COMMON BUCKTHORN
Rhamnus cathartica

Name and Family

Common buckthorn, Rhamnus cathartica, is a member of the buckthorn family (Rhamnaceae).

History and Habitat

  • Common buckthorn was introduced from Europe as an ornamental shrub. 
  • It is an aggressive, invasive shrub with multiple stems and can grow to be 25’ tall. 
  • Common buckthorn is most often found in drier soils but can tolerate a wide range of habitats such as riparian areas, woodland edges, old fields, ditches, and open grassy areas. 
  • Spreads mainly by sexual reproduction, with seeds often consumed and spread by birds. 
  • On PEI, common buckthorn is widespread in western PEI, particularly in the Richmond area. Observations of this species in the central and eastern regions of PEI have increased in recent years.

Identification Guide

  • Common buckthorn is a perennial deciduous shrub or tree. It typically reaches a height of 2-3 metres but can be as tall as 6 metres. 
  • This shrub has multiple stems with grey-brown spotted bark. The spots are known as lenticels.
  • A key ID feature is the sharp thorn found at the tip of most branches. 
  • Its leaves emerge early in the spring and remain on twigs into late fall after other species have died back. 
  • Leaves are mostly opposite or sub-opposite, dull green, and oval-shaped with finely serrated edges. 
  • Leaf veins curve inward towards the leaf tip. Leaf tips are often semi-folded.
  • Flowers bloom in May or June. They are very small, star-shaped, white to whitish-green, and are found growing along leaf axils in small clusters. 
  • Green berry-like fruits called “drupes” replace the flowers beginning in July.
    • As they ripen, they turn black. 
    • Drupes appear in discrete clusters
    • The drupes are produced from July to September and persist into the winter.

Look-alike species

  • Glossy buckthorn, Frangula alnus, is a related invasive lookalike of common buckthorn that is found extensively across PEI. 
  • Common buckthorn has slightly toothed leaf edges, while glossy buckthorn leaf edges are smooth. 
  • Common buckthorn has a thorn at the end of each branch, while glossy buckthorn lacks thorns. 
  • The drupes of common buckthorn grow in large clusters, while glossy buckthorn drupes grow in smaller clusters and are more dispersed along the branches.
  • The fruit of common buckthorn changes from green to black directly, whereas glossy buckthorn fruit changes from green to red to black as they mature. 
  • The leaf veins of glossy buckthorn extend straight toward the leaf edge (in a herringbone pattern), whereas common buckthorn leaf veins are curved and end near the leaf tip.
  • An additional non-native buckthorn was reported to be present on PEI. Dahurian buckthorn, Rhamnus davurica, which looks very similar to common buckthorn.
    • This plant is thought to be not as invasive as glossy and common buckthorn
    • Leaves of Dahurian buckthorn are approximately twice as long as common buckthorn leaves (e.g., common = 3-7cm, Dahurian = 6-13cm)
    • Common buckthorn has 3-4 seeds per berry, while Dahurian has only two seeds per berry.
    • The last and only report of this plant was from 1952 when it was found to be growing at the experimental farm in Charlottetown. It may no longer be present on PEI.
  • A native buckthorn species, alder-leaved buckthorn, Endotropis alnifolia, is found on PEI.
    • This plant looks significantly different from common buckthorn as alder buckthorn’s leaves are shiny, there are no terminal thorns, it grows in wetter areas, and the plant only grows up to 1m tall,
    • Note that alder buckthorn looks much more similar to glossy buckthorn, but can be differentiated from that species by its low growth (only up to 1m) and serrated leaf margins. 

What it does in the ecosystem

Outcompeting native species to reduce biodiversity

  • Like many invasive species, common buckthorn has adapted to thrive in a wide range of habitats and growing conditions. 
  • It can monopolize light in a canopy and out-compete native species.
    • Common buckthorn is fast-growing and densely leafed. 
    • Seedlings germinate early in spring, before most other understory plants.
    • The leaves on larger buckthorn shrubs emerge early in the spring, blocking sunlight from reaching lower-growing species such as native wildflowers. 
  • Buckthorns are allelopathic, which means they release chemicals from their roots that negatively impact the growth of competing plants and increase the buckthorn’s fitness.

Direct impacts on wildlife

  • Common buckthorn seed production is prolific and the berries stay on the shrubs over winter, increasing the chance that foraging birds will eat them.
    • Unfortunately, buckthorn berries are low in nutrients and contain a natural laxative. 
    • The laxative ensures that seeds are quickly dispersed by birds who eat buckthorn berries. 
    • Low nutritional value and the added laxative means that birds have to expend much more energy to feed.
      • This can be a serious concern for birds leading into the winter months. 
      • Weakened energy reserves can impact successful migration or the overwintering of resident birds. 
  • An interesting study by Lewis et. al. (2017) found that common buckthorn leaves that fall into rivers can impact the macroinvertebrate community (i.e., invertebrates that live on the bottom of streams and rivers and are visible to the naked eye).
    • Their study, titled “Amphipods, Gammarus pseudolimnaeus, prefer but are harmed by a diet of non-native leaves”, found that amphipods overwhelmingly chose to eat the leaves of buckthorn over other leaves presented. 
    • After feeding on buckthorn leaves, amphipods were found to be smaller and have higher mortality rates. 

Management

GENERAL CONSIDERATIONS

  • Before selecting a control method, consider the size of the infestation, your available resources, and the amount of effort you are willing to expend. 
  • Often, multiple control methods are used simultaneously. 
  • Surveying and management for common buckthorn is best done in the spring, as this plant produces leaves very early.
    • This way, you will avoid trampling native species that might be present in the area. 
    • You will have plenty of time in early summer for your management before common buckthorn goes to seed.
  • The most important aspect of glossy buckthorn management is that the berries are prevented from establishing. This is because berries are common buckthorn’s main method of spread. 
  • Cutting and burning tend to lead to vigorous re-sprouting from left-behind stumps, so these methods are not recommended control measures.
    • While these treatments temporarily prevent seed production, they do not kill the buckthorn. 
  • Removal sites must be closely monitored for several years for regrowth, and regularly repeated control efforts will likely be required to effectively control an infestation.

PULLING OR DIGGING

  • Physical removal of buckthorn infestations is the most effective method of manual control available. 
  • Plants less than 1 inch in diameter and about a metre in height can be pulled by hand or with the help of tools. 
  • The Extractigator® is a tool designed specifically for this purpose.
    • This is a lever system designed to uproot tough shrubs and bushes.
  • Spades and shovels can be used to dig a circle 1-2ft out from the shrub’s trunk.
    •  Another person rocks the shrub back and forth to show what intact roots remain necessary to cut. 
  • Using an axe can help to chop through the roots making it easier to remove large shrubs. 
  • Regeneration is unlikely if a significant portion of the root system is removed.
  • After removal, tap disturbed soils back into place and plant suitable native species to discourage the further establishment of invaders.
    • The right species to plant will vary based on site conditions (e.g. light, soil drainage, etc.). Reach out to the PEIISC for advice on selecting a replacement to plant. Native alder and willow grow similarly.
  • Pulling buckthorn plants early in the season is ideal, as it helps to reduce competition for the light and nutrients needed by other plants. 
  • If the plant has berries, it is important to remove the branches and bag them before pulling the stem out.
    • You do not want to shake the berries off the branch during management, as this will make the seed bank last longer 
    • Laying out a ground sheet below your work area can help catch any berries that detach. 
  • Pulling buckthorn infestations is labour intensive, so this method may not be feasible for large infestations.

GIRDLING

  • Girdling involves removing a two-inch-wide ring of bark from near the base of a tree. 
  • Remove the outer bark and the green cambium layer but avoid cutting into the hard core of the trunk. 
  • It is key to cut the trunk to the appropriate depth because if the cut is too deep (i.e., the core xylem layer is disturbed) the shrub will respond by resprouting and if the cut is not deep enough the tree will heal. 
  • This technique allows the roots to nourish the crown but prevents the crown from sending nourishment back to the roots, leading to eventual root death. 
  • Trees that have been girdled should continue to be monitored and new sprouts that develop below the girdled site should be removed. 
  • Girdling buckthorn infestations is labour-intensive, so this method may not be feasible for large infestations. 
  • Girdling should be reserved for large individuals that cannot be pulled or those along waterways where it is the only species along the bank. Disturbance along waterways may cause bank destabilization, affecting water flow and quality. 

CHEMICAL CONTROL

  • Chemical control methods exist for common buckthorn, but the PEI Invasive Species Council does not provide advice on these measures at this time. 
  • Surrounding vegetation and wildlife may be damaged by herbicides. 
  • Herbicide use is prohibited near wetlands and watercourses on PEI.
  • If using chemical control methods, it is important that all local legislation and manufacturer’s instructions be followed.

Follow-up and Disposal

  • After work is completed, it is important that all equipment be thoroughly cleaned and that all plant parts are removed from boots and clothing before leaving the infested area. 
  • If the plants have fruit, all branches with berries must be bagged and properly disposed of. 
  • If management occurs before berries are formed, or if the plants have not yet matured to produce berries, it is important to ensure that the roots of the plant are left in such a way that they cannot root themselves back in the soil (e.g., The PEIISC often hangs buckthorn plants upside down by their roots from nearby trees)
  • Each time you manage a site, collect, bag, mark, and dispose of all branches with berries properly.
    • Bag the branches in clear plastic bags.
    • Mark the bags boldly with a permanent marker. Write “INVASIVE PLANT” or “COMMON BUCKTHORN” on the bags. 
    • If the bags are thin, double bag them to prevent accidental dispersal during transport.
    • Tie the bags tightly and place them in your usual residential waste collection (THE BLACK BIN)
      • Never place invasive plants into your compost bin. This may allow them to become established once they reach the compost heap. 
  • With any treatment option, the site will need to be revisited regularly to control any new growth that may appear.

Map

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References

(PDF) The Biology of Canadian Weeds. 139. Rhamnus cathartica L (researchgate.net)

Buckthorn – Ontario Invasive Plant Council (ontarioinvasiveplants.ca)

Common Buckthorn – Profile and Resources | Invasive Species Centre

Common buckthorn | ontario.ca

NCC: European buckthorn (natureconservancy.ca)

Rhamnus cathartica (buckthorn) | CABI Compendium (cabidigitallibrary.org)