Common buckthorn (Rhamnus cathartica)
Common buckthorn is a small deciduous tree or large shrub. Native to Europe and Asia, it was brought to Canada in the 1700s for use in agriculture as a windbreak in farm fields. It has since spread aggressively throughout Ontario and east to Nova Scotia, invading areas such as roadsides, woodland edges, prairies, and meadows.
Common buckthorn is capable of growing in a wide range of soil and light conditions, and can thrive in shaded forests and swamps. It can grow up to 6 m in height with a trunk diameter of 25 cm. The bark is a grayish brown, and the twigs have raised, lighter patches called lenticels. The twigs usually end in a thorn-like tip, for which the plant is named. The inner bark is yellow with orange heartwood. The plant has simple, egg shaped, dark green leaves that are shiny with small rounded teeth around the edges. The leaves are sub-opposite in arrangement, since they are nearly opposite, but not quite in most cases. The plant produces yellowish green flowers in the spring (May to June). This species is dioecious, having male and female flowers on separate plants. When pollinated, female flowers develop into berries that are green at first but ripen to a black colour. The berries and leaves persist late into the fall after many native species have shed their leaves.
The common buckthorn is very successful at outcompeting native woody and herbaceous plants for space and light. It forms dense thickets that prevent the establishment of shade tolerant native species. Many natural areas are threatened by common buckthorn because it is shade and moisture tolerant and grows well across a variety of soil types including dry and moist conditions. It can completely dominate the understory and shrub layer. Common buckthorn also has allelopathic properties, meaning it changes the soil chemistry making it more difficult for other plants to grow. Additionally, it is a threat to agriculture because it is the alternate host of oat crown (a fungus) and a wintering host of soybean aphids. Both these species impact crop yields.
Since this plant will grow to the size of a small tree, early detection and removal is the best strategy for preventing establishment. Young plants can be manually removed by pulling or digging them out. The sharp spines are not usually a problem if precautions are taken (such as wearing gloves). A follow up visit is necessary since common buckthorn reproduces prolifically by seed and seeds can remain viable in the soil for up to six years.