The genus Rhododendron contains over 800 tree and shrub species worldwide. Of these, 21 are native to North America and 2 are native to California. In my last post, “Western Azalea” on 09-28-18, I discussed a Rhododendron native to California. The second native native California Rhododendron is Pacific rhododendron, Rhododendron macrophyllum. This specimen was blooming in May along the Myrtle Creek Botanical Trail in Six Rivers National Recreation Area (Del Norte County CA).
Pacific rhododendron grows in moist coniferous and mixed forests up to about 3,300 feet. It can be found in British Columbia, Washington, Oregon and California where it is most abundant in coastal areas.
A member of the Heath Family, Pacific rhododendron is an erect, scraggly, single or multiple-stemmed shrub. Sometimes this rhododendron assumes a tree-like form and can reach heights of up to 25 feet. The stems arise from a large, tuber-like root crown slightly under the surface of the ground. From this crown fine root hairs go into the ground. The root system is very shallow.
Pacific rhododendron leaves are evergreen, alternate, and leathery with a short petiole (stalk). Elliptical in shape, the leaves have entire margins which are slightly rolled under. The upper surfaces of the leaf blades are dark green and hairless with a sunken midvein. The lower blade surface is pale green, occasionally with rusty hairs.
The inflorescence is a large, showy, terminal cluster of pink to rose-purple, bell-shaped flowers. There is a rare white albino form of Pacific rhododendron. The five sepals and five petals are fused at the base. The petals are wavy-margined and one upper petal is usually spotted. The single pistil is reddish and silky. The stamens number 10. The flower buds are large and pointed with overlapping scales.
Pacific rhododendron fruits are woody capsules.
Pacific rhododendron forms dense ridge-top thickets on nephaline syenite, an unusual rock type composed of a type of granite without quartz that contains a jade mineral. Pacific rhododendron is believed to have mycorrhizal associations which aid in nutrient uptake, evidenced by this shrub’s ability to thrive on poor soils.
Following injury, Pacific rhododendron sprouts vigorously.
Because of its showy flowers and resistance to deer browsing, Pacific rhododendron is often used as an ornamental in landscaping. Mountain beaver in the Coast Ranges eat Pacific rhododendron. However, the foliage may be poisonous to sheep.
This shrub is the state flower of Washington. Other colloquial names for R macrophyllum include California rose bay, big leaf rhododendron and coast rhododendron. The species name, macrophyllum, means “big leaf” in Latin.