On May 22, 2024

The Outside Story: Jesup’s milk-vetch: A rare beauty

By Emily Haynes

A few ledges along the Connecticut River are home to a rare plant commonly known as Jesup’s milk-vetch (Astragalus robbinsii var. jesupii). In fact, this species, which has been listed as federally endangered since 1987, only grows at six sites along a 16-mile stretch of the river in New Hampshire and Vermont. But conservationists are working to boost the plant’s presence. 

A member of the pea family, Jesup’s milk-vetch has pinnately-compound leaves comprising many small oval leaflets. Its blue-violet flowers bloom in clusters. Even in the few areas where it grows, the average paddler, angler, or hiker is unlikely to encounter this plant; the rock outcrops it favors can be literal cliff-hangers and are sometimes encircled with poison ivy.

Jesup’s milk-vetch depends on habitat shaped by ice scour. This occurs as ice moves along the river in winter, pushing over the rocks and ledges that line the waterway. The abrasion scrapes away vegetation, exposing open soil that allows the plant to root into small crevices in the rock. A perennial, Jesup’s milk-vetch must survive for three years before it can flower and reproduce. 

Three of the sites where Jesup’s milk-vetch grows are naturally occurring. Three others were established through conservation work by the nonprofit Native Plant Trust, New Hampshire Natural Heritage Bureau, Vermont Fish & Wildlife Dept., U.S. Fish and Wildlife Service, and other groups. Since 1989, the conservation team has managed the plant’s habitat and transplanted seedlings grown from seeds collected from the natural sites and stored in Native Plant Trust’s seed bank. 

Not all seedlings planted at the restoration sites survive, said Michael Piantedosi, conservation director at Native Plant Trust. The growing conditions are harsh, especially as climate change leads to rising temperatures and more frequent droughts. Challenges also include both nonnative and native plants — such as poison ivy, European alder, and Japanese knotweed — encroaching on Jesup’s milk-vetch habitat.

Of the natural sites, one stands out for its vigor: it contained some 1,900 flowers at last count, surpassing its conservation goal and boasting the largest population observed in years, according to Grace Glynn, botanist for Vermont Fish & Wildlife. “It was just a sea of purple pea flowers,” she said. “It was really beautiful.”

Because Jesup’s milk-vetch prefers ledges near the waterline, unseasonal flooding, such as the torrent the region experienced in July 2023, poses another threat to the plant. Some populations suffered huge losses during that flooding, and as the climate warms, flood events are likely to become more common year-round.

“It was pretty disheartening. We went out right after the flood to a couple of the sites, and they were decimated,” Glynn said. “What’s so unprecedented about these flooding events is that they’re occurring in the summer, and that’s not the time of year that these plants are adapted to experience flooding.”

During normal springtime floods, the plant is small and leafy, making it less likely to be ripped from the soil as the water surges. But when the Connecticut flooded this past July, Jesup’s milk-vetch was blooming, and the bigger, flowering plants are much more vulnerable to swift water.

When Piantedosi collected seeds from a Jesup’s milk-vetch site in New Hampshire days after the flood, he noticed another strange occurrence: in each plant, roughly 60% of the seeds had begun germinating while still enclosed in the fruit, likely due to excessively wet and humid conditions caused by the flooding.

“The seed was producing what’s called vivipary, essentially living while in fruit,” Piantedosi said. He noted that this phenomenon greatly diminished the population’s ability to expand on its own, since the seedlings-within-fruit were unlikely to survive. Seeds need to be dormant to survive in a seed bank’s low-temperature environment, and the high rate of vivipary meant Piantedosi was able to harvest far fewer seeds than he had anticipated. 

The flood set back conservation efforts, but the silver lining has been the increased silt it deposited on Connecticut River ledges. Glynn said this could provide better conditions for transplanting seedlings this spring. She, Piantedosi, and others remain committed to protecting this plant, even against long odds.

“It’s just so exciting for me to see such diversity and such wonder and so much of the beauty in

nature in just a tiny, diminutive pea,” Piantedosi said. 

Emily Haynes is a writer currently living in Washington, D.C. An avid birder and hiker, she loves exploring the forests of the Northeast. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation: nhcf.org.

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