Of the 100+ Rhamnus species worldwide, the Northwest’s representative is a lovely small tree or tall shrub. The first thing you may notice about Cascara (Rhamnus purshiana, syn. Frangula purshiana) is its texture: Silvery gray bark that’s nearly smooth, and oval, glossy green leaves with veins so prominent that they make the surface wavy and crinkled-looking. But Cascara’s charm doesn’t stop there. Springtime brings loose clusters of small, pale greenish-yellow flowers that later become small red fruit (a drupe, each containing 2 or 3 seeds) that ripen to the deepest purplish-blue possible. Later in the year, its leaves turn yellow to orange in autumn and may hang on in areas with mild winters.
The genus name, Rhamnus derives from the ancient Greek “rhabdos,” which means stick, rod or twig. The epitaph, purshiana, commemorates Frederick Traugott Pursh, a remarkably well-traveled (often on foot)18th century German-American botanist who made major contributions to North American botany.
How it grows
Cascara naturally occurs along the Pacific coast from British Columbia south into northern California, as well as parts of Idaho and Montana. It’s found in moist to dry shady forests and mixed woodlands, often along streams and in moist ravines, at low to middle elevations, as well as floodplains. It grows up to 30 feet tall and about half as wide.
The dried bark of Cascara has been used for hundreds of years as a laxative—first by indigenous peoples and then commercially (sold as Cascara sagrada)—and the high demand for it has led to unethical harvesting from wild trees, which deprive the plants of their protective and essential bark. It is probable that this practice has heavily reduced cascara populations.
Pollinators—such as hummingbirds and native bees—come to Cascara’s late spring flowers. Birds, including band-tailed pigeons, robins, tanagers and grosbeaks, as well as mammals such as raccoons and coyotes, are attracted to the pea-sized fruit. Birds like bushtits, kinglets, warblers and chickadees forage on insects found on leaves, twigs and bark. Cascara is a host plants for the larvae of gray hairstreak and swallowtail butterflies, which feed on leaves. Mule deer and other mammals may use it as browse.
Try it at home
Cascara is a great choice for small yards or where large trees wouldn’t thrive, and I don’t know why it’s not planted more often. Besides it’s beauty and wildlife appeal, it’s a fast grower that can take full sun to full shade, but does best in partial shade. Though it is drought tolerant when established (especially in shade), it will look and do its best with somewhat moist, well-drained soil that’s rich in organic matter (add leaf compost!). In general, trees planted in hot, sunny areas will need more water. Like us, Cascara shows sensitivity to toxic gases and tiny sooty particles that are belched out of fossil fuel powered vehicles, so keep it away from busy streets and highways. It is reportedly fire resistant.
When planting multiple trees, place them about 15 feet apart (about 10 feet apart for shrubs used as a hedgerow). Cascara shrubs are a good substitute for invasive English laurel or Portugal laurel shrubs where they can be left unpruned.
Grab a partner
Cascara may grow in the understory of trees such as big leaf maple, Douglas-fir, and western hemlock, where it might live alongside vine maple, red alder, willows, and red-twig dogwood.
It’s worth noting that some Rhamnus species, such as R. cathartica (“common buckthorn,” native to parts of Europe, northwestern Africa and western Asia), are invasive outside their natural range. R. cathartica was introduced as a garden plant and is now naturalized in parts of North America, probably because it leafs out earlier than native species, often contributing to their downfall.