Gardeners rejoice; the winter solstice is coming!
Also called midwinter, the winter solstice is actually an astronomical event that happens at a specific time; it lasts only a moment. This year’s solstice (in the northern hemisphere) falls on Friday, Dec. 21, at 4:23 p.m. The event occurs when the sun is directly above the Tropic of Capricorn. This is the longest night of the year in our area of the world, after which there will be an indiscernible lengthening of the day. Unfortunately, that doesn’t correlate with our Wisconsin temperatures; our coldest winter weather is usually after the days have become noticeably longer.
The shorter daylight hours from June to December are due to the Earth’s positioning in relation to the sun; this time of year, the northern hemisphere is pointing away from the sun. This causes less sun to reach us, resulting in shorter days and eventually those cooler temperatures. While we’re experiencing the winter solstice and the least amount of daylight hours, our counterparts in the southern hemisphere are celebrating the summer solstice with the most daylight hours.
I could certainly envy our southern neighbors with their long, warm days. Instead, in those moments when I’m missing the extra daylight I remind myself that our neighbors north of the Arctic Circle will have 24 hours of darkness on Dec. 21. In Barrow, Alaska, the darkness lasts 65 days! It’s all about perspective, isn’t it?
So why do gardeners care about the winter solstice? We have quite a bit more wintry weather before it’s time for spring planting, after all. While the slight changes don’t really impact plant growth immediately, there are times when daylight length signals some plants to shift from vegetative to reproductive growth. This phenomenon is called photoperiodism and can be defined as the way plants respond developmentally to the relative lengths of dark and light periods. When the phenomenon was first discovered in the early 1900s, scientists didn’t realize that the important part in relationship to plant development wasn’t the length of day — but the amount of darkness. That discovery came later.
Plants use photoperiod to begin the process of winterizing. They prepare for the long, cold winters by producing amino acids and sugars that act as a sort of antifreeze. This minimizes — and even prevents — cold damage. The plant’s ability to produce chlorophyll decreases when photoperiod decreases, which causes the leaves to change colors and drop in the fall, long before the truly cold temperatures arrive. They also use this to wake up from their long winter nap and prepare for the coming spring. That’s where the winter solstice comes in; that imperceptible lengthening of the day after 4:23 p.m. on Dec. 21 will eventually lead to noticeably longer days, and then … spring.
Be happy, be grateful; next week we celebrate the return of the light!
Rae Punzel is a Kenosha writer and horticulturalist. She owns Bennu Organics, a horticulture services and consulting business. Contact her at firstname.lastname@example.org.