August 1, 2022
Roundpod primrose-willow (Ludwigia sphaerocarpa) photo on the left by Paul Marcum. Cypress swamp photo by Michael R. Jeffords.

Botanists Discover Plant Not Seen in Illinois for More Than 160 Years

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By Sheryl DeVore
A tall bushy plant with green oblong leaves and a wood like stem with peeling bark is in front of tall green vegetation.
Roundpod primrose-willow (Ludwigia sphaerocarpa). Photo by Paul Marcum.

Relocating the roundpod primrose-willow in Illinois, a plant not seen in Illinois in more than 160 years, was a shot in the dark.

But the light went on one summer day in 2019, when Illinois botanists Paul Marcum, Eric Ulaszek and David Ketzner were surveying Round Pond, a Pope County tract that had been purchased by the Illinois Audubon Society.

Wading up to his waist in a bald cypress-tupelo swamp, avoiding the cottonmouth snakes, Marcum pointed out a species to his colleagues that looked like a specimen he had seen in a Field Museum collection. It was the roundpod primrose-willow (Ludwigia sphaerocarpa).

“It has a unique growth form,” said Marcum, a botanist with the Illinois Natural History Survey, part of the University of Illinois’ Prairie Research Institute. “It’s woody and is inflated and peeling at the base of the plant. It’s spongy, feeling very soft. These are adaptations to growing in a really wet environment.”

The botanists were excited but decided they couldn’t clinch the identification until they returned in September to see the plant in fruit. Finding the plants fruiting, they rejoiced.

“It was the highlight of my career in Illinois,” said Marcum. “It’s the coolest thing I’ve ever seen.” He was further struck by the unique habitat discovered at the north edge of the Audubon property—a floating mat of decaying vegetation in an opening in the swamp. It’s a perfect growing regime for primrose-willow and other rare plants.

The botanists saw five primrose-willow plants in 2019, and when Marcum returned in 2020, he saw 16 plants with a total of 50 stems.

“This is a viable population,” Marcum said, adding when the next list of state threatened and endangered species is updated in about four years, the primrose-willow will likely be added.

Eric Ulaszek, associate scientist/botanist for the Illinois Natural History Survey who helped on the survey, added, “This is a really high-quality area that has a high diversity of native plants in it. The one thing that was notable, with few exceptions, was that we saw very few invasive plants in the floating mat habitat.”

Rediscovering a lost species is no easy feat, but Marcum was well-prepared to do so. Before he surveyed Round Pond, he and other botanists worked on a project to locate herbarium records of plant species observed in Illinois that had not been seen recently.

A dark and light green map of the state of Illinois with the very southern tip in white indicating the coastal plains natural division.
Coastal Plains Natural Division. Illustration by Sarah Marjanovic.

Marcum discovered two specimens of the primrose-willow at The Field Museum. They were collected Aug. 18, 1860 in Cook County.

The species’ typical range is along the Atlantic and Gulf coastal plains, so Marcum considers the Cook County specimens part of a former disjunct population. However, the coastal plains habitat extends up the Mississippi River to the southern tip of Illinois, and that’s where you’d expect the primrose-willow, he said.

Classified as a shrub, the primrose-willow has a spongy, peeling tissue at the bottom that helps gas exchange from the roots to the plant. The adaptation works in a low-oxygen environment found in a floating mat in a swamp.

The perennial plant dies back at the end of each growing season.

The Round Pond site where the plant was found was inventoried in 1977 and again in the early 2000. Rare species were found, but not the primrose-willow. It’s likely that spending more time at the swamp during the 2019 survey increased the chances of discovering it. But it helped that Marcum knew about the plant.

Marcum said the floating mat of decomposed plants, such as sedges and smartweeds, provided a challenge when surveying the swamp.

“You can’t stand on the mat. You’d fall through,” he said. “Surrounding the mat are shrub-sized trees—bald cypress and red maple and swamp tupelo. We would circle the edges and poke in a few places where we could, trying not to break up the sensitive mat.”

A researcher wades in a swamp and pauses to investigates a sedge plant. In the background are green cypress trees.
Paul Marcum wading next to Carex decomposita at Round Pond Swamp, Pope County, IL. Photo by Greg Spyreas.

Throughout the 347-acre property, they found a total of 619 plant species, including 10 on the Illinois threatened and endangered species list. Several grew on the floating mat along with the primrose-willow.

Marcum researched similar habitats in Arkansas and learned the floating mat is likely short lived.

“Once the trees start growing, the trees sink the mat, so it’s important that there are multiple examples of floating mats at different stages,” he said.

Marcum is organizing a search at sites adjoining Round Pond, including one owned by The Nature Conservancy, to search for more primrose-willow.

“I suspect there probably are more,” he said. “I hope there are.”

“Ideally, we will find more examples at different stages,” Marcum said. “If it is down to one floating mat, then we have to be creative about protecting the species in the long term.”

Finding the primrose-willow is a reminder that more discoveries can be made of rare habitats and organisms in the wild lands of Illinois, adding to our further knowledge and enjoyment of nature in the prairie state.

“This discovery says there are still a lot of nice areas left in Illinois that need more exploration,” Ulaszek said.


Sheryl DeVore writes environment and nature pieces for regional and national publications and has had several books published, including “Birds of Illinois” co-authored with her husband, Steven D. Bailey.

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