Origin and Arrival
Common buckthorn (Rhamnus cathartica) is native to Europe and western Asia. It was introduced to North America in the nineteenth century, initially as a hedgerow shrub and ornamental plant. Its hardiness, fast growth, and early leafout made it attractive for windbreaks and garden borders before its invasive behaviour was fully understood.
By the mid-twentieth century, buckthorn had naturalized across much of southern Ontario and was recognized as a significant threat to native woodland communities. It is now listed as an invasive species under Ontario's Invasive Species Act, 2015, and its sale, propagation, and intentional introduction are prohibited.
Why Buckthorn Spreads So Effectively
Several traits combine to make common buckthorn highly competitive in Great Lakes woodland settings:
- Early leafout and late leaf drop — Buckthorn leafs out several weeks before most native trees and retains leaves several weeks after they drop in autumn. This extended growing season gives it a light-capture advantage over native competitors during spring and fall.
- Prolific fruit production — Mature buckthorn shrubs produce large quantities of small dark berries that are eaten by birds. Seeds pass through intact and are deposited widely across the landscape.
- Soil chemistry alteration — Decomposing buckthorn leaf litter tends to increase soil nitrogen and alter pH, creating conditions that favour buckthorn seedling establishment over native species.
- Shade tolerance — Buckthorn can persist and grow in moderately shaded understory conditions where many other invasive plants cannot establish.
- Resprouting after cutting — Cut stumps resprout vigorously and repeatedly unless the root crown is treated or removed.
Effects on Oak Woodland Structure
In an unmanaged oak woodland, buckthorn typically establishes first in the shrub layer and mid-understory. Over years, it forms a near-continuous canopy from about 1 to 5 metres in height, blocking light to the ground layer. Native woodland herbs, grasses, and sedges decline sharply under this shade.
Young oak seedlings and saplings are particularly vulnerable — they require moderate light levels to establish and cannot compete with buckthorn in deeply shaded conditions. Over time, sites with heavy buckthorn pressure lose the capacity to regenerate oaks naturally, even if mature oaks in the overstory persist for decades.
Identification Notes
Common buckthorn can be distinguished from native shrubs by several features: opposite or nearly opposite leaves with curved veins converging toward the tip; small, sharp-tipped thorns at branch ends; dark, smooth bark with orange-brown inner wood visible when cut; and clusters of small black berries in late summer and fall.
A similar invasive, glossy buckthorn (Frangula alnus), is also present in Ontario and shares many of the same competitive traits. Its leaves are alternate and its bark is speckled with pale lenticels.
Manual and Mechanical Removal
For small plants and seedlings, hand-pulling during or after wet soil conditions is effective. The entire root must be removed; broken roots can resprout. Seedlings up to about pencil-diameter can often be pulled with minimal equipment.
Larger shrubs and trees require cutting. A common approach in Ontario restoration sites uses hand saws, loppers, or brush cutters to remove stems at ground level. Stumps are then treated with an approved herbicide concentrate applied directly to the cut surface immediately after cutting to prevent resprouting. The cut-stump treatment method limits herbicide exposure compared to foliar spraying and allows selective treatment of target plants in a mixed-species understory.
Mechanical tools including skid-steer mounted brush grinders can be used on high-density buckthorn at larger sites. This approach allows faster clearing but requires follow-up to address root-sprouting and the seed bank already present in the soil.
The Seed Bank Problem
Even after adult buckthorn is removed, a substantial seed bank persists in the soil from previous years of fruit production. Germination can continue for several years after a site has been cleared. This makes follow-up monitoring and re-treatment critical — a site that appears cleared in year one will typically show a new cohort of seedlings in year two and three.
Restoration practitioners commonly describe buckthorn control as a multi-year commitment rather than a one-time project. Follow-up visits at least once or twice per growing season are generally needed for the first several years after initial clearing. The frequency and intensity of follow-up required tends to decrease over time as the seed bank is gradually depleted.
Coordination with Other Restoration Activities
Buckthorn removal is rarely the only intervention in an oak savanna restoration. It is typically followed or accompanied by reintroduction of native ground-layer species — grasses, sedges, and forbs that will compete with buckthorn seedlings and stabilize the restored community. Prescribed burning, where feasible, adds an additional tool: surface fires preferentially kill buckthorn resprouts and seedlings while mature oaks survive.
The sequence of activities matters. Removing buckthorn without introducing native cover can temporarily favour other weedy species, including garlic mustard (Alliaria petiolata), which also thrives in disturbed forest edges. Restoration planning typically considers these interactions explicitly.