photo of field full of pumpkins

13 Days of Halloween: Plant biology eventually gave the world the feast of ghosts and goblins.

Halloween is thirteen days away. A quick scan of your local Walmart or Target will tell you it’s one of the big commercial events of the year. Skeletons, spiders, zombies, vampires, mummies, cobwebs, and coffins decorate shopping mall walls, storefront displays, and pretty much any place with heavy foot traffic. For children, it’s become a day of costumes and trick-or-treating. For adults, it’s become a day of costumes and heavy drinking. The origins of Halloween, however, are a far cry from what we know today.

In terms of the traditions that eventually morphed into Halloween, the timing of the holiday is a big tell. It takes place in the fall, the season when agrarian societies around the world take out their sickles and scythes and harvest the crops they’ve spent the spring and summer nurturing. According to one theory, Halloween’s earliest iteration was a Celtic harvest festival called Samhain. The day symbolized the transition point between the light of summer and the dark of winter, life and death.

The autumn harvest was always an urgent affair and communities would have an all-hands-on-deck approach with everyone sharing responsibilities on each other’s farms. The later in the season the harvest took place, the more danger the dropping temperatures became. Cold weather can ruin crops en masse. At its essence, the plant’s delicate biology dictated the existence of harvest festivals.

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So what is it about plants that make them so sensitive to climate?

During the summer, plants store energy in their roots, bulbs or corms (aka rhizome)—the part where the stem and root meet. That stored energy allows it to go dormant once the cold weather sets in. 

Perennials, especially prairie plants, are known for their extensive roots that reach deep down into the soil. Those roots, storing energy, help them survive wildfires and drought. Those roots also help keep soil in place, preventing it from eroding away.

Low temperatures can damage plants in several ways. First, the cold reduces the plant’s rate of metabolism and growth. If the temperature remains low for an extended period, plant quality will suffer, and death may occur. A period of cold weather may also alter plant growth, as when certain vegetables “bolt,” or produce seed stalks, in response to several days at low temperatures.

Another type of injury occurs if the temperature falls below freezing. Below 32° F, the water within and between the plant’s cells freezes. Ice crystals form inside cells. Plants try to protect themselves from freezing by concentrating solutes like sucralose to lower the freezing point inside their cells, this only works for a while. If the temperature dips below 20 degrees, all bets are off.. After that point, the water in cells can actually freeze into crystals and can puncture the cell wall membranes, leading to widespread destruction.

Intercellular ice formation also means that water is unavailable for the plant’s metabolic processes and leads to desiccation, a sort of cellular dehydration.

Vegetable plants differ in their hardiness to cold temperatures. Warm season crops, such as tomatoes and snap beans can be severely injured by even a light frost. On the other hand, cool season crops, such as broccoli, cabbage, peas, and onions can tolerate frost and light freezes with little damage.

Much of the death and dying iconography of commercial Halloween has its roots in harvest festivals which were also essentially about dying. With the spread of Christianity, those harvest festivals lost their pagan elements and became days to celebrate people who have passed away and pray for their souls. The names of harvest festivals may have changed, but their essences did not. They were still about death.

And that brings us right back to Halloween in the 21st century when the only harvesting going on is done by companies stealing your data as you shop for candy corns or ghost shaped marshmallows.



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