Of the 35+ Frangula species worldwide, the Northwest’s representative is a lovely medium-sized tree or tall shrub. The first thing you may notice about Cascara (Frangula purshiana, syn. Rhamnus purshiana) is its texture: Thin, silvery gray bark that’s nearly smooth but with a patchy look, 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. In autumn, its leaves turn yellow to orange and may hang on in areas with mild winters.
Frangula purshiana is a member of the Rhamnaceae family; the species name relates to frangulanin, a peptide alkaloid. The epithet, 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 or in moist ravines at low to middle elevations, as well as floodplains. It grows up to about 30 feet tall and roughly half as wide.
Cascara and red alder look a bit alike; you can tell them apart mainly by their fruits and leaves. Cascara produces a red to deep purple drupe, while alder’s fruit is an inch-long woody fruit that resembles a cone, known as a strobile. The leaves of Cascara are shinier and smoother than those of alder, which are tightly rolled under on the edges.
Conservation
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.
Wildlife value
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 plant for the caterpillars of gray hairstreak and swallowtail butterflies and more than a dozen moth species, which feed on its leaves. Mule deer and other mammals may use it as browse.
Try it at home
Cascara is a great choice for small yards or places where large trees wouldn’t thrive, and I don’t know why it’s not planted more often. Besides its beauty and wildlife appeal, it’s a fast grower that can take a fair amount of sun to full shade, but it 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. 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 it may be best to 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 grows 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.
Do you have a primary source for a study that proves that cascara is sensitive to air pollution? There are a few plant experts who are saying that this just isn’t so and who think planting them alongside a freeway is appropriate. I can find lots of secondary sources saying that they’re sensitive to air pollution but not a single one says where they got the information. The only studies I’ve found are from the 80s and 90s that say a cascara epiphyte is sensitive to air pollution.
Cindy, I doubt that there ever was a formal study on this topic. A USDA page does mention that Cascara can be susceptible to crown rusts and certain herbicides, but chances are, the hesitation to grow cascara trees near highly polluted areas comes from Art Kruckeberg’s writing following his personal and insightful observations: “… in the urban setting, the leaves rarely achieve their natural size and texture, nor their bright green hue. In the environment of cities, urban pollution and aphid attacks suppress the best expression of the handsome foliage. Cascara, then, is for the rural woodland garden or suburban park, either at home there by nature’s chance or by gardener’s intent.”
Cascara is shade-tolerant, a moist-site indicator, and is often found naturally growing in the understory, not in harsh sunlight along roads. Of course, there are a great many plants that are also likely sensitive to air pollution and will do much better in an intact ecosystem without high levels of pollution. So the recommendation to not grow them in densely populated areas with heavy traffic is just that: a thoughtful recommendation, not a rule.
Hi,
Do you know whether topping (pruning top meristems) of lonely and etiolated (talla en skinny) individuals in small gardens are sensible options to build strength?
Thanks!
Topping trees is never a good idea. Here’s why: https://www.oregon.gov/odf/Documents/forestbenefits/WhyToppingHurts.pdf
Hope that helps; thanks for your question.
thanx