Common Buckthorn is an introduction from Europe two centuries ago by the nursery trade, for use as hedge material, that is now the scourge of every woodland in the entire United States and Canada north of the southern tier of U. S. states. There is variability in the leaves and flowers as explained below.
Stems and bark: Growing as a small shrub or to a 20 foot tall tree, the plant is identifiable on younger specimens by the dark bark marked with lenticels (raised portions of bark that are air pores). Old bark is dark gray, scaly and blocky. Sapwood is pink and heartwood is yellow to orange. There are spiny tips on many branches. Bark on young twigs and stems is gray to reddish brown. Buds are reddish-brown to dark brown and appressed to the twig. Twigs angle in many directions.
Leaves are dull green, elliptical to oval, stalked, with a round base, blunt tip and a toothed edge. They have 2 to 4 pair of up-curved veins. Most are opposite but some can be sub-opposite. Leaves are usually restricted to the branch tips. The leaves will remain until frost, thus in late fall Buckthorn is easily identified by their presence.
The inflorescence is a dense cluster at the leaf nodes of the current years growth just after the leaves unfold. Buckthorn is dioecious, that is, the male and female flowers are on different plants, however, a few plants may have perfect flowers.
Flowers are small, 1/4 inch wide, with a yellow-green calyx that has 4 spreading lobes and 4 inconspicuous petals. Male (staminate) flowers usually number 2 to 8 per cluster (but may have up to 40) and have 4 stamens. Stamens have reddish brown filaments and yellow anthers. The female (pistillate) flowers usually number 2 to 15 per cluster (but may have up to 30) and have a single pistil whose style has a 4-parted tip. At the base of the style is a reddish nectar ring.
Fruit: Fertile flowers mature to blue-black 1/4 inch drupes in late summer. Each contains 3 to 4 egg shaped black seeds, 5 to 6mm long, that are flattened on one side and tapering to a short beak. The seeds are long lived and were it not for the birds that love the berries, control would be easier, but our avian friends spread the seeds from within the berries far and wide. The drupes have strong laxative properties - even for birds.
Habitat: Like many invasive plants, Common Buckthorn adapts to a wide variety of habitats - from dry open areas, to woodlands, to moist wet meadows. As it grows best in full sun, it is usually restricted to woodland edges. Roots are not deep, but are angled making pulling difficult. Once established the dense growth of seedlings crowd out all other plants. Eradication by hand pulling is necessary for smaller seedlings as the roots will re-sprout new stems if not removed or killed. Large specimens must have the cut trunk treated to prevent re-growth. Controlled areas must be revisited as seeds in the soil will continue to sprout as long as they are viable, which may be up to 10 years.
Names: The genus, Rhamnus, is an old Greek name for Buckthorn. The species name, cathartica, refers refers to a purgative, which the berries are if eaten. The author name for the plant classification, 'L.', refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: Common Buckthorn is most easily confused with the native Alder-leaf Buckthorn, Rhamnus alnifolia, where the leaves also have serrated edges but differ with 6 to 7 pairs of leaf veins and the flowers are 5-parted. Another species is Glossy Buckthorn, Frangula alnus, whose leaves are not serrate, but shiny, and the berries develop along the branch going through a red stage before turning black, the flowers are 5-parted and the branches do not have thorns. All three retain their green leaves late into autumn when most deciduous species have shed theirs.
Above: 1st photo - The flowers occur in clusters around the leaf nodes. 2nd photo - Individual male flowers are quite small with 4 stamens and 4 spreading lobes of the corolla.
Below: The pistillate flowers have a style with a 4-parted tip. Below is a reddish nectar ring.
Below: 1st photo - Leaf structure - toothed edges and blunt tips. 2nd photo - Typical hedge use of Common Buckthorn. Trimming reduces the amount of berry production.
Below: 1st photo - Brownish-gray bark and thorns as usually found on mid-size plants. 2nd photo - Tall hedges will produce more berries. Buckthorn hedges were first established in the first quarter of the 20th Century and people are reluctant to remove them.
Below: 1st photo - The scaly bark of older specimens - may be confused with Black Cherry if the rest of the tree is not considered. 2nd photo - The green flower buds emerging at the leaf nodes in early spring before the leaves have matured. 3rd photo - 5 Weeks later the young fruit is formed. Early and rapid development is a general characteristic of invasive species.
Below: Each drupe contains 3 to 4 black angled seeds, 5 to 6mm long.
Below: 1st photo - Twig with typical reddish-brown buds appressed to the twig. Leaf Comparison: 2nd photo is the leaf of Common Buckthorn with the toothed margin and 3 pair of veins; 3rd photo is the leaf of Glossy Buckthorn with the smooth margin and up to 10 pair of veins. Both are invasive.
Below: Buckthorn fruit of late summer, early autumn. Photo courtesy of Kathy Connelly
Notes: Common Buckthorn is not native to any part of North America. It has spread from material used for landscape plantings and shelter-belts and has resulted in many states banning the sale or importation of the plant. It is considered noxious in most states and subject to eradication where found. It was not reported to be in the Garden at the time of Martha Crone's 1951 plant census, but had made great inroads thereafter resulting in the start of a continuous program of eradication by Gardeners Ken Avery and Cary George. Today, Garden Curator Susan Wilkins and staff with the help of volunteers of The Friends Invasive Plant Action Group have made great strides in eradicating it from the Garden and are working on the surrounding buffer zone in Wirth Park. New growth from seed is removed each year as it appears. Within the South Wirth restoration project, The Minneapolis Park & Recreation Board has published some information of their approach to Buckthorn. Here is an excerpt from their material:
There is no biological control agent available for Buckthorn. Removal of the many small Buckthorn seedlings after the initial clearing is the first wave of post removal control. Control of the many small seedlings will take several years and will involve using a combination of herbicide treatment, hand pulling, mowing and potentially small test areas of prescribed burns to achieve some level of control. Research has shown that Buckthorn alters the soil nitrogen levels. It takes time for soil to return to "normal." This presents challenges to restoring some native plants. Research into this phenomenon and more studies are still being conducted regarding this by research institutions associated with the University of Minnesota and the Nature Conservancy.
More Background: Buckthorn seed was available as early as 1807 in Philadelphia. In the early 1900s it was being recommended for shelter belts in the Great Plains area. By the 1920s it was recognized in the agricultural community that the plant was an alternate host for oat crown rust (Puccinia coronata), but by then it had already escaped cultivation.
An interesting use: No matter any of the above - if you are a honey lover, the bee people say that Buckthorn honey has a pale nutty flavor with caramel notes - a definitely pronounced flavor.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"