While plants need sunlight for performing critical processes like photosynthesis, they also need a period of darkness to rejuvenate themselves and perform other essential respiratory functions. The amount of darkness required by plants varies depending on the species, growth stage, and other factors.
Short-day plants such as chrysanthemums need darkness between 14 and 16 hours to bloom, while long-day plants such as poppies need only 8-10 hours of dark. Day-neutral plants like sunflowers can adapt and rely on environmental clues other than darkness to thrive.
During the nighttime, plants regenerate phytochrome, an essential compound in regulating plant growth and development. In this comprehensive guide, I’ll explore other topics related to plants and darkness, including why plants need darkness. I will also cover a few tips on providing artificial night for your plants—let’s dive in!
Factors That Affect How Much Darkness Plants Need
While plants need a period of darkness to aid in respiration and regeneration of phytochrome, several factors affect how many hours of darkness they need, including:
The Type of Plant (Species)
Different plant species require different amounts of darkness. Flower plants, such as tomatoes, petunias, and impatiens, need 8-10 hours of dark to bloom. The absence of light helps these plants produce the hormones they need for growth and flowering.
There are three kinds of plants, each with specific light and darkness requirements.
Short Day Plants
Short-day plants respond by utilizing photoreceptors to the presence of more extended darkness—or nighttime conditions. These plants will not bloom or develop unless the dark periods exceed the hours of light, roughly 14-16 hours a day.
They use photoreceptors to measure phytochrome levels and can either trigger or inhibit flowering according to day/night length.
Examples of these plants include:
Long day Plants
Long-day plant species demand a shorter length of darkness to thrive. On average, they require only 8-10 hours of darkness daily. Plants in this category include:
- Sugar Beet
Day Neutral plants
These plant types will flower regardless of the night/day division—keeping in mind that no plant that relies on photosynthesis can live without a light source. Day-neutral plants can live and bloom in daylight between 5-24 hours.
Plants in the day-neutral category include:
- Some species of strawberry
The Season Affects Plant Darkness Requirements
The season also affects how much darkness plants need. During the winter, when days are shorter, plants experience more extended periods of darkness, affecting their photosynthetic rate.
Some plants enter a state of dormancy in winter to conserve energy. To help plants grow during the winter, gardeners often use artificial lights to extend the length of daylight plants receive.
Why Plants Need Darkness
For many years, people believed that it was the exposure to light that causes plants to develop and bloom—but this is not the whole story. The absence of light plays just as vital a role as the presence of light.
At night, plants focus on performing respiratory and metabolic functions for their growth and development. Here are key reasons why plants need a period of darkness:
Plant Respiration Occurs at Night
During the day, plants perform photosynthesis, using sunlight to convert carbon dioxide into food. Chlorophyll, the green pigment in plants, is essential for this process. The plants absorb sunlight through their leaves and use it to split water molecules and produce oxygen gas.
But at night, photosynthesis stops, and the process reverses. Plants take in oxygen and release carbon dioxide through a process called respiration. Respiration is essential for plants to create the energy they need to fuel their growth.
During respiration, plants break down the glucose they create during photosynthesis to release energy. Plants have adapted to take advantage of cooler night temperatures to respirate at peak efficiency—that’s why you often see plants “sweating” at night or releasing water vapor.
Plants also use respiration to produce the compounds they need for growth, including:
- Amino acids
Regeneration of Phytochromes Occurs at Night
Phytochromes are critical plant photoreceptors that help them regulate their growth and development in response to light. There are two forms of phytochrome:
- Pr (red) with an absorption peak of 670 nanometers ( nm)
- Pfr (far-red) with an absorption peak of 740nm
These two interchangeable photoreceptors play an essential role in regulating metabolic processes and flowering of plants—each with unique requirements. Sunlight has more Pr, which collects the red light during the day, and converts it to Pfr. Pfr is the active phytochrome that regulates whether specific metabolic processes will be activated or inhibited in a plant.
In the nighttime, the phytochrome is unstable and reverts to Pr form. The plant measures the length of day and night through the balance of these phytochromes.
Scientists call this the photoperiodic stimulus and this nuanced reception travels via the leaves, releasing a hormone called florigen that travels to the buds to stimulate flowering.
Darkness Is Essential for Plant Metabolism
In addition to respiration, plants also perform various metabolic processes at night. These processes are essential for plant growth and development and include:
- Synthesis of hormones
- Growth of root systems
- Production of enzymes
Seed germination, for example, is triggered by the production of enzymes. These enzymes are produced in the dark and help break down carbohydrates to provide energy for the seed to sprout.
Plants also produce hormones at night that regulate their growth. One of the most important hormones is auxin, which helps plants grow taller. Auxin is made in the dark and transported to the plant’s tips, where it promotes cell elongation.
Additionally, plants use the dark to grow their root systems. While the tops of plants may appear to grow taller at night, the roots are growing longer and thicker. This growth allows plants to anchor themselves in the soil and absorb more water and nutrients.
Effects of Too Little or Too Much Darkness
While darkness is critical for plant growth, too much or too little can have adverse effects. Plants use darkness to reset their daily growth cycle and perform essential processes.
Too low darkness can disrupt the production of hormones and enzymes plants need for growth. It can also inhibit the regeneration of phytochrome, causing plants to experience stunted growth.
Too much darkness can also disrupt plant growth. Prolonged periods of darkness can cause plants to become spindly and weak. This reaction is because they try to grow towards the light and become etiolated. Additionally, since there is less light for photosynthesis— plants may not be able to produce the food they need to survive. Ultimately, the plants will wilt, turn yellow, and die.
Too much darkness can also cause plants to produce small, pale, and deformed flowers. This process happens because darkness interrupts the process of photosynthesis, which is essential for producing chlorophyll. If you have houseplants, you must move them to a brighter location to ensure optimal growth.
If you find your plants show stunted development, you can learn what to do in my guide on dealing with undergrown plants: How to Fix Stunted Growth in Vegetable Plants
How To Provide Artificial Darkness for Plants
Although you may not have much control over plants in your garden, you can provide artificial darkness for plants indoors or in a raised garden bed. This method is necessary if you are growing plants that require extended periods of darkness or live in an area with long summer days.
You can use several techniques to provide artificial darkness for plants, including the following.
Covering Plants With a Black Tarp or Cloth
One of the simplest ways to provide artificial darkness for plants is to cover them with a black tarp or cloth. As the name suggests, this method involves covering plants with a black tarp or cloth to exclude light.
A black tarp is a heavy-duty, waterproof tarp often used to cover gardening equipment. A black cloth is a lighter-weight fabric that you can use to protect smaller plants or seedlings. This method is suitable for growing plants in a raised garden bed or large pot.
To use this method:
- Choose a black tarp or cloth that is large enough to cover your plants.
- Place the tarp or cloth over your plants and cover them completely.
- Secure the tarp or fabric with bricks, rocks, or heavy objects.
- Leave the tarp or cloth in place during the dark period.
Building a Dark Box or Cabinet To Provide Darkness to Plants
Alternatively, you can build a dark box or cabinet to provide artificial darkness for plants. This option is a relatively simple DIY process that requires a few supplies, including:
- A cardboard box or cabinet
- Black paint
- Utility knife
- Tape measure
To use this method:
- Determine the size of your plant so you can design a box or cabinet that will accommodate it. Use a tape measure to measure the length and width of your plant.
- Cut the cardboard to size using the utility knife. Join the pieces of cardboard together with screws to make a box or cabinet.
- Paint the inside and outside of the box or cabinet black. This color will help to absorb and reflect light, creating a dark environment for your plant.
- Place the plant inside the box or cabinet.
- Close the lid or door, making sure you securely fasten the door.
- Leave the plant in the box or cabinet during the darkness period.
If you found this guide helpful, I recommend my complete guide on growing crops in a greenhouse. You’ll learn everything you need to know about growing healthy vegetables and plants in a greenhouse: How to Grow Crops in a Greenhouse: The Ultimate Guide
Plants require a period of darkness to grow and thrive. The amount of darkness they need depends on whether your plant is a short-day, long-day, or day-neutral species. Too much—or too little darkness—can cause plants to become spindly, produce small and pale flowers, and eventually die.
If you are growing plants indoors or in a raised garden bed, you can provide artificial darkness by covering them with a black tarp or cloth or by building a dark box or cabinet. These methods are relatively simple and will help your plants to grow and thrive.