There are many factors that affect when and how well plants grow including temperature, water, nutrients, plant spacing, and even the presence of pests or diseases.
However, many gardeners often overlook the importance of day length and what impact it has on plant development. To be a better gardener, it helps to understand day length and how it affects flowering and growth of plants.
So, how does day length affect plant growth? Day length tells plants what season it is, and thus when to produce flowers. When a plant produces flowers will depend on whether it is a short day, long day, or day-neutral plant. Short day plants need less than 12 hours of daily sunlight to flower, and long day plants need more than 12 hours of daily sunlight to flower. The flowering of day-neutral plants does not depend on the amount of daily sunlight.
Of course, changes in day length will depend on the seasons and on what part of the world you live in (latitude). Let’s take a closer look at how day length affects plant growth, along with some plants of each type (short day, long day, and day-neutral).
How Does Day Length Affect Plant Growth?
Changes in day length (the number of hours of sunlight in a day) give plants information about the seasons. This helps them to figure out when it is time to start growing, flowering, and going to seed (trying to reproduce).
Photoperiodism describes what a plant does in response to changing day lengths. As days get longer, the nights get shorter, and plants respond to the length of darkness accordingly, depending on whether they are short-day, long-day, or day-neutral (more on this later).
In fact, plants need uninterrupted darkness in order to grow properly. For more information, check out this article on photoperiodism from Wikipedia.
Note: do not confuse photoperiodism (defined above) with phototropism, which describes when a plant grows and moves in response to light. Usually, the shoots above ground bend towards light, while the roots below ground bend away from light.
How Is Day Length Related to Seasons?
As the seasons change, day length fluctuates between shorter and longer days (and nights). To give an example, we’ll look in detail at how day length relates to seasons in Boston, Massachusetts, U.S. (located in the northern hemisphere).
In Boston, Massachusetts (U.S.), the longest day of the year is around June 21 – this is called the summer solstice. The day length on June 21, the summer solstice, is 15 hours, 17 minutes, and 2 seconds long.
The shortest day of the year is around December 21 – this is called the winter solstice. The day length on December 21, the winter solstice, is 9 hours, 4 minutes, and 37 seconds.
This means that there is a difference of 6 hours, 12 minutes, and 25 seconds between the longest and shortest days of the year in Boston.
Each day between December 21 and June 21 gains some daylight. The fastest rate of increase is on the spring (vernal) equinox, which is around March 21. On this day, the lengths of day and night are equal – that is, 12 hours of light and 12 hours of darkness.
On the other hand, each day between June 21 and December 21 loses some daylight. The fastest rate of decrease is on the fall (autumnal) equinox, which is around September 21. On this day, the lengths of day and night are equal – that is, 12 hours of light and 12 hours of dark.
If you look at a graph of day length plotted against time, you would see that the slope of the graph is steepest at the spring and fall equinoxes. (The slowest rates of change in day length are on the summer and winter solstices, June 21 and December 21 – at these points, the slope of the graph is flat).
For more information on daylight and day length, check out this article on daytime from Wikipedia.
Obviously, day length affects how much light is available to plants for photosynthesis, which defines how much energy they can produce. However, day length can serve as an indicator of the season for plants.
Day length helps plants to regulate their internal clock. Day length informs plants about the seasons and tells them when it is time to start growing.
Day length also tells plants when it is time to go to seed. In that case, they are trying to reproduce before freezing to death or dying of heat!
Understanding day length is also important because many plants are started or grown entirely indoors with artificial light. Indoor growers can use this information to their advantage to change the behavior of plants in a greenhouse.
Using automated lighting systems, the intensity and duration of light can be changed depending on the plant type and desired behavior (for example, flowering or not). You can also use day length to help you figure out what to plant, and when, in your area.
How Much Light Do Plants Need?
Most plants need at least 10 hours of light per day to grow. Otherwise, they go into a state of dormancy and stop growing until they start getting enough light per day to continue growing.
There are three categories of plants, depending on how they respond to day length: short day, long day, and day neutral plants. Let’s take a closer look at each one, along with some examples. We’ll start with short day plants.
What Are Short Day Plants?
Short day plants are those that need less than 12 hours of sunlight in a day to flower. To say it another way, short day plants need more than 12 hours of uninterrupted darkness in a day to flower. (The second statement is more appropriate, since plants respond to length of darkness rather than length of daylight).
Short day plants flower in the summer or fall in the northern hemisphere, since the days are getting shorter at this time (between the summer solstice and fall equinox, or June 21 to September 21 around Boston).
When growing short-day plants indoors, you can encourage them to flower by providing less than 12 hours of artificial light, and giving them over 12 hours of uninterrupted darkness.
On the other hand, you can discourage flowering by providing less than 12 hours of uninterrupted darkness per day.
Some examples of short day plants are:
- sweet potatoes
- some onions
To see a longer list of short day plants, check out this article from Washington State University on day length.
What Are Long Day Plants?
Long day plants are those that need more than 12 hours of sunlight in a day to flower. To say it another way, long day plants need less than 12 hours of uninterrupted darkness in a day to flower. (The second statement is more appropriate, since plants respond to length of darkness rather than length of daylight).
Long day plants flower in the late spring or early summer in the northern hemisphere, since the days are getting longer at this time (between the spring equinox and summer solstice, or March 21 to June 21 around Boston).
When growing long-day plants indoors, you can encourage them to flower by providing more than 12 hours of artificial light, and giving them less than 12 hours of uninterrupted darkness.
On the other hand, you can discourage flowering by providing more than 12 hours of uninterrupted darkness per day.
Some examples of long day plants are:
- some onions
To see a longer list of long day plants, check out this article from Washington State University on day length.
What are Day-Neutral Plants?
Day-neutral plants are those that do not flower in response to changes in day length. Instead, they flower in one of two cases:
- when they reach a certain stage of maturity (size, age, etc.)
- in response to other environmental factors, such as vernalization (a period of low temperature)
Some examples of day-neutral plants are:
- Brussels sprouts
- some onions
To see a longer list of day-neutral plants, check out this article from Washington State University on day length.
By now, you have a much better idea of how day length affects plant growth. You also know about the three different types of plants (short day, long day, and day neutral), along with the light and darkness needs of each one.
I hope that you found this article helpful – if so, please share it with someone who can use the information. If you have any questions about day length and plant growth, please leave a comment below.