Buckthorn--A New Highly Invasive Shrub Found in Princeton

For years, I've traveled between Michigan and New Jersey and noted how some invasive plant species that have run rampant in the eastern U.S. have yet to show up in the midwest. That is changing. Last year, I encountered a dramatic example of stiltgrass spreading down a hillside in Michigan, and there are reports of lesser celandine gaining a foothold there as well. Similarly, Princeton had remained free of the common buckthorn--the most invasive shrub clogging midwestern forests. That, too, is changing.

The fateful day came on September 16, 2025. I was standing in a spot I'd passed by many times, near the entrance to the Herrontown Woods parking lot, when I happened to look down and saw that characteristic leaf of common buckthorn--the first sighting, by me and perhaps anyone, of this uber-invasive shrub in Princeton. 

The leaves of common buckthorn (Rhamnus cathartica) have arc-shaped veins, and are described as "sub-opposite," because they can appear to be paired on the twig but are slightly staggered.

Buckthorn gets its name from the two terminal buds, which make the shape of a buck's hoof, with a thorn-like protrusion between them.

This photo from upstate NY shows there can be thornlike protrusions along the trunk as well.

Shunned by deer, buckthorn's combination of massive seed production and shade tolerance allow it to clog forests and bury whole native plant communities under its dense growth. The richly diverse bur oak savannas of the midwest were nearly lost beneath a rising sea of buckthorn. Only botanical sleuthing and the hard work of clearing buckthorn, honeysuckle and other invasive plants from beneath the massive oaks, along with reseeding and prescribed burning, has brought back that plant community. 

The small cluster of young shrubs I spotted at Herrontown Woods has fortunately not yet produced seeds. But anyone who has lived in the midwest knows the potential of buckthorn to grow, seed and spread.

An email to the Stewardship Roundtable group of land managers in NJ yielded some responses. Duke Farms has it, as does the Watchung Reservation. The Friends of Great Swamp have an info sheet on their website, so I suspect it has established itself there as well. Mike van Clef, who deals with invasive plants all across New Jersey, writes: 

Duke Farms definitely stands out! From iNaturalist, there are 64 research grade observations, heavier toward northern NJ but all over the state. I often find it as single immature individuals, especially in northern NJ...which is always a head scratcher because I never seem to find a nearby large fruiting individual...

This photo shows the lingering green of buckthorn in upstate NY. Like many invasive shrubs that evolved in a different climate, buckthorn keeps its leaves longer that native species in the fall. 

Given New Jersey's already long list of invasive shrubs clogging our forests--among them multiflora rose, Photinia, privet, and Linden Viburnum--it's hard to imagine another having much of an impact. Having seen what buckthorn does in other areas of the country, all I can say is "Watch out!"

Early detection and rapid response are key to stopping biological invasions. This is true both of the immune systems protecting our bodies and of land managers caring for nature preserves. At Herrontown Woods, we've done very well with that creed of early detection and rapid response. Lesser celandine and garlic mustard--the bane of many a nature preserve--are now vanishingly rare. Vigilance each year in August has helped keep many areas free of stiltgrass, the most rapid spreading of all. People who care and take action can make a difference.

On a town-wide scale, some early interventions have helped keep the thorny Mile a Minute vine from spreading across Princeton. Though Princeton has hired contractors to help counter at least some invasions in their early stages, it's hard to get private residents--often disconnected from the yards they own--to act collectively to knock out new invasive species before they become a problem. Having fought the good fight, my advice about buckthorn is: be informed, be on the look out, and be proactive. There's also a super handy, targeted, and frugal way for homeowners and professionals to cut and treat buckthorn and other invasive shrubs. Appropriately enough, it's called a Buckthorn Blaster. 

Mile a Minute--A Wave Growing Across NJ's Countryside

You can spot it a mile away. Early summer, and already this annual, thorn-covered vine called Mile-a-Minute is rising like a wave along fencelines in New Jersey's countryside. Thus far, in my ramblings around Princeton, I have found only two tiny patches--at the Battlefield and along the driveway into Rogers Refuge--both of which have been knocked out the past two years. Is this sort of early intervention and annual followup worth it? The answer becomes abundantly clear just outside of town, halfway to Hopewell, where Mile-a-Minute vine is demonstrating just how much of a prickly menace it can be if not caught early.

It's a plant that seeks to be seen everywhere, and with all the other players on the plant scene. Here it is growing up a tree,

and sprawling over another invasive, garlic mustard.

Even those thistles with their prickly personalities aren't off-putting for a Mile-a-Minute vine.

It's said to have been an accidental introduction from eastern Asia via the nursery trade, originally gaining a foothold in York County, PA, in the 1930s and spreading from there.

Rampancy rules in this photo, as mile-a-minute swarms an autumn olive--a highly invasive shrub. When mile-a-minute's around, the curtain doesn't fall on other plants, but rises, in a wave of triangular leaves.

Here's Mile-a-Minute chasing the growth tip of a blackberry. Check back in a month to see who won the race.

Here, a privet's growing a prickly skirt.

Those pink flowers are Canada thistle, invader of many a garden bed, which is about to meet its match.

Long-time ubiquitous invasives, multiflora rose and Japanese honeysuckle, are joined by Mile-a-Minute.

You'd think perennial vines like wild grape would have a big advantage over an annual vine that has to spring anew from the soil every year, but Mile-a-Minute is looking up to the challenge.

Note the holes in the Mile-a-Minute leaves. Those are most likely from a weevil that was introduced as a biological control. The hope is that the weevil will become numerous enough, and consume enough triangular leaves to slow the wave of Mile-a-Minute engulfing the countryside.

Thus far, the Mile-a-Minute looks undeterred, growing over the slowly maturing fruits of wineberry,

and the pale stems of native black raspberry.

Beyond any ecological impact of such rampancy, it's interesting to reflect on the aesthetic and emotional impact of seeing a landscape being overrun by Mile-a-Minute. A healthy native prairie, for example, teaming with many species of grasses and wildflowers, all reaching for the sun with no inclination to crawl over one another, gives a feeling of striving, freedom, diversity, peaceful cohabitation, tolerance of one species for another. In contrast, a vine like Mile-a-Minute creates a smothering effect, a sense of clutter and thorny entanglement, a suppression of difference, an oppressive dependency that plays out as a punishment for any plant that dare reach a sturdy stem for the sky.