Understanding the Role of Photoperiod in Plant Growth

From the first spring crocuses peeking through the snow to the autumnal cascade of color in the trees overhead, the seasonal rhythms of the natural world showcase the remarkable influence photoperiodism has on plant life cycles. The relative lengths of light and dark in a 24-hour cycle, also known as the photoperiod, act as a potent signal enabling plants to synchronize their growth and development with seasonal changes. 

However, not all plants have the same light requirements. Depending on their species, cultivar, and stage of development, different plants need different photoperiods of light to thrive. Some plants are short-day plants, which means they flower when the day length is below a certain threshold. Some plants are long-day plants, which means they flower when the day length is above a certain threshold. Some plants are day-neutral, which means they flower regardless of the length of the day.

In this blog post, we will help you understand how photoperiod affects plant growth, and how you can use horticulture lighting products to manipulate these factors for optimal results. Let's dive in!

What is photoperiod?

Photoperiod is the relative length of the light and dark periods in a day. It is measured as the amount of interrupted light time in a given day. For example, a plant can have a long duration of light of 18 hours, but a short photoperiod of 8 hours, if the light is interrupted by a dark period of 10 hours.

Photoperiod influences plant growth by affecting flowering initiation, vegetative growth, dormancy induction, and other developmental processes. Different plants have different photoperiodic responses: short-day, long-day, or day-neutral. These terms describe how plants respond to changes in the photoperiod for flowering. Long-day plants require longer photoperiods, typically over 12 hours, while short-day plants initiate flowering when the photoperiod falls below 12 hours. Day-neutral plants essentially ignore photoperiod cues.

Effects of photoperiod on plants

The effects of photoperiod on plant growth and development are profound and play a crucial role in shaping various physiological processes.

One of the primary effects of the photoperiod is flowering. Many plants exhibit photoperiodic responses, meaning their flowering is influenced by the length of light and darkness they experience. Short-day plants require longer periods of darkness to initiate flowering, while long-day plants require longer periods of light exposure. By manipulating the photoperiod, growers can control the timing of flowering, enabling them to synchronize bloom cycles or extend flowering periods as desired.

Vegetative growth is also influenced by the photoperiod. The length of light exposure affects the rate of photosynthesis, which in turn impacts plant growth. Longer durations of light provide more energy for photosynthesis, leading to increased growth. By adjusting the duration and photoperiod, growers can regulate the vegetative growth of plants, promoting lush foliage or controlling the size and shape of plants.

Circadian rhythm, the internal biological clock in plants, is also affected by photoperiod. Plants have an innate ability to perceive and respond to changes in light and darkness. These responses are regulated by the circadian rhythm and influence various physiological events, including growth, metabolism, and defense mechanisms.

By providing the appropriate photoperiod, horticulturists can promote healthy plant growth, regulate flowering, control vegetative development, enhance seed germination, and synchronize plant responses with their environment.

How do we optimize photoperiods for crops?

When looking to optimize photoperiods for specific crops, it is important to understand whether the plant utilizes short days, long days, or day-neutral flowering responses. Photoperiod optimization allows growers to maximize yields, schedule harvests, and ensure consistent plant growth.

Flowering ornamentals like poinsettias, chrysanthemums, and kalanchoes are short-day plants that initiate blooming and coloration when exposed to photoperiods under 12 hours. Providing poinsettias with 12 hours or less of uninterrupted darkness beginning in October will ensure the flowers develop fully in time for the winter holidays.

Long-day plants like spinach, beets, and most cucurbits require photoperiods exceeding 14 hours to avoid premature flowering and bolting, which reduces yields. Supplementing natural day length with LED grow lights allows growers to maintain the vegetative growth of long-day plants year-round.

Photoperiod adjustment also optimizes yields in fruiting vegetables like tomatoes, peppers, and eggplant. Providing a 14-16 hour photoperiod during flowering and fruit set boosts the number and size of marketable fruits on these long-day plants. Day-neutral plants like tomatoes, cucumbers, and strawberries largely ignore photoperiod cues, allowing them greater flexibility in production scheduling.

So, adapting light and dark periods to a crop's photoperiodic needs allows for improved plant health, maximized yields, and greater control over plant development timelines.

Innovative photoperiodic horticulture lighting

Photoperiodic horticulture lighting has revolutionized the way we control and optimize light exposure for plant growth. This advancement has provided growers with greater flexibility and precision in manipulating duration and photoperiod, leading to improved crop performance and increased yields.

In recent years, there have been many innovations in photoperiodic horticultural lighting, especially with the development and diffusion of LEDs. One of the main innovations of LEDs is the possibility to tailor the light spectrum to suit the specific requirements of different crops and developmental stages. By using the customized spectrum, indoor plant growers can create light recipes that can enhance various aspects of plant growth and development, such as:

• Photosynthesis and biomass production
• Flowering initiation and duration
• Plant morphology and architecture
• Secondary metabolite accumulation and quality traits
• Stress tolerance and resistance

While white LEDs have a fixed light recipe and the light intensity of each color is not changeable, variable spectrum LEDs allow growers to adjust the intensity of each color spectrum. Take one of our variable spectrum lighting solutions - a four-channel adjustable spectrum LED grow light as an example. This variable spectrum LED grow light has 4 spectrum channels: white light, deep red, UVA, and far red. Growers can adjust the grow light to emit white light, deep red light, UVA, or far-red light alone. They can also control the grow light to emit all colors together.

Another innovation of LEDs is the possibility to regulate the light intensity and duration according to the plants’ needs. By using dimmable LED grow lights, indoor growers can create dynamic lighting regimes that can mimic natural light conditions or create artificial photoperiods. For example, indoor plant growers can use:

• Low-intensity night-break lighting to prevent flowering in short-day plants or induce flowering in long-day plants.
• High-intensity supplemental lighting to increase photosynthesis and growth in low-light conditions.
• Variable-intensity cyclic lighting to modulate plant metabolism and water use efficiency.

Photoperiod is a key environmental factor that influences plant growth, development, and flowering. Whether it is extending or shortening the day length, manipulating the light spectrum, or creating artificial night breaks, horticultural lighting can provide effective solutions for controlling photoperiodic responses. At Atop Lighting, we offer a range of high-quality LED grow lights that are designed to meet your specific needs. If you are interested in learning more about how our horticultural lighting can help you improve your plant performance and productivity, please contact us.