European buckthorn – Rhamnus carthartica

What is it?

The European buckthorn, common buckthorn, or just buckthorn, is an invasive flowering tree native to Europe that was introduced in the 1880s to Canada, and has since spread across the country, becoming widespread throughout eastern Canada by the 1900s. They remain common across Ontario, having been naturalized in many regions and are one of the most iconic invasive plant species.

Unlike our previous plant guest, buckthorn is well named for the sharp thorns at the end of its branches hidden by its lush green foliage. Since the leaves of this prickly tree species tend to stay a little longer than its native colleagues in the forest, they can stick out to hikers planning to enjoy a colourful autumn at this time of year. Watch out though, though their thorns aren’t poisonous, they’re certainly not dull!

Why you should care

Common Buckthorn (Rhamnus cathartica), not to be confused with the similarly named but native Alder Buckthorn (Rhamnus alnifolia), is one of the most iconic and harmful invasive species, and it isn’t just because they can poke your eye out if you fall into one by accident!

Much like other naturalized invasive species, the common buckthorn and its relatives like the Glossy Buckthorn (Frangula alnus) were often deliberately planted and spread by humans to serve as ornamental plants, hedgerows and windbreakers. Additionally, its berries, which are mildly toxic to humans, are sometimes consumed by birds and small mammals that then disperse the seeds that pass through their intestinal tracts and can be spread over a wide area to quickly germinate.

Combined with its hardy nature, being able to repeatedly resprout from just its roots, and tolerance of a very wide range of soils and low light conditions, the common buckthorn can thrive in many environments. As a result, it has become extremely common in Ontario, and can be found along roadsides, fences, the edges of urban forests, and other locales.

Beyond being the occasional nuisance, the spread of the invasive common buckthorn has had serious negative effects on the landscape. Not only does this prickly bush take up space that could have otherwise housed native trees and shrubs, but its dense thickets, which develop leaves before and last long after most native plants, can crowd and shade out new undergrowth, including important wildflowers and herbs, and even alters soil nutrients to discourage potential native competitors while favoring other invasive species like garlic mustard. This has also diminished local water quality as many native plant species are critical to reducing erosion and runoff.

Additionally, though buckthorn berries are often consumed by small wildlife, it has little nutritional benefit, has laxative properties, and is toxic in high quantities, being eaten only because, like our prickly enemy’s leaves, they persist even into winter, being a seemingly easy source of food for unsuspecting mammals and small birds.

Worse, this nasty invader has also had an impact on Canadian agricultural output! The common buckthorn can serve as hosts for at least three agricultural pests. One is the Oat Crown rust (Puccinia coronate spp. avenae), which appropriately harms oat crops (Avena sativa). Another is the spotted wing drosophila (Drosophila suzukii), a fruit fly pest that has damaged many soft fruit and berry crops that Ontario is famous for. The third, and more dangerous pest that the common buckthorn invites is the soybean aphid (Aphis glycines). This nasty critter not only hurts soybeans, but other vegetable crops as well, by consuming them and using them as host plants to survive the winter. Even worse, the soybean aphid itself has facilitated the invasion of the multi-coloured Asian Lady Beetle (Harmonia axyridis) which feed on the aphid and whose presence has led to the decline of native lady beetle species.

As a result, there should be no surprise that the common buckthorn listed as a noxious weed in Ontario’s Weed Control Act, with growing awareness and efforts in recent decades to stamp it out and restore our native ecosystems!

What can we do?

The best way to help reduce the impacts of the common buckthorn is to help identify and report infestations. Much like other invasive species, early detection is critical in controlling its spread as younger shrubs are much more easily removed compared to older growths.

How to identify Buckthorn

The common buckthorn is one of the more distinctive invasive plants out here in Ontario but can sometimes be confused with the similar glossy buckthorn, which is also invasive, and should also be removed, so in this case, mix-ups between the two are no great loss, though this is unlikely to happen as while the common buckthorn prefers well-drained soils, the glossy buckthorn prefers to grow in wetlands.

However, you should not confuse either for the native alder buckthorn, though it is more similar in appearance to the glossy buckthorn than the common buckthorn.

To identify a true common buckthorn, look out for the following characteristics:

Height
- Depending on the site, the common buckthorn can grow as a short shrub two to three metres tall, but under optimal conditions, can grow into a tall six-metre tree.
Stem
- Woody twigs with sharp thorns at the tip rather than on the sides like a rosebush
- At its full size at 6 metres, the central trunk can be up to 25 centimetres in diameter
- Younger shrubs will consist of multiple stems with flakey bark
Leaves
- Glossy dark green on top, and pale green on the bottom; retains its colour even in late autumn when it becomes more obvious
- Short stalk per leaf
- Opposite, with fine toothed margins, and parallel veins curving into the tip
- Leaves generally elliptical ranging from two-point-five to six centimetres long, rounder at the base, and narrowing into a curved pointed tip.
- Develop early in the spring, and only loses its leaves in late autumn, sometimes persisting into early winter
Flowers
- Small, inconspicuous with two – six light yellow-green petals
- Flowers during May and June
Fruit
- Distinctive clusters or singular stalked glossy purplish-black round (5 – 6 mm) berries
- Juicy with stony seeds that can germinate quickly
- Develop during in July and August, but can persist into the winter despite shriveling

Can you remove Buckthorn?

If it is on your property, instances of common and glossy buckthorn can and should be removed and replaced with native species like these.

As we stated earlier, because buckthorn shrubs are easiest to remove when young, monitor your property for any emergent saplings. Like other invasive species, any instances of common buckthorn shrubs should be removed in their entirety, from root to stem. This can be accomplished by manually pulling up the buckthorn saplings and is easiest in mid-October when the common buckthorn is not only easiest to identify by its distinctive persistent leaf cover, but when the soils are loose and damp, and collateral effects on other species will be minimal. Stronger tools such as weed wrenches may be used to clamp onto the stems of buckthorn saplings and pull them out.

When removing the pulled-up buckthorn plants, do not discard them in the compost, or deposit them in natural areas as they are extremely hardy, and may resprout roots, or overlooked seeds in the flowers may germinate, allowing the infestation to persist.

However, if the stand is too well developed, pruning can be used to slowly cut back the extent of the infestation, until the plant either dies or can be more easily removed. Legal herbicides may also be used, either separately or in conjunction with pruning efforts.

In any case, the hardiness and extent of common buckthorn infestations in Ontario are generally so serious that you should be prepared to spend several years battling recurring and persistent infestations. While it can be tempting to use limited burning practices, as many city officials and conservation authorities sometimes do, we DO NOT recommend attempting this as it can quickly become a serious fire hazard.

A more detailed set of management practices for controlling and removing common buckthorn infestations can be found below:

On public properties, many conservation authorities organize expeditions to collect and remove emergent saplings. Check with your local conservation authority to see if you can participate!

Additionally, you can also help prevent the spread of invasive plants like the common buckthorn by staying on trails and keeping pets leashed, reducing the chances that your clothing or pet’s fur will pick up and spread its berries and seeds.

If you see any plants that match the description above, please contact the Invading Species Hotline at 1-800-563-7711 or the Natural Resources Information Support Centre at 1-800-667-1940 or 1-800-387-7011 to submit your observations! Also consider visiting the EDDMapS Ontario which has partnered with both organizations above to report sightings!

Additionally, importation of the common buckthorn and other invasive plants is illegal and can be reported to the MNRF TIPS line at 1-877-TIPS-MNR (847-7667) toll-free anytime. You can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477).

Interested in learning more? See also:

Ontario’s Invading Species Awareness Program – Common Buckthorn (Rhamnus cathartica)
Ontario Weeds: European Buckthorn
Nottawasaga Valley Conservation Authority – Common & Glossy Buckthorn
Ontario.ca – Common Buckthorn
Friends of the Mississippi River – Buckthorn: How can a shrub be so harmful?
Grow Me Instead
Invasive Common (European) Buckthorn (Rhamnus cathartica) – Best Management Practices in Ontario