Don’t Pass the Buck(thorn)

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Rhamnus cathartica

Buckthorn (Rhamnus cathartica) in fruit.

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North American distribution of Rhamnus cathartica (Source: Environment Canada).

Buckthorn (Rhamnus cathartica), also known as common buckthorn, is a large shrub or small tree of about 6 to 15 feet (2 to 5 m) in height, sometimes more. It is native to Europe, western Asia and northern Africa where it is only considered moderately invasive. But in North America, where it was introduced in the 1800s as an ornamental and medicinal plant, it has no natural predators and has caused a huge degree of environmental degradation. In many areas, it now makes up almost the entirety of the shrub layer of deciduous forests and little grows in its shade other than baby buckthorns: a disaster for biodiversity!

It’s no friend of farmers, either: buckthorn is the alternate host of oat and barley crown rust (Puccinia coronata) and the winter refuge of the soybean aphid (Aphis glycines).

A Quick Portrait

Even though it belongs to an entirely different plant family, the Rhamnaceae or buckthorn family, buckthorn is often mistaken for a wild plum (Prunus spp.), a plant in the rose family (Rosaceae family). The two are similar in foliage, fruit and general overall appearance.

Both are about the same dimensions, with a similar rounded habit (when grown in full sun; in shade their habit is less symmetrical) and both the buckthorn and wild plums bear thorns (domesticated plums usually don’t).

Buckthorn leaf (note the curved veins) versus plum leaf.

Both also have dark green elliptical or oval finely toothed leaves, which can be alternate or opposite and are about the same length (1 to 4 inches/3-10 cm for buckthorn, 2 to 4 inches/5 to 10 cm for plums). To tell them apart, note that the veins of buckthorn leaves are curved, while those of the plum are straight. Also, buckthorn leaves hang on until late in the fall and don’t change color before they drop, while plum leaves turn yellow early in the fall and quickly drop off.

Buckthorn fruits start green, then turn reddish yellow before becoming black at maturity. They are significantly smaller than plums at about 0.2–0.4 inches (6-10 mm) and contain 2 to 4 seeds while a plum has just one single pit and even a wild one is two to four times larger.

When it comes to bloom, the two are a snap to tell apart: buckthorn flowers are small, green and insignificant while plum are larger, very showy and white.

Toxicity

Buckthorn fruits are slightly toxic to mammals, including humans, causing cramps and having a purgative or cathartic effect (which is where the species name cathartica comes from). Every year, people are poisoned by mistaking the fruit for plums or wild cherries. No, they don’t die, but the effect is very unpleasant. Birds, on the other hand, can eat the ripe fruit with impunity.

Buckthorns are Bad Neighbors

Birds relish buckthorn fruit and distribute the seeds far and wide in their droppings, which is why you often find them sprouting in places where birds like to roost, like under tree branches and power lines and along fences. They germinate readily in sun or shade and will grow in just about any well-drained or even dry soil. They are often especially numerous in forested areas as they are highly shade tolerant.

To make matters worse, just about everything that falls from a buckthorn plant, from its leaves and fruit to its branches and bark, is allelopathic, that is, toxic to other plants. Buckthorn is especially harmful to native North American plants as they have not developed any resistance to the toxins buckthorns release (including emodin).

Add to that the fact the buckthorn leafs out first thing in spring, creating dense shade at the one season when the sun usually penetrates right to the ground in deciduous forests. This deprives lower-growing plants of a major source of energy. So if allelopathy doesn’t kill them, lack of light usually will.

The result of this is that, over time, fields fill with buckthorn shrubs while the composition of established woodlands switches from a wide range of mostly native species to a buckthorn monoculture. Even native trees can no longer germinate readily under buckthorns and are not replaced when they die.

Dealing with a Thorny Issue

20160809BIsabelle Bergeron

Buckthorn fruits starting to change color. Photo: Isabelle Bergeron.

Ideally North American landowners would destroy any buckthorns found on their property. Municipalities should do the same and in fact, some have indeed adopted programs to exterminate buckthorn from their parks.

Unfortunately, though, buckthorn is not easy to eradicate.

Young seedlings are the easiest: just pull them out. But once the plant is well rooted, its tough-as-nails root system simply doesn’t let go.

Cutting back is the logical alternative, but even if you cut the shrub on the ground, it quickly resprouts… and will do so repeatedly each time you cut it. If you have just a few shrubs to deal with, keep at it and eventually they’ll run out of energy and die. If you’re dealing with a woodlot that is invaded (often the case), you’ll need to do more.

20160728EThe most effective treatment is to paint a systemic herbicide such as glyphosate onto each freshly cut stump. It will then migrate down into the roots and kill them. Spraying a herbicide in a sensitive environment is not recommended, as it can harm or kill surrounding plants.

You’ll have to combine whatever treatment you choose with uprooting seedlings, because even after the adult shrubs are gone, seed can remain viable for 2 years (sometimes up to 6 years).


A Quick Tip

It is easier to spot and suppress buckthorn either early in the spring, since it leafs out so early, or late in the fall, as its leaves will remain green longer than any of the plants.


Don’t Pass the Buckthorn!

Buckthorn can be present on your property whether you live in a city, a suburb or the countryside. Be alert and ready to act if you do find it. The faster you act, the easier it will be to control. And don’t hesitate to notify neighbors if you see it on their side of the fence. Sometimes it takes a neighborhood effort to control a noxious weed.

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