Red and blue light are the colors plants use most, so both are essential for plants to grow. Blue light stimulates chlorophyll production and healthy growth, and red light boosts flowering and fruit production. A good grow light delivers both, along with the rest of the spectrum.
How plants use light
Plants do not use every color of light equally. Photosynthesis runs mainly on light between about 400 and 700 nanometers, a range that scientists call photosynthetically active radiation, or PAR. Inside that range, chlorophyll, the green pigment that captures light energy, absorbs blue and red light most strongly. Green light is absorbed less at the surface of a leaf, which is why leaves look green: it is the color they reflect back to our eyes.
That is the reason red and blue light are the most utilized by plants, and why many early LED grow lights were built with only red and blue diodes. But red and blue do more than feed photosynthesis. Plants also use light as a signal. They sense the colors around them and change how they grow in response.
Blue and red light at a glance
- Blue light: stimulates chlorophyll production, encourages healthy growth and helps keep plants from stretching too tall.
- Red light: boosts flowering and fruit production and tends to make plants grow taller.
What blue light does for plants
Blue light stimulates chlorophyll production and encourages healthy plant growth. It also acts as a signal. Blue-sensing photoreceptors, called cryptochromes and phototropins, tell a plant which way to grow toward the light and help control the tiny pores on the surface of a leaf, called stomata, that let carbon dioxide in and water vapor out.
Growers put this to work. It is a common practice to use a bluer spectrum light to help prevent plants from stretching too tall before switching over to flowering and fruit production. Seedlings and young plants that get plenty of blue light tend to grow compact, sturdy and leafy rather than tall and spindly.
Where blue light helps most
- Seedlings and cuttings, which need to stay short and strong
- Leafy greens and herbs, which are grown for their leaves
- The vegetative stage of fruiting plants, before they begin to flower
What red light does for plants
Photoreceptors in plants are sensitive to red light. The phytochrome pigments absorb the red and far-red portions of the light spectrum, and they are primarily known for boosting flowering and fruit production. They also tend to make plants grow taller. Red light is also among the most efficient colors for driving photosynthesis itself, which is why it is so valuable in a grow light.
Why red and far-red light change how tall a plant grows
Phytochrome works like a switch that flips between two forms: one responds to red light and the other to far-red light, which sits just beyond the red that we can see. In nature the balance between the two tells a plant what is around it. Sunlight is rich in red light. Light that has passed through or bounced off other leaves has had much of its red removed, so it contains relatively more far-red. A plant that senses this shade signal stretches upward to get above its neighbors. That is why red and far-red light tend to make plants grow taller.
Red light, night length and flowering
Phytochrome is also how many plants keep track of the seasons. Short-day plants such as chrysanthemums and poinsettias flower when nights are long enough, and phytochrome is the pigment that measures the length of the night. Growers can even delay flowering by interrupting the night with a little red light. Other plants, including tomatoes and cucumbers, are day-neutral: they flower as they mature rather than by day length, but they still depend on strong light to set and ripen fruit. Read how many hours of light plants need.
Why red and blue alone is not enough
If red and blue are the colors plants use the most, why not build a light with only those two? Some lights do. They are often called blurple lights because of their pink-purple glow. Plants will grow under them, but there are drawbacks:
- You lose the rest of the spectrum. Plants use green light too. It travels deeper into a leaf and a canopy than red or blue, so it helps feed the lower leaves that sit in shade.
- Your plants are hard to read. Under pink-purple light every color looks wrong, which makes it harder to spot pests, nutrient problems and disease.
- It is hard on your eyes. Working for long under intense colored light is uncomfortable.
Even NASA saw this. The Veggie plant growth system on the International Space Station launched with red and blue LEDs, and NASA's team added green LEDs so the plants would not look an alarming purple and the crew could judge their health by eye. Red and blue, one of the scientists said, are the minimum needed for good plant growth.
Being full-spectrum white LED light, Samsung LM301b/h LEDs have a more natural color to the human eye compared to the blue spectrum of MH lights and the yellow and red of HPS lights. That helps you, the indoor grower, monitor your plants for pests and disease control more accurately. Read more about full spectrum LED grow lights.
Matching light color to the stage of growth
Growers have long adjusted the color of their light as plants mature. A bluer light is common for the vegetative stage, when you want strong, compact leafy growth. A redder light is common for flowering and fruiting. The two classic grow lights show the difference: Metal Halide (MH) lights give off a bluer light and High-Pressure Sodium (HPS) lights a yellow and red light, so growers switched from one to the other as their plants moved from vegging to flowering.
With a full-spectrum LED there is no need to worry about that switch. The same light covers every stage of growth and the dimmer switch lets you adjust the intensity as your plants change. See which stage of vegetable and fruit growth you can use LED lights at.
Common questions about red and blue light
Do plants prefer only red and blue light?
Red and blue are the most utilized by plants, but plants use the rest of the spectrum too. A full spectrum gives them the red and blue they use most, plus the colors that help them grow evenly and help you see them clearly.
Is more blue light always better?
No. Blue light helps plants stay compact, but plants also need red light to flower and set fruit. The right balance changes as the plant matures, which is why growers adjust the color of the light for each stage.
Why do some grow lights glow pink or purple?
Those lights use mostly red and blue diodes, and the mix of the two looks pink-purple to our eyes. A full-spectrum white light looks natural instead.
Key takeaways
- Chlorophyll absorbs blue and red light most strongly, so they are the most utilized colors.
- Blue light supports chlorophyll production and compact growth; red light supports flowering and fruit.
- Red and far-red light act through phytochrome to signal shade and night length.
- A full spectrum is better than red and blue alone: it adds green light and lets you see your plants.
- A bluer light suits vegging and a redder light suits flowering, but a full-spectrum LED with a dimmer covers both.
Protect your eyes
Grow lights are bright, and light that is good for plants is not always comfortable for people. UV eye protection is recommended. Just like the sun, you do not want to look directly at the light, and if you feel you need them, a good pair of sunglasses or the green-tinted glasses made for LED lights will do the job.
Back to how plants use light.