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Feeling Thankful for Cranberries’ Quirky Evolution

wild cranberries, not yet ripe, in a Vermont bog

For Thanksgiving, a quick post singing the praises of that humblest fruit, the cranberry.

Growing up, I never cared for the gelatinous cranberry sauce my relatives turned out of a can each year at Thanksgiving.  The wobbly, alarmingly-colored goo made me leery of cranberries for years, in fact.  I didn’t discover the delights of dried cranberries until my early twenties, when I started studding my oatmeal with them at breakfast.  And it wasn’t until last Thanksgiving that I discovered how I love to eat fresh cranberries: minced into a relish with a whole orange and a bit of sugar, no cooking required.

My partner Dave was washing this year’s cranberries this morning as he prepared to make our Thanksgiving batch of cranberry-orange relish (based on a recipe at, but we include the orange peel and decrease the sugar).  He turned to me with a cranberry in his hand.

“Is it okay to use the soft ones?”  He held a cranberry up to my face and gave it a quick squeeze to demonstrate its mushiness.  Instead, it let out a little poof of air and collapsed, startling both of us into laughter.

“Whoa!” he said, looking bewildered down at the cranberry.  “What was that?”

Somewhat to my surprise, I knew the answer.  “The berries have air pockets inside to make them float!  I’d forgotten that.”

Wild cranberries grow in the cool, moist, acidic environments of bogs and fens.  There are a few different species of cranberry, and all are edible.  The cranberries in commercial cultivation are cultivars of Vaccinium macrocarpon (macro = big, carpon = fruit) while the wild ones I know best are Vaccinium oxycoccos.  Either way, they’re tasty members of the heath family (the Ericaceae), and are close relatives of blueberries and huckleberries.

Although I grew up in New Jersey, one of the leading commercial cranberry-producing states, it wasn’t until the summer of 2010 that I saw my first wild cranberry plants at a fen in Vermont.  It was in August, just as I started graduate school, and those berries (pictured above) were whitish in color, not yet ripe.  They grew on a trailing, delicate vine with dainty leaves.  The berries seemed disproportionately large in comparison with the rest of the plant, and rested on a spongy, wet bed of sphagnum moss.

I learned just a few weeks later that the big berries have a sizeable amount of empty space inside.  Like buoyant little balls, the berries will float at the surface of the bog if the water level rises, which is important for seed dispersal.

cross-section of cranberries showing air pockets; Christian Science Monitor photo by Melanie Stetson Freeman

Cross-section of a fresh cranberry, showing air pockets and seeds within the fruit. Christian Science Monitor photo by Melanie Stetson Freeman, from the charming photo gallery called “Cranberries: the juicy rubies of fall!”

There are two likely mechanisms for seed dispersal.  The first and most obvious is that cranberries aren’t just tasty to humans: a variety of wild birds and mammals like to feed on them in the fall.  As with many berries, the undigested seeds are pooped out to grow elsewhere.  But wildlife can’t get to the berries if they’re submerged, so it seems likely that the air bladders evolved to keep the cranberries afloat at the surface and accessible.

But that isn’t all there is to the story.  Once the berries had evolved to float, it turned out that floating was itself a handy means of seed dispersal—one that could, in fact, work even more efficiently.  A berry could detach from the parent plant and be carried away to settle somewhere else moist, where it would probably have a better chance of establishing than if it had been cast randomly into the environment by a pooping animal.  So cranberries don’t rely exclusively on being palatable to wildlife anymore.  Instead of pumping lots of energy into sugar production, cranberry plants fill the berries with tannins that give them their tart taste.  This makes the fruits less appealing to animals, but it hasn’t thwarted humans.

And that’s why we needed to add sugar to our cranberry-orange relish for Thanksgiving!  Have a happy holiday.

3 Comments Post a comment
  1. And on the subject of cranberry sauce: read The Cranberry Sauce Has Something to Say, from The New Yorker

    November 22, 2012
  2. Janet Johns #

    A cool kids book about the cranberry and why it is red, “The Legend of the Cranberry”. Happy Thanksgiving! We love that relish too.

    November 22, 2012
    • Thanks for the book recommendation, Janet. I’ll have to try to track that down … especially since it has mammoths in it as well as cranberries. Hope you’re having a happy Thanksgiving!

      November 22, 2012

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