Once you start growing vegetables & other plants, you might wonder how to make them grow better--and what effect light and different colors of light has on plant growth.
The color of light does indeed affect plant growth, but the effect is more noticeable under low light intensity. Red and blue light are most effective for plant growth, while green has minimal effect. UV light can damage plants, causing leaves to burn. Greenhouse growers use supplemental light for optimal plant growth.
Understanding visible light and how it relates to plant growth
The light we can see--visible light--is only a tiny fraction of the entire spectrum of electromagnetic energy. Electromagnetic energy--light--can come in a wide variety of forms, including visible light, gamma rays, infrared light, radio wave, microwaves, x-rays, etc. While some forms and colors of light are beneficial and even required for plant growth, other forms of light can harm plants.
Here's a graphic showing how visible light is just a tiny slice of the entire electromagnetic spectrum:
So, why do plants need light?
Simple: Plants use light to manufacture their own food through photosynthesis; chlorophyll--the green pigment in plant leaves & stems-- converts carbon dioxide into food using the energy from visible light.
Because visible light is used for plant growth, can we adjust or enhance how much light we provide so that our plants grow faster, or healthier?
The short answer is yes.
Quick science lesson
So we've discussed how each color affects plants but I think it's also important to understand the science behind why the colors affect plants the way they do. Photosynthesis, chlorophyll and wavelengths are all factors to consider for how different color lights affect plant growth.
Photosynthesis is the process that plants go through to change light, water and carbon dioxide into glucose and oxygen. Glucose is a type of sugar and is used as a food for the plants. The byproduct of photosynthesis is oxygen.
Photosynthesis cannot take place without chlorophyll. Chlorophyll is what makes plants green and allows plants absorb light energy and turn it into food. Since chlorophyll is green, colors on the opposite side of the color spectrum are most efficient for plant growth including red and blue.
Light is a type of energy and it varies based on the wavelength of the color. Ultraviolet has the shortest wavelength so it has a higher energy, this is followed by violet, blue, green, yellow, and lastly red which has the longest wavelength and is least energetic. This is important when considering which light to use for plant growth. Since UV light is so energetic, it's actually harmful to plants. And since red light is less energetic, it is less effective when used by itself.
All this is important to understand how light affect plants. Without the proper energy from light, plants would not be able to go through photosynthesis and produce food to sustain the plants.
What the heck is DLI, and why should you care?
Commercial greenhouse growers and horticulturalists often look at DLI, since it gives them a benchmark to know who healthy their plants are. So what exactly is DLI? DLI--the daily light integral--is essentially a measure of how much light a plant gets that it can use for photosynthesis.
Not all light is used by plants for photosynthesis. Some colors and wavelengths are used differently by plants, and some wavelengths can't really be used at all. DLI tells how much light is available to the plant for photosynthesis--which is light in the 400-700 nm range--in other words, visible light.
The more DLI, the more your plants can photosynthesize, and the more they can grow.
Greenhouses, because of the glazing, allow less light for plants. Researchers from Purdue University and Michigan State University found that supplemental lighting--in other words, grow lights--were the only way to increase DLI in a greenhouse to be able to produce high-quality plants (original article here).
So, to ensure your plants grow optimally, you'll want to keep an eye on DLI, and how much daily light each type of plant that you're growing requires.
Out of the colors in the visible spectrum, red and blue are the most important colors for optimal plant health and maximizing plant growth. Ultraviolet light is actually harmful for plants, and green and yellow light have minimal effects on plants. Details for each light color and its affect on plant health and growth are listed below.
Ultraviolet light, also known as UV light, is commonly known as being harmful for humans. If a human is exposed to too much UV light they will get sunburned and can have other negative effects. Just like prolonged exposure to UV light is harmful to humans, high-intensity or long exposure to UV light is harmful to plant growth. The less exposure to UV light, the better your plants will grow.
Violet light is beneficial to plants. Violet light has a higher amount of energy so when plants are exposed to violet light the plant color, fruit and vegetable taste is enhanced, the plants have a better aroma, and increases the amount of antioxidants that the plant produces. So overall, violet light helps plant growth, and can be used to protect the plants from damage.
Plants benefit from blue light primarily during the early stages of growing. Blue light helps with the plant's production of chlorophyll--the green pigment that traps light energy and is integral to photosynthesis. In other words, blue light is easier for a plant to absorb and use the energy in photosynthesis. So, blue light increases plant growth and makes the plant reach maturity faster. This is why blue light is especially important for seedlings and young plants. Exposure to blue light can help the seeds germinate, and ensure robust growth for roots, stems, and leaves.
Plants absorb many types of light but reflect green light. This is why plants typically have green leaves. Mostly green light is used to enhance the production of chlorophyll making the plants greener. While green light isn't as integral as red or violet light to plant growth & health, green light can make plants appear greener, and therefore is useful in commercial greenhouses where customers are viewing & selecting plants to purchase.
Yellow light is similar in its effects on plant growth & health as green light. The major difference yellow light has as opposed to green light is that, while yellow light doesn't directly help produce chlorophyll, some studies have shown that removing yellow light from greenhouses may produce healthier plants than growing the same plants outside, where they're exposed to full-spectrum sunlight.
Red light carries less energy than the other colors and is not very effective on its own, but when combined with blue light it is very helpful for mature plants. Red light helps flowers bloom. When combining red and blue light, the amount of leaves that a plant grows is greatly increased.
Specifically, a study by Michigan State University floriculture extension team found that if plants are grown only using red light, they'll have a stretched, enlongated appearance, and their leaves will become thin and large. Adding 10-20% blue light allowed plants to grow much healthier, with a compact appearance.
There is also far-red light which is has wavelengths that are lower than normal red light--similar to near-infrared wavelengths. Far-red light helps the plants produce greater yields. The far-red light does this by speeding up the process a plant goes through in the night. During the night is when the flowers ripen. By speeding up the nighttime process, red light helps produce greater yields for your plants.
How color corresponds with the plant lifespan
When plants are just seeds and are first sprouting, blue light should be used. Blue light is very important to the beginning of plant growth because it increases the amount of chlorophyll that is formed. Some seeds won't sprout without blue light. Red light and blue light are important when the plants are mature and beginning to flower. This helps them produce more leaves and more crops. Violet light can is used at any point in the plant's life to enhance the color and taste of a plant.
Choosing the right light
Although it may be tempting to choose a light that is concentrated on color, plants need to have a bit of each color. Providing your plants with each type of light will benefit the plants the most. If you have a higher concentration of blue and red light rather than yellow and green light there will be enhancements in the function of your plants.
Other considerations when choosing a light is how intense the light is and how long the plants are exposed to the light. Higher intensity increases the amount of photosynthesis the plant goes through. The duration of light outside is based on the sun, but in a greenhouse, it's recommended to keep the lights on for 12 to 14 hours per day to achieve the best results.
There are a few good options for lights:
- High intensity discharge lights are used mostly by major growth operations, because they use more electricity and produce more heat.
- Fluorescent lights are longer lasting and produce a lot of blue light. They may not provide the full color spectrum but for leafy plant such as herbs and lettuces they are effective. They are often chosen because they are long lasting, energy efficient, and do not produce a lot of heat.
- LED lights are designed to produce red and/or blue light. They are increasingly popular for hobbyist indoor greenhouses, but they don't produce very high intensity, so shouldn't be used for large-scale growth operations.
- Incandescent lights are not ideal for a greenhouse. These are often used for aesthetics and not function. Although they are the cheaper than other types of lights, they don't provide the needed intensity that plants require.
How Far Should Grow Lights Be From Plants?
Distance from the light to the plant depends on the type of light used:
- High intensity discharge lights: 5 to 8 feet above the plants
- Fluorescent lights: 4 to 6 inches above the plant
- LED lights: 14 to 30 inches above the plants
You can make you own grow light using light boxes, chains, and light bulbs. The chains are used to adjust the distance the light is from the plant. As the plants grow you will need to adjust the distance the light is above the plant.
Does leaf color affect photosynthesis?
Yes, green leaves have a higher rate of photosynthesis than other colors of leaves because green leaves have more chlorophyll. Plants with predominantly reddish leaves tend to be less productive--in other words, they tend to grow slower and have less biomass that green-leaved plants.
Does light intensity affect plant growth?
Yes, higher intensity light tends to increase the rate of photosynthesis. This is because the higher intensity of light will speed up the reactions needed for photosynthesis to occur. Greenhouse glazing decreases the intensity of light available to plants for photosynthesis, so supplemental lighting is recommended.
Do plants grow under white light?
White light--in other words, full-spectrum light--is what plants normally have available to them, since the sun produces full-spectrum light. Plants grow optimally in natural sunlight, and white light can also be used in a greenhouse, though you might need to experiment to see what type of light works best for the plants you're growing.
Does green light affect plant growth?
Green light tends to be the least effective for plant growth. You can tell that green light isn't absorbed by plants because plants appear green--green light is reflected from, rather than absorbed by, plants. That said, green light is still useful for photosynthesis, but isn't nearly as essential as red or blue light.