6 Things You Need to Know About Long-Day Lighting in Horticulture

Lighting plays an important role in regulating the flowering process in plants, offering an effective means to enhance profits. You might be familiar with long-day plants, which flower when the days are longer, typically during spring or early summer before the equinox. These plants require more than the critical duration of light (usually 14-16 hours) to initiate flowering. Long-day lighting refers to the lighting strategies that extend photoperiods or day length to regulate the flowering of plants. Long-day lighting is effective in regulating flowering.

How does it work? Does it work for my crops? How do I implement it in my greenhouse and indoor farm? What are the differences between long-day lighting and supplemental lighting? In this blog, we’ll explore six aspects of long-day lighting. Let’s find the answers.

1. What is long-day lighting?

In simple terms, long-day lighting refers to the delivery of lighting to create "long days" for plants, which is crucial for managing the flowering of plants. When natural daylight is insufficient due to shorter days, growers will use artificial lighting to prolong the photoperiods or day length to create long days. This lighting can promote the flowering of long-day plants and prevent or delay the flowering of short-day plants. Generally, day-extension or night-interruption lighting (also called cyclical lighting) is used to create long days. However, the definition of a long day is not indeterminate. The threshold for what constitutes a long day versus a short day varies depending on the critical photoperiod of each plant species. Normally, days longer than 12 hours are considered long, whereas those shorter than 12 hours are deemed short.

2. Effects of long-day lighting on plants

For long-day plants, extended light exposure can lead to increased growth and photosynthesis. By mimicking a longer summer day, long-day lighting provides these plants more time to produce energy through photosynthesis, potentially leading to higher yields. Day-neutral plants are not affected by day length. Long-day lighting won’t necessarily harm them, but it also won’t provide any particular benefit. However, long-day lighting can stress short-day plants, especially if the light intensity is too high or the dark period is too short. This stress can manifest as stunted growth, wilting, or leaf drop.

3. What crops respond to long-day lighting?

Since day-neutral plants are not affected by day length, they will not respond to long-day lighting. However, there is still a long list of crops that respond to long-day lighting. Here are some crops that respond positively to long-day lighting:

• Leafy greens: lettuce, spinach, kale, arugula, Swiss chard
• Herbs: dill, cilantro, parsley, chives
• Flowers: asters, lilies, snapdragons, phlox
• Vegetables: carrots, onions, beets, cabbage, garlic, wheat

Short-day plants will extend the vegetative growth phase and delay flowering when they are treated with long-day lighting. These plants include cranberries, chrysanthemums, poinsettias, Christmas cactus, sweet potatoes, etc.

It is important to note that the response to long-day lighting can vary not just between species, but also between different varieties or cultivars within a species. For example, June-bearing strawberries are considered short-day plants, while Everbearing strawberries are known as long-day plants.

4. When to deliver lighting to create long days

When to create long days in your growing environment depends on your goal for plant development. Are you aiming to promote flowering in certain crops, or perhaps delay it in others?

Most long-day plants are triggered to flower when they receive at least 13 hours of light per day. Conversely, short-day plants initiate flowering when the daily light period is less than 12 hours. The timing of creating long days should be based on the natural daylight hours in your location.

To encourage flowering in long-day plants, you can use artificial lighting to extend the day length when natural daylight falls short of 13 hours. For short-day plants, if your goal is to prevent or delay flowering, you can prolong the day length when the natural light period is less than 12 hours.

Generally, implementing long-day lighting is most beneficial in early spring. You can stop this practice in late March or early April, depending on your location and the specific needs of your crops. Once the desired total photoperiod is achieved, you can end the long-day lighting.

5. How to deliver long-day lighting

There are two ways to create long-day conditions: day-extension or night-interruption lighting.

Day-extension means using artificial lighting to extend the day length or photoperiod. This can be achieved by providing light either before sunrise or after sunset. This requires the lighting to be long enough to promote or inhibit flowering. For most plants, a 16-hour photoperiod is considered a long day, so this is used as the target day length. In practice, this often means operating lighting for about 4 hours during the night.

Night-interruption, on the other hand, is the use of lighting in the middle of the night to interrupt the dark period. A common approach is to apply long-day lighting from 10 pm to 2 am. While one or two hours of lighting can be sufficient to stimulate a long-day response in some plants, a 4-hour night-interruption lighting period ensures a long-day response in nearly all crops.

In addition to photoperiod manipulation, light intensity, and quality are crucial factors in creating effective long-day conditions. Blue, far-red, and red lights are effective in encouraging flowering in long-day plants. The minimum required light intensity is generally accepted to be 2 μmol/m²/s. Light levels below this threshold may be ineffective for plants to perceive and respond to the lighting. However, it's important to note that more intense light does not necessarily yield better results.

6. Difference between long-day lighting and supplemental lighting

Since both long-day lighting and supplemental lighting are used to supplement lighting for plants. You might be confused about them. Let’s clarify the differences to assist you in selecting the proper lighting strategy.

Purpose

Long-day lighting: Manipulates the photoperiod to regulate flowering.

Supplemental lighting: Increase the overall amount of light plants receive to reach an optimal DLI for photosynthesis.

Focus

Long-day lighting: Aim to extend the duration of light exposure.

Supplemental lighting: Increase the intensity of light to boost photosynthesis.

Light duration

Long-day lighting: Extending the total light period to around 14-16 hours per day.

Supplemental lighting: The grow lights turn on during the day, potentially extending the natural light period.

Light intensity

Long-day lighting: The minimum required light intensity is about 2 μmol/m²/s

Supplemental lighting: Bright enough to achieve the target DLI needed for optimal photosynthesis.

Application

Long-day lighting: Often used for photoperiodic crops or to prevent premature flowering.

Supplemental lighting: Used for most crops, especially in low-light environments or seasons.

Here is a table about the differences between long-day lighting and supplemental lighting.