Natural Connections: Freezer Burn

This week's featured outdoors column from Emily Stone.

Natural Connections: Freezer Burn

Reaching into my chest freezer, I pulled out a quart-size zip-top bag full of dark green leaves. Or at least leaves that had once been dark green. The frilly edges of my kale were now a little pale in places, and ice crystals crunched brittlely inside the bag. Last summer I’d harvested grocery bags full of kale from my garden, blanched them briefly in boiling water, and then quenched them in two cold water baths. After stuffing a handful into a baggie, I rolled it from the bottom to squeeze out extra water and air, and firmly pressed the closure.

Despite my best efforts, several months in storage had led to freezer burn. Ice in the leaves had sublimated, turning from solid to gas without passing through the liquid state. The water that was once in the leaves had become the ice crystals in the bag. Unworried, I thawed the kale and chopped it finely to add to a soup simmering on the stove. Freezer burn isn’t dangerous to eat. It can affect the flavor of food, but I wasn’t counting on kale for flavor anyway.

The following afternoon, I headed out for an afternoon walk. With bright sunshine, the temperature had risen just above freezing, but now a brisk wind was making the lengthening shadows quite chilly. As usual, I paused to admire the mosses growing along a steep, north-facing bank. On this day they weren’t very pretty.

Whole patches of moss were crinkled and brown, while others were brown at their tips, and some leaves were ghostly pale…not unlike my freezer burned kale. I nosed around a bit in the moss patch, taking photos and investigating the damage. Later, I emailed Joe Rohrer, Professor Emeritus of Biology, University of Wisconsin-Eau Claire. Joe taught a moss ecology workshop for the Museum in 2019, and will be teaching it again this October.

“Honestly, I can't cite a single academic paper on this topic,” wrote Joe. But he’s also been noticing chatter about this very topic in the moss-themed social media groups he’s part of. “The consensus seems to be that some mosses show winter dieback regularly but sprout new green growth in the spring. The moss gardeners see this a lot with Atrichum and Polytrichum species. The leaves of the previous year do seem to die, but new growth from the tip restores their green color. Other mosses just turn a rather ugly golden brown, such as Thuidium, probably similar to the red coloration we see in some vascular plants when they get winter sun but are shaded during the growing season.”

Uploading my moss photos to iNaturalist to identify them, I was able to confirm that the pattern he outlined seemed to hold true on my driveway.

“My guess is that drying out is probably more harmful than freezing temperatures,” Joe concluded.

And I agreed. One of the benefits of the Subnivean Zone, which never had a chance to truly develop on my driveway this winter, is that a blanket of snow holds moisture close to the soil. According to horticulturists, winter burn is caused by low soil moisture, freezing temperatures, and blowing wind. Not only does that magical space hold the temperature steady near 32 degrees, it also eliminates windchill, and provides a high humidity habitat. Without it, moisture sublimates from the moss leaves just like from my kale.

It's not that mosses haven’t prepared for this. As drought sets in, their cell membranes shrink like vacuum-sealed freezer bags. This winter has been especially rough, though, with many nights below freezing without snow on the ground. Hopefully the mosses were able to synthesize a big enough supply of enzymes for cell repair to manage the damage from this weird winter.

Next to that sad patch of moss were several rosettes of fern leaves. Evergreen wood fern doesn’t die back in the fall. Instead they flatten to the ground and let snow cover them. Concentrated sugars act like antifreeze in temperatures 5-10 degrees below freezing, and special proteins keep them from being damaged as temperatures plummet further. Even in a normal winter, these leaves never stand back up. They are replaced by fresh, new leaves in the spring. But for several months they can continue to do photosynthesis—while leaves are off the trees—and give the plant a head start on new growth. Maybe.

The tips of the evergreen wood fern fronds near my sad mosses are curled up and look dry. Water is essential for photosynthesis. I can’t imagine they are very productive right now, and I wonder if they will even recover if it eventually rains.

Farther up the road, I stopped near another nearly vertical bank. Dozens of wintergreen plants poked stiffly up above the dry brown leaves. They looked pale and dehydrated, just like the ferns. Would their waxy leaves be tough enough to survive the dry cold? They are well-adapted to hunkering down in the Subnivean Zone and even photosynthesizing there. These many weeks of exposure to dry cold must be challenging, even to them.

I wasn’t looking for wintergreen, though. One of my favorite spring flowers, trailing arbutus, grows on this bank, too, but spotting them is always a challenge. Finally, I glimpsed the brighter green, broader leaves peeking out from under the duff. They didn’t look freezer burned. Perhaps the secretive, ground-hugging nature of this little plant is a way to survive winters just like this one. Soon they will bloom, and I’ll plant more kale, and most of the mosses will recover. Soon, this weird winter will be over.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is closed until May 1 to construct our new exhibit: “Anaamaagon: Under the Snow.” Our Winter/Spring Calendar of Events is open for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Last Update: Mar 12, 2024 7:07 am CDT

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