as a poultice for cuts.

White-Flowered Rhododendron as an Ornamental

Sometimes Cascade Azalea is used as an ornamental shrub in gardens where conditions are right. It is a low-maintenance plant with gorgeous fragrant flowers. The Thompson native peoples also used the plant as a fragrance at times.

Ethical Wildcrafting of Rhododendron albiflorum

Check the status in your state before harvesting since it is imperiled or vulnerable in several states. See the above section on Status. Alternatively, grow it in your garden for both its lovely fragrant flowers.

If you are harvesting Rhododendron albiflorum from the wild, as always, use the 1 in 20 rule of Ethical Wildcrafting. Pick one in every 20 flowers, branches, buds, or plants you see.

Wildcrafting

If you are wanting to grow Rhododendron albiflorum in your garden, air-layering is the most successful method of propagation and the least impactful on the wild populations.

Air layering is very simple. All you need is some moss (which can be collected on-site), water, plastic, garden twine, and a knife. Use a knife to cut a small wound into a branch where you want it to root. Be sure not to cut too deep and never cut all the way around a branch, or it will die above the cut. Then wrap the wounded area in moistened moss. Wrap the moss in some plastic. I just cut thick plastic bags into pieces for this purpose. Then tie it on with some garden twine to ensure it stays on there.

Create several plants at the same time and return about 14-24 months later to harvest your new plants by cutting them off just below the plastic and twine. Bring the plastic and twine home with you to dispose of accordingly. Then simply pot up your new plants or plant them directly in the ground in suitable soil in a suitable location.

Picked buds and leaves can be placed in a basket, bowl, or paper bag and brought home for processing. If you are harvesting multiple products on the same day, be sure to label the roots in a paper bag so that you do not confuse different plants.

Processing

To dry the leaves or buds, simply place them on a rack or screen in a single layer and allow them to dry. Once dried, the leaves and buds can be stored in a jar for later use. Label your jar with the species’ name and the date of harvest. I also usually add the location of the harvest for my own reference. Do not grind or crush the leaves or roots until you are ready to use them to keep them as fresh as possible and preserve their medicinal properties. When you pre-grind, even if stored in glass jars, this increases the oxidation rate and rapidly degrades the medicinal properties so that they are rendered ineffective in a shorter amount of time than if left as whole as possible.

References and Resources

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – by Lyrae’s Nature Blog https://lyraenatureblog.com/blog/dictionary-of-botanical-terms/

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

IUCN Red List https://www.iucnredlist.org/

Native American Ethnobotany http://naeb.brit.org/

NatureServe Explorer https://explorer.natureserve.org/Search

USDA Plants Database https://plants.sc.egov.usda.gov/home

Useful Temperate Plants on Rhododendron albiflorum http://temperate.theferns.info/plant/Rhododendron+albiflorum

Willis, Lyrae (Unpublished).  Plant Families of North America. 

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