Heather Kolich, ANR Agent, UGA Extension Forsyth County

An orange sky full of migrating geese.
Shortening daylight hours stimulates migration in some wildlife species. Photo by Barth Bailey on Unsplash.com.

Despite clear weather, I didn’t need sunglasses to drive to work this week. People are contacting the Extension office for information on fall gardening. School started. Just six weeks into summer, the days are still hot, but fall is in the air.

Summer solstice is the day each year when we experience the longest amount of daylight and, in counterpoint, the shortest night. It’s the official beginning of summer, but it’s also the beginning of incrementally shorter days. It’s a subtle difference – just 1-3 minutes less daylight each day – but it triggers changes in plants, animals, and people.

For plants, the important change in photoperiod – the length of exposure to light within a 24-hour period – is the longer dark period. Longer nights stimulate biochemical processes in tree leaves. In deciduous trees, production of green-pigmented chlorophyll dwindles, allowing the true color of the leaves to show. In preparation for leaf drop, leaves close vessels that deliver water from the roots. Within the tree, wood matures to resist damage from freezing temperatures.

Herbaceous plants rely on photoperiod to know when to flower. Fortunately, some plants flower when the days are long and others flower when the days are short (or really, when the nights are long), which is why we have flowers in all seasons. All plant growth effectively halts, however, when daylight exposure drops below 10 hours. This Persephone period, as it’s known in gardening circles, begins in Forsyth County this year on December 5 and continues until January 8, 2024.

A bunny whose fur has adapted with white to blend into snow more effectively.
As daylight hours lessen, some animals change fur or feather color to blend better with the winter landscape. Photo by Atharva Tulsi on Unsplash.com.

In animals, shorter photoperiods affect reproductive behavior, increase fur or wool production, and may initiate a molt to change summer fur or feather color to a color that blends better with the winter landscape. The combination of lower daylight and lower temperatures signals some mammals and birds to migrate, while others begin to conserve energy in preparation for hibernation, or a lighter sleep called torpor in mammals and diapause in insects.

Mammals detect changes in photoperiod through the optic nerve. Over the past six weeks since the summer solstice on June 21, when we had a high of 14 hours and 26 minutes of daylight, we’ve lost nearly an hour of sunshine. While we may not consciously notice that days are getting shorter, our brains – and the brains of other mammals – register the difference and respond with hormones that cause physiological changes.

a side view of a woodchuck in the wild.
Waning daylight increases the risk of roadway collisions with woodchucks and other wildlife that forage in the twilight hours of dawn and dusk. Photo by Andy Holmes on Unsplash.com

One of these hormones, melatonin, helps regulate waking and sleeping cycles. In the evening, as daylight wanes, a tiny gland in the brain releases melatonin to help make us sleepy. In the morning, as our optic nerve senses light from the rising sun, the brain stops delivering melatonin. Longer dark periods mean our brains release melatonin earlier in the day and later into the next morning. With less stimulation from sunlight, we can feel less alert, lethargic, moody, depressed, and/or hungrier than usual. For most people, these effects are sporadic and mild, but for a few, the collection of symptoms can cause a more persistent condition called Seasonal Affective Disorder (SAD).

While our daily time schedules don’t typically change with the seasons, those of crepuscular animals adjust as dawn occurs later and dusk arrives earlier. We end up commuting in the dark, in low visibility, when we’re feeling sleepy, at the same time that deer, rabbits, armadillos, and other twilight foragers are out looking for food. This increases our chances of roadway collisions with wildlife. Reducing driving speed and increasing watchfulness can reduce this risk.

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