A Visit with Minnesota's Rarest Plant

Rice County, Minnesota - April 27, 2025

No more than 10,000 years ago, a White Trout Lily (Erythronium albidum) mutated into Minnesota's only endemic flowering plant.

About 10,000 years ago, the Des Moines lobe of the Wisconsin glaciation retreated from the Big Woods region of southeastern Minnesota. As the land rebounded, the Cannon River cut its way through young deposits of crushed limestone - exposing sandstone cliffs, cascading over limestone bedrock, and forming extensive floodplains. Somewhere in the floodplains, a common spring wildflower mutated into what is now Minnesota's rarest plant, known from as few as 400 clones in three counties along a short stretch of the Cannon River watershed.

This plant was the object of my late April visit to Minnesota, but it was cloudy and blustery when I woke up on Sunday morning, guaranteeing that its flowers would be closed for at least a few hours. With partly sunny skies in the forecast, decided to visit another gem of the Cannon River first in order to wait for the skies to clear and the rarity's flowers to open. I headed to McKnight Prairie, one of two gravel terrace prairies along the Cannon River and home to my favorite Eastern Red Cedar (Juniperus virginianus), massive glacial erratics, the globally rare Kittentails (Veronica/Besseya bullii), and the federally threatened Prairie Bush Clover (Lespedeza leptostachya; flowering in August). This incredible site rises well above the surrounding farmlands and is a notable spot on the landscape for miles around.

Throughout the growing season, there are always special things happening at McKnight Prairie. I missed the Pasqueflower show, but on my visit, I did find a number of Kittentails, a few flowering Ground Plums (Astragalus crassicarpus, at the eastern edge of their range in this region), and hundreds of Prairie Smoke (Geum triflorum) in bloom. No matter what time of year, I always admire a slope of eroding sand which has slowly exposed over a dozen glacial erratics and supports a surprising population of Beach Heather (Hudsonia tomentosa). I think the Beach Heather and Kittentails are especially notable. Both of the Cannon River's remaining terrace prairies have exceptionally large populations of Kittentails, a rare plant found only on gravel prairies in a handful of Great Lakes/Upper Midwestern states. It is highly unusual to find Beach Heather away from large areas of shifting sand, usually along major rivers, Great Lakes shorelines, and inland sand dunes. This population may indicate that the lowlands surrounding McKnight Prairie were once an extensive sand flat.

After birding and botanizing the prairie for an hour and a half, the skies had begun to clear. This was my sign to follow the Cannon River 20 miles to a preserve dedicated to protecting Minnesota's rarest plant: the Dwarf Trout Lily (Erythronium propullans). I parked at the dead end of a gravel road and descended onto the verdant forest floor of a floodplain valley. I have searched for this plant three times now, and to me the experience is rewarding and maddening in equal parts. The floodplains it inhabits are a joy to walk through, but are also home to massive populations of the vegetatively identical Erythronium albidum. Searching through thousands of trout lily leaves eventually results in severe frustration and desperation (see middle picture below). Even in a relatively small floodplain, hemmed in by sandstone exposures to the west and limestone cliffs to the east, it took me two hours to find my quarry.

Each time I've looked for this species, I've been tricked into combing through the abundant leaves of Erythronium albidum. This turns out to be unnecessary, though, as E. propullans simply does not produce the same masses of sterile leaves. In fact, it is named (propullans = "sprouting forth") for a unique reproductive strategy whereby it sends out a single rhizome, well below the ground, which travels horizontally for some distance before giving rise to a new stem. In E. propullans, each stem is much likelier to flower than in its sister species, which leads to its clones having a totally different structure. E. albidum produces dense masses of sterile leaves with occasional flowering plants, while E. propullans produces looser clusters of leaves with a much higher proportion of fertile plants. Once one adjusts to the counterintuitive strategy of searching where trout lily leaves are less abundant, finding the Dwarf Trout Lily becomes much simpler. Next time I'll need to remember this from the start to save myself the frustration and strained eyes.

E. propullans occurs in loose clones where trout lily leaves are counterintuitively less abundant.

The astute reader will have already noticed a much more obvious difference, as well as deduced it from the common name: the flowers are absolutely tiny! They are no more than a centimeter across and also have fewer tepals - three to five as opposed to their larger cousin's six. The tiny rice-grain-sized flower buds and seed capsules are also held upright, as opposed to nodding in E. albidum. The last and perhaps most striking difference is that this species is apparently unable to sexually reproduce. Incredibly, this suggests that its scattered populations may be remnants of one massive clone which once ranged throughout an unbroken floodplain system, propagated by rhizomes and occasional fragments floating short distances downstream.

This mutation could have probably occurred anywhere within the White Trout Lily's range, but one has to wonder if there's something special about the middle Cannon River drainage that allowed it to persist. It arose at the southernmost edge of the Big Woods ecoregion, surrounded largely by prairie ecosystems to the south and west and a massive area of unbroken woodlands to the north and east. Its range lies very near the westernmost extent of the last glaciation, meaning that E. albidum may have been able to move into the region sooner than elsewhere to the north and east, and that the Cannon River drainage may have had more time to stabilize. Populations of E. propullans inhabit sheltered valleys with well-developed terrace systems, which may provide the stability needed to support populations of such an unusual plant - especially given that floodplains can often be subject to extreme amounts of disturbance. Does this all mean that a mutated, slowly propagating trout lily is more likely to persist here than any number of other floodplain systems across North America? I really don't know, but it's interesting to think about.

Regardless, this rarity has indeed persisted at this site, confined to a narrow floodplain pinched between exposures of sandstone and limestone. Its above-ground existence is likewise confined, like any spring ephemeral, to two months or less each year. The Cannon River rambles on - past McKnight Prairie, beneath Driftless bluffs, and into the Mississippi River - just as spring rambles on, withering all visible evidence of Erythronium propullans by the first week of June.