Buckthorn: A prickly problem

Common buckthorn is known as one of the most troublesome plants in the landscape, causing as much as $3.6 billion in damage to Ontario’s agricultural industry.

In the late nineteenth century, Ontario farmers thought fondly of common buckthorn (Rhamnus cathartica), a small, shrubby tree brought here from its native range in Europe and Asia. It grows quickly, forming dense thickets that make excellent windbreaks and hedgerows. As an added benefit, it was considered medicine for various conditions including constipation and rheumatism. As farming spread throughout Ontario so did buckthorn, framing fields throughout the province.

In the twenty-first century, buckthorn is known as one of the most widespread and troublesome plants on the landscape. Able to thrive in sun or shade and a wide range of soils, it’s now found in woodlots, hydro corridors and along roadsides and riverbanks across all of southern Ontario. Whether you're driving a sideroad outside Exeter or wandering the trails at the Pinery, there's likely a buckthorn in sight both ahead and behind you.   

The problems with buckthorn are plentiful. Classified as a noxious weed under Ontario’s Weed Control Act, our farmers are probably aware of the agricultural issues. Over winter, buckthorns host the soybean aphid (Aphis glycines matsumura), an invasive insect that feeds on the crop, reducing yields. It also hosts the fungi that causes oat crown rust (Puccinia coronata corda. f. sp. avenae) and barley crown rust (Puccinia coronata var. hordei), which cause serious damage to these grains. For these reasons alone, buckthorn is an often-used example when discussing the economic impacts of invasive species on industry which the Invasive Species Centre recently estimated could be as high as $3.6 billion each year in Ontario.

Buckthorn's impact on our natural world is more severe. As is the case with many plants introduced from abroad, buckthorn is of little ecological value to local insects and wildlife. Deer, for example, avoid browsing buckthorn in favour of just about anything else. This puts additional pressures on other food sources, reducing opportunities for those species to spread while more and more buckthorn fills the void. On the flip side, many of our most common birds, like American robins (Turdus migratorius) and cedar waxwings (Bombycilla cedrorum), will feed on buckthorn berries. Unfortunately, these provide them little nutrition and have a notable laxative effect. The birds fly away full but poorly nourished and quickly pass the seeds elsewhere furthering the spread of buckthorn.

Once buckthorn has taken root it's likely to form dense thickets with alarming speed. The shade produced can severely limit nearby plants access to sunlight. For all but their most shade-loving neighbours, this is likely a kiss of death.

Buckthorn's impact on soil is just as vicious. Its leaves are high in nitrogen, and when they decompose they deliver that nitrogen to the soil. This may sound like a good thing, but it's important to understand that our native plants, species that could have been found here long before settlement, generally prefer much lower levels of nitrogen than those brought here from elsewhere. Buckthorn's impact on the soil makes it less suitable for natives and more friendly to exotic species. Where the spread of buckthorn leads, other species of questionable ecological value follow.

As one might expect, Ausable Bayfield Conservation Authority (ABCA) routinely works to manage buckthorn on their properties. In 2024, they conducted an important management trial in Hay Swamp supported by a grant from the Invasive Species Action Fund. The trial evaluated three different techniques involving varying degrees of mechanical mulching and herbicide application to assess their success rates and costs per acre. Trial results will be compiled later this year.

Public lands, like those managed by ABCA, benefit from large-scale solutions and the experience of practiced professionals. Private lands are a different beast. Most of us may not even recognize buckthorn to see it, let alone understand how best to battle this scourge.

Buckthorn is a tall understory shrub with multiple stems that can eventually reach the heights of a small tree, some six to eight metres. Its grey-brown bark is often cracked or flaky and dotted with small spots called lenticels. Leaves are egg-shaped with jagged edges and veins that curve towards the tip. You'd think its characteristic thorns would be the tell-tale sign but there's a more distinctive trait: look just beneath the bark for a layer of orange tissue called the cambium. Any of the popular plant identification apps should prove reliable in confirming identification.

Strategies for removal depend on size. The Ontario Invasive Plant Council offers a comprehensive Best Management Practices document on their website, ontarioinvasiveplants.ca, which is full of excellent advice.

Seedlings and small plants are easy to pull by hand. Be sure to get as much of the root as possible and tamp down the disturbed soil after. Saplings will likely require use of a weed-pulling tool as the roots hold soil firmly. Removed materials are best stashed in a sealed bin or contractor bag for a year before disposal. When that's not possible, hang the plant upside-down from a nearby branch with its roots exposed to sunlight.

Significant challenges come when you're dealing with mature buckthorn. The most common approach is to cut it down close to soil level. Girdling can work but is not nearly as effective. With either approach, be prepared to manage resprouts for up to three years. Stumps can be tarped over to minimize resprouting.

Making the best use of your time is key to long-term success. Removing a dozen trees is a great start, but preventing three dozen from spreading their seeds may be an even greater victory. Prioritize the removal of females before they start fruiting in July. Lob off all branches in reach while waiting for girdling to do its job.

The next two years are critical. Buckthorn can produce a huge number of seeds, which stay viable for many years after they fall. Expect those to sprout. The best defense is to introduce competition by planting natives. An outstanding choice is black elderberry (Sambucus nigra) which has leaf-out and leaf-fall timing similar to buckthorn. Another suitable choice might be northern spicebush (Lindera benzoin) which is fast-growing and plentiful around these parts. Look for local native plant sales and nurseries to source these and other suitable choices. If planting isn't an option, consider mulching or tarping the exposed soil to help suppress that next generation of buckthorn.