Beyond Your Backyard: Now that’s fen-tastic

Nothing makes my heart fill with awe in winter more than a calcareous fen (ok maybe a snow-covered prairie gets me too). These rare, peat accumulating wetlands are fed by a constant supply of upwelling groundwater and formed after the last glaciers receded. You can find them in western Minnesota with the remains of glacial features, the Minnesota River Valley, or the limestone dominated parts of the southeast (karst anyone)? In addition to aging well (we’re talking beautiful, at a minimum of 4,000 years old in western MN or 10,000 years old in the Minnesota River Valley on the better with age scale), they are also home to rare and uncommon plants.

Not only are these plants rare, they’re Minnesota tough. It’s hard to live in a calcareous fen because the pH is so high. Most plants prefer a range of 5.5-6.5 pH (slightly acidic). Whereas most Calcareous fens range around 6.8 or higher. Yikes—that’s a lot of calcium! In fact, it’s so much that if you’re so lucky to visit one of these ancient features, many of the plants are covered in a fine white dust because they are “sweating” calcium or releasing it through transpiration if you want to sound cool around the dinner table. Some plants have developed unique adaptations to handle it, like this tiny nutrush that forms its seed from the extra calcium making it look like an adorable miniature snowball.

State-threatened whorled nutrush that we found! It uses the calcium in the groundwater to form a shell around its seed! (You can tell because it’s the tiny white circle).

I always think their open pools with inter grading islands of low vegetation make them look like miniature Minnesota Everglades. Healthy calcareous fens have sustained groundwater flow. Think of it like an uncapped fire hydrant. There’s so much pressure in there the water comes out in a rush. Because fen groundwater upwelling is spread out across the system, the pressure is the same, but it’s not quite as dramatic unless you tried to channel that water into a well (but who would do such a thing?). In winter, the warm groundwater (roughly around 50 degrees) bubbles up and hits the cold winter air causing the water to freeze. All the water underneath is still a balmy 50 degrees so over time, you end up with a frozen, hollow dome with a secret stream hidden underneath.

Minnesota miniature Everglades. A calcareous fen with iron-stained groundwater pools and low growing islands of vegetation in between.

Calcareous fen dome at the Sioux nation Calcareous fen. 

Fens are important not just because they look neat, but because they’re part of a larger groundwater system and like most wetlands are filtering and cleaning water. They have been one of the most stable points on the landscape and there is nothing in nature that can destroy them short of plate tectonics. Some people even call them the Goldilocks habitat because they need their water supply and chemistry just right in order to exist. When they disappear it’s a dire signal to us that we have disrupted our groundwater supply. The same water supply that gives us clean drinking water, and provides us with showers, and cooking water—not to mention we share that water with our wildlife friends.

Frozen groundwater channels among the islands of sedge vegetation at the Sioux Nation fen.

Calcareous fens are both strong and fragile. They serve as a beautiful reminder that finding a delicate balance and maintaining connection is critical. Nature is part of us even when making a cup of morning coffee. Now that’s fentastic! #calcareousfens #fentastic

Burke wildlife management area Calcareous fen nestled in the hillslope awaiting the thunderstorm.

Kalm’s lobelia (Lobelia kalmii) a Calcareous fen indicator plant grows happily with a bunch of sedges on a tiny hummock.

American grass of parnassus (Parnassia glauca) a calcareous fen indicator plant.


All photos courtesy of Megan Benage.