Scientific Name
Rhamnus cathartica
This small tree, native to Europe, has escaped cultivation in the U.S. and aggressively colonized many landscapes in the northern states. The plant may either take the form of a large shrub or a small tree reaching about 20 feet with a rounded spreading crown; the Colorado champion is 25 feet tall.
The leaves are “subopposite,” meaning some grow from the twigs in opposite pairs while others are slightly offset. As you might expect, the twigs and branches carry stout thorns about 1 inch long, though they are not quite as dangerous as those of old-style hawthorns or honey locusts. The tree is very adaptable to different soils, hardy to 30°F below and tolerant of drought. Buckthorn is rarely planted — and it doesn’t need to be — because birds eat the fruit and spread the seeds widely. The fruit is a quarter-inch berry that matures to a shiny black color in the fall on the female trees (these trees only produce either male or female flowers). Birds gradually consume the fruit during the winter but cannot digest the hard seeds. The fruit is often produced in large numbers and can stain pavements and decks wherever it falls, either by gravity or with the help of the birds’ digestive systems.
Unfortunately, European buckthorn has overtaken several areas on the Regis campus. It is a very difficult plant to eradicate, especially without injuring nearby plants that are more valuable. Chemical control methods have well-known drawbacks, and mechanical eradication is time-consuming and requires a long-term effort.
It is somewhat surprising that this tree is not on the Colorado invasive species list. One reason may be that its spread may depend on irrigation, making it primarily a problem in urban areas. Even so, the well-formed buckthorn tree north of Carroll Hall that was selected for this group stands alone and presents a relatively benign appearance compared with the dense infestations found on the west side of the O’Connell Grotto and in other areas on the campus.