Even with the warmer weather this past week, the sun is still bobbing below the treeline by mid-afternoon. After long days of holiday festivities last week, I often found myself looking at the clock, expecting it to be nearing close to bedtime, only to discover that it was barely past 7 o’clock. We recently passed the winter solstice, the shortest day of the year, on Dec. 21, and now we’re gaining a few more minutes of light every day. This turn into the light feels like a relief in these days where the night still far outpaces the day. While it has been shown that the modern human response to the length of the day exists more in our experiences than in our behaviors, many plants are intricately tied to this seasonal availability of light.
In the greenhouse where I work, we notice the plants starting to slow their growth by mid-to-late September, when the daylight begins to dip under twelve hours a day. While there are some fluctuations in our greenhouse climate depending on the seasons, we mostly keep the temperature and humidity as consistent as possible — the real difference is in the daily amount of light.
Above the soil, the two main influences on plant growth are temperature and light. Temperature is relatively straightforward: Every plant has a range of temperatures in which it will grow, and within this an ideal temperature for optimal growth. The effect of day length on plants’ growth is more nuanced, but understanding it helps us understand the needs of our plants in a fuller way and can help fill in our understanding of the timing of garden plantings, especially when it comes to many common annual vegetables and flowers that respond to shorter or longer days.
Even though horticulturalists tend to speak about a plant’s sensitivity to day length, plants are actually being influenced by the length of uninterrupted dark, or night. This phenomenon is called photoperiodism, and plants generally fall into three categories: short day, long day, or day-neutral. Short day plants begin to flower when daylight is less than 12 hours; some short-day plants like Christmas cactus need a full 14 hours of darkness to produce blooms. Other common short-day plants are Poinsettia, Kalanchoe, soybeans and many varieties of scallions.
Long-day plants begin flowering when daylight is greater than 12 hours per day, and plants that are not sensitive to the availability of light are called day-neutral. Many common long-day vegetables — like lettuce, spinach and turnips — are grown in the spring and the fall, both because they prefer the cooler temperatures and also because the long days of summer induce the plants to flower, which in the case of all three of these vegetables renders them unharvestable.
Rudbeckia, commonly known as black-eyed Susans, are long-day flowers that if planted in spring will grow vegetatively until the longer days prompt them to send up strong, long, harvestable flower stalks. If planted later in the summer, however, Rudbeckia will grow but might not flower, or if so will likely flower on shorter, flimsier stalks. Timing our plantings to correspond with temperature and day length can go a long way towards optimizing the plants we grow in the garden, and much of this knowledge is baked into the common-sense timing and practices we use to plan out our annual vegetable and flower gardens. If you have tried growing an annual garden plant without success, it is worth considering whether the timing you offered it corresponds to its needs around the length of day.
For me, even working in a greenhouse, the shortness of light in this part of the season definitely takes its toll. I find that the days leading up to the winter solstice are a time of drawing in, taking stock, and retreating. The cold and dark evenings make it hard to spend much time outside, and my body naturally craves more quiet time and rest. As I’ve gotten older, I have found more peace in this retreat. In a way, it is a comfort to know how intertwined this darkness is in the life cycles of the plants and animals around me, and also that this period is distinct and will eventually turn into brightness again. I find support in being tied to this cyclical balance that comes season to season.
Now, as we turn into the light, I can already feel the subtle shift away from retreat and back out into the world. I will begin my garden planning for next season soon. On warmer sunny days, I feel pulled outside and am already daydreaming about the walks and hikes I will go on this coming year. Spending time outside now, when most plants and animals are in hibernation, amps up the dramatic reveal of spring when everything pops back into life. We still have several months of long nights ahead of us, but the days are finally getting longer, and hopefully that little extra light will bring us closer to our own version of unfolding, whatever that might be.
Hitchcock Center Living Building Tour Program
Jan. 3 at 4 p.m.
Free, please register in advance online.
The Hitchcock Center’s living building is designed to model systems in nature, is net-zero energy, net-zero water, has composting toilets and has been made with responsibly sourced non-toxic materials. Join the staff for a tour of the building and the systems in it. Tours typically last from 1-1.5 hours. Spanish-language translation is available for building tours. Please contact the Hitchcock Center in advance for scheduling purposes.
Movie Night: Eastern White Pine: The Tree Rooted in American History
Arcadia Wildlife Sanctuary
Wednesday, Jan. 15, 7–9 p.m.
Led by Ray Aselin, naturalist and filmmaker, and Bob Leverett, Native Tree Society co-founder.
$6 members, $8 non-members
This new documentary tells the story of our native white pine and the significant part it played in American history, using archival footage, photography and aerial views. After the movie, the producers will discuss the making of the film and take audience questions.