Invasive tree taking root in N.H. may trace back to Harvard

A castor aralia seedling appeared on the UNH campus.
Tom Lee/University of New Hampshire
A castor aralia seedling appeared on the UNH campus.

Call it the case of the alien tree.

When Tom Lee noticed a nonnative species with fine-tipped leaves taking root in flower beds around the University of New Hampshire campus five years ago, the associate professor of forest ecology just had to know its origins.

So when a team of students came to him asking for research ideas, Lee recommended they do a bit of sleuthing to help find out why the invasive greenery was spreading on the Durham, N.H., property.


What they found was a tree mystery that they believe branches back to the Arnold Arboretum of Harvard University, which gave specimens of the exotic plant to institutions around the country as part of its centennial celebration in 1972.

Get Metro Headlines in your inbox:
The 10 top local news stories from metro Boston and around New England delivered daily.
Thank you for signing up! Sign up for more newsletters here

“It’s an interesting trail. The whole thing has been like a detective story,” said Lee. “They may have inadvertently spread this.”

The tree, known as the castor aralia, has shown up elsewhere, as well. Researchers in the mid-Atlantic are tracking its spread, watching with their colleagues in New Hampshire to see whether it could threaten native trees along the East Coast.

Lee and his associates began their research in 2011, by examining the ancestry of the invasive trees on campus. The castor aralia, which is an Asian ornamental shrub related to ginseng and English ivy, had also sprouted on UNH’s nearby Thompson Farm, they found.

The inquiry was funded by the New Hampshire Agricultural Experiment Station, and was part of a broader study of nonnative plants and their potential effects on the forest ecosystem.


Their findings were recently published in the journal Invasive Plant Science and Management.

Researchers tracked the spread of the exotic tree, more formally known to botanists as the Kalopanax septemlobus, to a single, ornamental specimen that was intentionally planted next to the school’s Spaulding Hall between 1971 and 1975.

The tree grew to around 25 feet before the school cut it down in 2013.

It was also in the 1970s that the Arboretum offered castor aralia plants to institutions and botanical nurseries across the country. The gifts were considered reverse-birthday presents.

“We did give it out, as one of 10 different trees, in 1972,” said Michael Dosmann, the curator of living collections at the arboretum.


While there’s no paper trail that shows UNH received a castor aralia from the park during the gift-giving, officials at both institutions say it’s a possibility.

‘It’s an interesting trail. The whole thing has been likea detective story.’

The arboretum has a separate set of records showing that they gave UNH scientists a sample of castor aralia in 1965, for research purposes, Dosmann said.

“My gut would say that if it wasn’t in 1972, it was probably from the 1965 distribution,” he said. “It could very well have come from us.”

Back then, specialists didn’t know what effects the species could one day have on the environment, or how easily it could spread. The subject is still being studied today.

“It wasn’t on our radar,” Dosmann said. “That’s been a more recent revelation. Not just by us, but by others with this particular tree species.”

In the studies of invasive plant ecology, there’s a term called the “lag phase,” in which a nonnative species can lay in waiting for decades before it spreads.

Some of the thousands of trees Lee and his team discovered didn’t start growing until years after the parent tree arrived. The trees were likely spread by birds who feast on their blue fleshy fruits.

The National Park Service has tracked a similar invasion of the showy ornamental shrub in the mid-Atlantic region, also citing the arboretum’s distribution in the 1970s. The agency placed the castor aralia on their “plants to watch” list.

Nurseries, horticulturists, and hobbyists have all likely contributed to the spread of the plant, however.

“While the Arnold Arboretum seems to have distributed the plant in 1972, material has no doubt been sent around by many different sources,” Lee said.

The aboretum actively culls castor aralias from natural and cultivated areas of the parkland when they’re spotted, although they have a few older ones in their collection for research purposes. The institution has learned to be much more careful with the castor aralia.

“We wouldn’t be distributing the castor aralia now that we know it can wander,” he said.

Steve Annear can be reached at Follow him on Twitter @steveannear.