Welcome The "Shortest" Day, The Winter Solstice.

Today is the ”shortest” day of the year (i.e. the day when the time between sunrise and sunset is shortest). It’s the winter solstice, December 21st.

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This date may surprise you as many people think the solstice is on December 23rd. The truth is it varies from year to year. In 2022 it is on the 21st of December, as it was in 2021. In 2023 it is on the 22nd of December then for the next three years it will be again on the 21st of December. The changing dates is not down to bureaucracy, it’s down to the movements of the moon and planets in the solar system.

And in reality, the winter solstice isn’t a day, but actual time when the tilt of the planet Earth reaches its maximum. That maximum occurs for a moment, not for a day, but it is impossible to see visually at that time. It’s only when the tilt of the Earth moves back that we see that it had reached its maximum. In 2022 the moment of equinox is actually 21.49 this evening.

But whichever day the solstice is it brings with it lengthening daylight hours. And as the days lengthen the days will theoretically get warmer, though we will of course still experience individual days that can be colder than before the equinox.

Winter Equinox or Winter Solstice?

I’ve seen lots of posts about the equinox but it’s actually a misnomer. Equinox means equal length whilst as we’ve seen today is the shortest day so its actually a solstice.

The Post Winter Solstice Impact On Plants.

Plants are affected by day length. It can initiate flowering and various other processes. More importantly, as the days lengthen, the plants have the opportunity of getting more daylight, simply because the sun is above the horizon for longer each day. More sunlight means more photosynthesis. Photosynthesis means more growth.

Therefore, the longer the day is the more growth potential there is for our plants.

But it isn’t quite as simple as that.

Plants vary in their need for light. Some are sun-loving, and some are shade- loving. Both sun and shade loving plants need just the right amount of light for them to be healthy. I often hear gardeners say that specific plants do well in particular parts of the garden and they often put this down to the soil. The reality is, it may well be down to the amount of light particular parts of the garden get. Give a specific plant, too much or too little of light, and it will not prosper as well as it would in what for it are perfect conditions.

Couple this with the fact that the amount of light in a specific area the garden gets can vary tremendously throughout the year. Both the duration of light changes from here from the season to season with less light in winter. Plus the strength of the sun will change as the sun gets higher in the sky, irrespective of the number of hours it shines.

Photosynthesis involves both the number of hours of light and the strength of that light during those hours. Sometimes a few hours of intense light is better for the plant than many hours of dull light on a grey day.

In winter, the amount of daylight doesn’t affect those plants that are dormant and even evergreen plants are not as responsive to the amount and quality of light as they are later in the year.

Some plants however, are actively growing this month. For example, my daffodils are now several inches high and the amount of light they get will affect their growth. In recent weeks. We have experienced grey days and bright sunny days with blue skies. Those sunny days have provided good levels of light plus a little warmth to the garden and this is good for those plants that are actively growing.

Photoperiodic Plants

Photoperiodic plants are strongly affected by the longer nights of autumn and winter. These are the plants with the ability to measure the number of hours of darkness that occur in a 24-hour period. And they respond to it.

Short day plants are those that respond to the long periods of darkness in winter. The shorter days and colder temperatures are what prompts deciduous trees to drop the leaves in autumn. And it is the lengthening days after the winter equinox prompts growth.

Commercial growers manipulate the light to trigger responses in certain plants. By applying the correct lighting the regime plants can be prompted to flower out of season. And those plants that prefer short days can be prompted to flower or change colour on the days to suit the human calendar.

Poinsettias are a plant sold at Christmas for the beauty of its bright red bracts (orange, yellow and white are other options). It naturally produces colour in winter, but commercial growers manipulate their lighting to ensure that plants are ready on the correct day to enable you to buy them for Christmas

Poinsettia - https://commons.wikimedia.org/wiki/File:Euphorbia_pulcherrima_0111.jpg — Poinsettia
Credit https://commons.wikimedia.org/wiki/File:Euphorbia_pulcherrima_0111.jpg

The coloured bracts need short daylight hours and are grown where they get at least 14 hours of darkness each day.

Sometimes manipulating light means adding a black out to prevent natural light and sometimes it means adding light via artificial lighting.

All year around (AYR) chrysanthemums are another crop where light is manipulated to get the crop flowering when needed.

Light striking the planet from the sun when low in the sky creates another issue. It gives longer shadows. Areas of land that get good sunlight in midsummer may get much reduced light in winter because of shadows. In my own garden I have a bed at the side of the house that gets really good light in midsummer but sees no sunlight in winter. It is perpetually shaded by the house. I have to plant accordingly. That’s not a problem because I know it happens every year.

Poor Light Can Be Good Light

However it’s not all bad. The slanting light, coming in from a low angle, can sometimes reach places it doesn’t in summer. My house is an example. I have an extension that is very much an indoor plant growing room. In summer, when the sun is high in the sky, it gets little direct sunlight, though light levels are still good. In winter, when the sun is low in the sky, the room is flooded in sunlight whenever we get a sunny day. The solar gain boosts the temperature and it’s both sunny and very warm. But as soon as the clouds appear the temperatures significantly drop. My plants are chosen to grow in these conditions and relish them. As I write I have Strelitzia reginae, The Bird Of Paradise, in flower. The plant loves these conditions.

It’s not just muy house that sees light getting in in winter. The floor beneath deciduous trees are also potential spots the sun can reach now the leaves have gone. Spring flowers in woodlands all benefit from winter sunshine to bring them on.

So enjoy the solstice, whichever date or time of day it arrives. It promises longer warmer days are coming and is actually needed by some plants.