Buckthorn. Credit: University of Minnesota Extension.
Guest Blog Post by ARCC Student Jessica E. Strand
Types of Invasive Non-Native Buckthorns
History of Buckthorn
Buckthorn is a large shrub that is native to most of Europe and west Asia.1 Buckthorn was intentionally introduced to North America from Europe as an ornamental species during the mid-1800s.2 Since its introduction it has spread rapidly throughout the United States, establishing itself as dense thickets. Due to its rapid expansion and establishment, it is illegal in many states to import, sell, and transport this invasive non-native. The import, selling, and transfer of buckthorn became illegal in Minnesota when it was added to Minnesota’s Noxious Weeds List in 1999.3
Family poses in front of buckthorn shrub for a photo. Credit: Mary Lahr Schier.
Common Buckthorn can be found in dry to wet soils, though it prefers well-drained soils.2 It grows in both shade and sun, thriving in partial shade.4 It can be found in forests, prairies, fields, parks, yards, and roadsides. 2
Glossy Buckthorn prefers wet soils but can be found in drier soils. 2 It grows in both shade and sun and can be observed in open forests, wetlands, and fields.4
|Common Buckthorn||Glossy Buckthorn|
|Growth||Can grow to be ten to twenty-five feet tall and the trunk can reach ten inches in diameter.2||Can grow to be ten to twenty-five feet tall and the trunk can reach ten inches in diameter.2 This type of Buckthorn is more upright in form 4|
|Bark||When young, the bark is gray to brown, as the species grows the bark becomes a brown-black color and is roughly textured.2 Lenticels run parallel to the bark and are lighter in color.2 Heartwood is pink to orange. Spines can be found at the tip of twigs.4||The smooth bark is gray to brown and has light colored lenticels that run parallel to the park and are slightly raised.2 Twigs can be a red-brown color and contain hairs.4 Heartwood is pinkish to orange.2 Glossy Buckthorn lacks the thorns observed in Common Buckthorn.4|
|Leaves||The leaves are dark green, can be glossy to dull, oval, serrated, and have a pointed tip.4 Veins are curved towards the tip and are in pairs of three to five.2 They are simple and grow opposite of each other, although can grow alternately. 2 Leaves stay green late into the fall.4||Glossy Buckthorn leaves are alternate but can be opposite.2 Leaves are untoothed, pointed at the tip, oval, smooth, and dark green.4 Leaves are glossy on the top side and can be dull and hairy underneath.2 Eight to nine veins form a V at the midrib.4 Leaves do change color but remain on the shrub longer than leaves on native plants.2|
|Flowers||Flowers are small and yellowish green in color. They have four petals and bloom late May to June.4||Flowers are small and pale yellow. Flowers contain five petals and bloom in late May to June.4|
|Fruit||Fruit clusters are round and black. They grow at leaf axils or along the stems. Fruits contain three to four seeds that are viable for up to three years.2 Berries ripen from August to September.2 The berries can remain throughout the winter and are toxic.2||Fruit is often in pairs but can be in clusters. Fruit is round and changes from red to black as it ripens.2 Each fruit has two to three seeds that are viable for two to three years.4 Berries ripen from July to September.2 Berries do not persist through the winter and are considered mildly toxic.4|
Native Species with Similar Characteristics
- American Plum – The American Plum can be mistaken for Buckthorn when young because the bark looks similar between the two species.5 The leaves of the plum are serrated but leaf veins move from the midrib to the outer edge, unlike with Common Buckthorn, which moves towards the tip. Leaves are also narrower than both types of buckthorns. Flowers and fruits are distinctly different from buckthorn.
- Black Chokeberry – Both Black Chokeberry and Common Buckthorn have serrated leaves and veins that run from the midrib to the tips of the leaf.6 However, the fruit of the Black Chokeberry hangs downwards from long stems while buckthorn fruits grow in short clusters. Additionally, this species has red, smooth, and pointed buds while Common Buckthorn has dark brown buds with scales and Glossy Buckthorn has red hairy buds.6
- Black Cherry – Black Cherry trees have bark very similar to both types of buckthorns when young. The cambium layer of Black Cherry is green while that of the buckthorns is orange.7 The leaves of both species are oval in shape but the leaves of Black Cherry tend to be narrower. Glossy Buckthorn has shiny leaves while Black Cherries do not, and the veins of the Black Cherry leaves go towards the outer edge while they go towards the tip in Common Buckthorn.8 Fruits look similar to buckthorn species but flowers are distinct.
- Chokecherry – Like with Black Cherry, the Chokecherry has bark like buckthorns but a green cambium layer underneath.7 Like with Black Cherry, the leaf similarities and differences are the same.8 Flowers looks different from buckthorn species and Chokecherry fruits have a reddish tone
Problems with Buckthorn
Buckthorn is a problem because it outcompetes other native plant species. Buckthorn produces heavy shade which limits the ability of native tree seedlings and saplings as well as other native ground layer plants to thrive. 2 In addition to light competition, this species also outcompetes native species for water and nutrients. As a result, the health of the habitat that Buckthorn invades is severely threatened. The introduction of Buckthorn limits habitat for wildlife. It also contributes to soil erosion because the species that would normally prevent erosion are not able to thrive. Buckthorn can function as a host for pests, such as the Crown Rust Fungus and Soybean Aphid, that may harm native species, such as native grasses, while Buckthorn itself lacks pests that would be able to serve as a control limiting its growth.4, 9 In addition to not having pests to control its growth, a mutualistic relationship with invasive earthworms may actually give Buckthorn a competitive advantage over its native counterparts. Research suggests that Buckthorn creates an optimal environment for earthworms by creating shade and leaf litter while the earthworms assist buckthorn with germination by exposing soil through their consumption of leaflitter.10 Not only does Buckthorn get a competitive advantage from earthworms but the presence of earthworms is actually a disadvantage to native vegetation because their consumption of leaflitter eliminates the duff layer that protects young native seedlings and other native woodland vegetation.11 Another interesting relationship that Buckthorn has is with native bird species. Birds are the primary dispersers of Buckthorn seeds.12 Birds consume buckthorn fruit as food, the fruit has a laxative effect on birds who then disperse the seeds in a new location with a natural fertilizer.12 The relationship that buckthorn has with birds helps to enable its spread and ultimately contributes to its invasion of the United States. These factors allow buckthorn to thrive making it a huge threat to native Minnesota flora and fauna.
Buckthorn is easiest to find in the fall because buckthorn leaves will still be green while native shrubs and trees will have started turning colors or lost their leaves.4 Since buckthorn seeds stay viable for multiple years, follow up control may be required until the seed bank is depleted. 4 Buckthorn can be controlled using mechanical and/or chemical methods.2 While research has been conducted to biologically control buckthorn using European agents, no promising agents have been discovered because their potential to become invasive is too high.13
Small plants can be pulled by hand or with leverage tools, such as a weed wrench or root talon, when the soil is wet.2 With larger shrubs, the shrub can be cut, and the stump can be covered for two to three years with a tin can or plastic which limits resprouting.4 Controlled burns are another option when the buckthorn population is dense and growing in a habitat where the native plant community is fire adapted, such as in a prarire.2 These are best performed during the spring or fall when foliage is dryer. Burns can be especially effective when there are many seedlings present.2
Buckthorn can also be controlled by using herbicides. Common chemicals used to control buckthorn include glyphosate (Round-up, Rodeo, etc…), triclopyr amine (Vastlan, Garlon 3, etc…), and triclopyr ester (Garlon 4 or Pathfinder II).4 Glyphosate and triclopyr amine are best used when temperatures are above freezing because they are water based while triclopyr ester is best used when temperatures are below freezing because it is oil-based.4 While all herbicides should be used carefully, users should be especially careful when using glyphosate because it is a non-selective herbicide which means that it will kill most plants.14 Triclopyr herbicides on the other hand target woody plants and broadleaf weeds, making it more plant selective.15 Care must also be taken when using herbicides near water sources, check that the chemicals are safe to use near water if planning to chemically treat buckthorn near water. Caution and awareness should always be taken when handling herbicides, make sure to read instructions and follow proper administration and safety practices. The most common methods of controlling buckthorn using herbicides are foliar treatment, basal bark treatment, and cut stump treatment.2 Foliar treatments are best for areas that are badly infested with buckthorn.2 The herbicide is applied directly to the leaves and should be done after a hard frost when native species have gone dormant for the winter.2 Buckthorn that is less than 5 inches in diameter can be treated using the basal bark method.2 With this method of treatment, the herbicide is absorbed through the bark.4 The herbicide used for this treatment must contain triclopyr ester which is oil based.2 The herbicide should be applied from the bottom of the shrub to about twelve to eighteen inches above the bottom. 4 Cut stump treatments are best applied when shrubs are larger than two inches in diameter. 4 This method involves cutting the trunk flush with the soil and then treating the stump with herbicide. The herbicide should be applied to the outermost growth rings.4 The best time to treat buckthorn using this method is from the late summer to the fall.2
- New York Invasive Species (IS) Information. ¨Common Buckthorn¨. 2 Jul 2019. https://nyis.info/invasive_species/commonbuckthorn/#:~:text=Origin,distribution%20until%20the%20early%201900s
- Czarapata, J. Elizabeth. Invasive Plants of the Upper Midwest: An Illustrated Guide to their Identification and Control. The University of Wisconsin Press, 2005.
- Minnesota Department of Agriculture. “Minnesota Noxious Weed List¨. 2022. https://www.mda.state.mn.us/plants-insects/minnesota-noxious-weed-list
- Minnesota Department of Natural Resources. Buckthorn: What You Should Know, What You Can Do. Ecological and Water Resources, 2019.
- Kortebein, Paul. “Species Spotlight: American Plum.” Three Rivers Park District, Nature Notes, 19 Jan. 2021.
- Holm, Heather. “A Native European Buckthorn Look-Alike.” Restoring the Landscape with Native Plants, 2015.
- Minnesota Wildflowers. “Rhamnus cathartica (Common Buckthorn)” 2022.
- Minnesota Board of Water and Soil Resources. “Black Cherry and Chokecherry (Prunus sp.).” BWSR Featured Plant.
- Braker, Nancy. “Common Buckthorn (Rhamnus Cathartica). Carleton, Cowling Arboretum, 3 Jun. 2022.
- Heneghan, Liam, et. al. “Interactions of an Introduced Shrub and Introduced Earthworms in an Illinois Urban Woodland: Impact on Leaf Litter Decomposition.” Pedobiologia, vol. 50, no. 6, pp. 543-551, 4 Jan. 2007.
- Holdsworth, Andy, et. al. “Earthworms.” Minnesota Department of Natural Resources, Jul. 2017.
- Craves, Julie. “Birds that Eat Non-native Buckthorn Fruit (Rhamnus cathartica and Frangula Alnus, Rhamnaceae) in Eastern North America.” Natural Areas Journal, vol. 35, no. 2, pp. 279-287, Apr. 2015.
- Minnesota Department of Natural Resources. “Biological Control of Buckthorn.” 2022.
- Henderson, A. M., et. al. “Glyphosate General Fact Sheet.” National Pesticide Information Center, Oregon State University Extension Services, Mar. 2019.
- Strid, A., et al. “Triclopyr General Fact Sheet.” National Pesticide Information Center, Oregon State University Extension Services, May 2018.