Rhamnus cathartica

Secondary compounds, particularly emodin, have been found in the fruit, leaves, and bark of the plant, and may protect it from insects, herbivores and pathogens.[17] The emodin present in R. cathartica fruit may prevent early consumption, as it is found most in unripe fruits, which allows seeds to reach maturity before being dispersed. Birds and mice significantly avoid eating unripe fruits, and if forced to ingest emodin or unripe fruit, the animals regurgitate the meal or produce loose, watery stools.[17] Rhamnus cathartica is a constitutive isoprene emitter [15]

Allelopathic effects of exudates from R. cathartica leaf litter, roots, bark, leaves and fruit may reduce germination of other plant species in the soil. Soils in buckthorn-dominated areas are higher in nitrogen and carbon than normal soils, which speeds up decomposition rates of leaf litter.[18] This can result in bare patches of soil being formed and R. cathartica performs well in such disturbed habitats, so this may be adaptive for the setting of its seed.[17]

The species is naturalised and invasive in parts of North America.[19][17][20] Rhamnus cathartica has a competitive advantage over native trees and shrubs in North America because of root competition, the shade it produces,[21] and it leafs out before native species.[22] Of the annual carbon gain in R. cathartica, 27–35% comes from photosynthesis occurring before the leaves of other plants emerge.[13] Soil in woodlands dominated by R. cathartica was higher in nitrogen, pH and water content than soil in woodlands relatively free of R. cathartica,[18][23] probably because R. cathartica has high levels of nitrogen in its leaves [24] and these leaves decompose rapidly.

Rhamnus cathartica is also associated with invasive European earthworms (Lumbricus spp.) in the northern Midwest of North America.[25] Removing R. cathartica led to a decrease of around 50% in the biomass of invasive earthworms.[26]

Soils enriched by extra nitrogen from decayed buckthorn leaves and additional earthworm activity have a negative impact on North American forest ecosystems: "Invasive earthworms, which need rich litter, break [buckthorn leaves] down rapidly, destroying beneficial fungi and exposing bare soils in the process. These soils provide ideal conditions for buckthorn germination and seedling growth but many native trees and shrubs need the beneficial fungi and will not reproduce without it."[24]

Common buckthorn can be found in most northern states of the United States and it is particularly prevalent in the Great Lakes states of Minnesota, Wisconsin, and Michigan. A current map of its distribution can be found at Early Detection and Distribution Mapping System (EDDmapS).[27] This species is largely bird-dispersed, but only a subset of bird species that eat the fruit are competent dispersers.[28] It is winter hardy in USDA zones: 3 to 8.[29] It is a prohibited species in a few US states; Minnesota lists it as a Restricted noxious weed making it illegal to import, sell, or transport the plant,[30] Connecticut, Iowa, Massachusetts, New Hampshire, also restrict or ban the plant, and Vermont lists it as a Class B noxious weed.[31]

Cutting board made from common buckthorn and Norway maple

Rhamnus cathartica is difficult to control in its invasive range, because it sprouts vigorously and repeatedly from the root collar following cutting, girdling or burning.[32] Herbicide application to newly cut stumps is a popular and effective control method, but seeds stay viable in the soil for several years before sprouting, so repeated treatments and long-term monitoring of infested areas is required.[33] Garlon and Tordon and their derivatives have been found to be effective chemical means of control. Roundup can be used but is less reliable.[13] An application of these chemicals in early winter reduces the risk of negatively affecting non-target species, as most have gone dormant by this time. It is also easier to spot infestations at this time of the year, as the leaves of R. catharticus stay out an average of 58 days longer than native plants.[13]

Mechanical control methods such as pulling and chopping of plants are more environmentally friendly, but also very time-consuming. Plants with stems less than half an inch in diameter or less than a metre (3') tall can easily be pulled, but pulling risks disturbing the roots of adjacent, native plants and harming them as well.[33] Propane weed-torches may also be used to kill seedlings, which will generally not resprout if burned in spring or early summer.[13]