Red Light vs. Blue Light: Which Light Color Is Better For Plant Growth
There isn’t really an answer to which light color is better for plant growth, since both red light and blue light are necessary to the health of your indoor plants. That being said, you can find more info on red light vs. blue light in this article.
Effects of Red and Blue Light on Plants
What we perceive as white light from the sun is actually made up of all the colors of the rainbow. The three major colors of light are red, blue, and green.
We can tell that plants don’t absorb much green light because it reflects off of them and into our eyes, making them appear green. The fact that leaves don’t usually appear blue or red means that they absorb those parts of the light spectrum and use them to grow.
The effect of blue light on plants is directly related to chlorophyll production. Plants that receive plenty of blue light will have strong, healthy stems and leaves.
Red light is responsible for making plants flower and produce fruit. It’s also essential to a plant’s early life for seed germination, root growth, and bulb development.
Red Light or Blue Light for Plants?
While outdoor plants in full sun will naturally receive both red and blue light, indoor plants might be lacking in it. Even plants next to a window may not be receiving enough of a certain part of the color spectrum.
If your plant is getting leggy or losing the green color in its leaves, odds are it’s not getting enough blue light. If it’s not flowering at a time you know it should (this is a particular problem for Christmas cacti that refuse to bloom at Christmas), it’s probably lacking in red light.
You can supplement blue light with fluorescent lamps. While using red light for plants is possible with incandescent bulbs, these often produce too much heat to be kept near houseplants. Use a broad spectrum fluorescent bulb instead.
Sometimes, pollution can block essential light. If your unhealthy plant is next to a particularly dirty window, the solution to your problem could be as simple as giving it a good cleaning to let in as much light as possible.
There isn?t really an answer to which light color is better for plant growth, since both red light and blue light are necessary to the health of your indoor plants. That being said, you can find more info on red light vs. blue light in this article.
7 Myths LED Grow Light Companies Tell You
So you’re interested in buying the best LED grow light for indoor plants? You have come to the right place.
To make an informed decision, you should understand how plants use light, especially from artificial light sources. We are going to deconstruct a few myths in the industry to help you identify the best grow lights on the market.
Myth #1: “HPS puts out over 50% wasted light”
“This is why our purple 12-band LEDs are more efficient! Our perfect 12-band spectrum puts out the only light plants really need.”
If you see this quote, run! 🙂 Some LED companies say that the reason why LEDs are more efficient than High-Pressure Sodium bulbs is because plants don’t use yellow or green light. In other words, the reigning champion of grow lights for several decades, the mighty High-Pressure Sodium, is putting out a lot of “wasted light”. What?
Here’s what the LED companies haven’t been telling you. There’s a phenomenon known as the green gap for narrowband LEDs — they can’t create green, yellow or infrared wavelengths very efficiently. So when you look at the graph below, you’ll suddenly understand why we have purple grow lights. It’s because red and blue are the only colors narrowband LEDs are good at making! Also notice how narrowband LEDs only create very small “points” of color, which is why you might see 8, 10, or 12 bands being advertised as a full spectrum led grow light – it’s a company’s way of trying to deal with LED technology developed in the 1990’s.
The efficiency percentage of narrowband LEDs at different colors (wavelengths)
Truth: LEDs are more efficient than HID bulbs because they create light directly from electricity, and do not have to heat up a bulb.
But really, where are they getting the idea that purple light is the best for plants? Read on to Myth #2…
Myth #2: Plants Only Need Red and Blue (Purple) Light
Here is where the LED company will pull out their favorite chart – the Chlorophyll A/B absorption graph (see “Pigment Extract” in the below image). They’ll show their spectrum on top of this chart and say that they’re giving plants only the colors of light that they can absorb, or 100% usable light. Sounds great right? It’s more like a marketing trick.
Remember the colors that narrowband LEDs could create from Myth #1? Notice how red (
450nm) conveniently coincide with the colors most absorbed by the “Pigment Extract” which is Chlorophyll. In actuality, the leaf itself is absorbing more than just red and blue. Green and yellow light (500-600nm) are getting absorbed by the leaf, too. In fact, these colors are very important for the full development of the plant since that is what they experience when growing under the sun.
Absorptance spectra of spinach leafs
If you look closely at the graph, notice how different parts of the plant leaf absorb varying portions of light. The “Chloroplasts” are the centers of photosynthesis — notice how much green, yellow, and orange they absorb. Green doesn’t just “bounce off” of the leaves after all!
If you can imagine green and yellow light as the “free agents” that can slip by Chlorophyll A/B to other parts of the leaf, think about how a red and blue spectrum could fail. Red and blue light cannot easily penetrate past the first few layers of plant cells, into buds, or to the leaves below because Chlorophyll A/B is blocking those colors! See the illustration below where the narrowband Red/Blue LED having most of its light blocked by the leaf (e.g. Chlorophyll A/B) whereas HPS or broadband light penetrates past the canopy through to the leaves below.
Myth #3: Blue is for Veg, Red is for Flower
This myth originates from the ritual of switching between Metal Halide (MH) and High-Pressure Sodium (HPS) bulbs when transitioning from the vegetative stage to the flowering stage of growth. Growers use MH because it has blue light which is needed to prevent stretching. HPS bulbs have more yellow/orange light, which is important for rapid growth during flowering. Neither of these bulbs is designed for plant growth, but each is more suitable for a certain stage of plant growth than the other.
Since growers are in the habit of switching bulbs between veg and flower, LED companies provide two spectrum modes to increase marketability. Usually, the veg mode is almost 80 percent blue light. Studies show that giving your plants mostly blue light during veg is wasting the potential of your harvest. Your plants will grow slower, create smaller leaves, and won’t be prepared to grow big for the final yield.
Check out the image below illustrating the vegetative growth of lettuce under varying amounts of blue and green light. Watch what happens when blue light passes about 20 percent — the plants shrink! Think about the results that you might get from a grow light with more than 50 percent blue light. LED companies that “match” the chlorophyll A/B graphs will provide this much blue light, seriously decreasing yields. For curious minds, check out the study (quoted below) here.
As the fraction of blue light increased, leaf size and plant growth decreased significantly. However, while the addition of green light considerably reduced the leaf photosynthetic rate, it did not reduce plant growth.
Myth #4: UV LEDS Help Plant Growth
First things first, UVB light has shown benefits to certain plants, but only when applied at the correct intensity, for the proper length of time, and at the right time. Otherwise, UV light can be harmful to plants, let alone your eyes or skin — it’s the reason we use sunscreen and wear sunglasses when going outside.
All UV Rays can lead to eye damage. If your grow light emits UV, wear eye protection !
That being said, if used properly, could UV LED’s help plant growth? It’s a toss-up of whether it’s worth it when reviewing scientific articles. Most UV LEDs actually only produce UVA light, which can be safer and less damaging than UVB. However, the reason why UVB light is beneficial to plants is because it introduces damage and the plant responds with a defense, so UVB light is ideal.
What’s more, UV LEDs are not as efficient, more expensive, and burn out more quickly than other LEDs. With that in mind, it bears the question if using UVA LEDs is worth it, especially if that power could be redistributed to colors that are proven to increase plant growth.
See the graph below from an LED chip manufacturer to see the limited output range of UV LEDs:
Leading UVA/UVB LED supplementation bars last about 15,000 hours (vs the standard LED lifetime of 50,000) and cost hundreds of dollars, whereas a UVB bulb such as the T8 Reptisun 10.0 UVB cost about $15 and will last you an entire year if used for the flower period of your grow.
Myth #5: 1000W LED = 1000W HPS
- The HPS bulb wattage the LED fixture is intended to replace.
- An addition of the “highest rated power output” of LED chips.
- The actual power draw in watts.
Myth #6: The More Lumens, the Better
Lumens are a measure of how bright light is to the human eye. Looking at the graph below, you can see that green/yellow light is the most visible to the human eye, with blue and red light being the least visible.
The sensitivity of the human eye to color. 400-500nm is blue 500-600nm is green/yellow/orange, and 600+nm is red
You may find white LED companies advertising Lumens because they can “beat” the other fixtures that emit mostly red and blue light. A Red/Blue light will score a low amount of lumens.
Each color of light has a different efficiency and plays a different part in a plant growth, so we need a plant-centric light measurement as a benchmark. This is called Photosynthetically Active Radiation (PAR), which weights all wavelengths from 400-700nm equally.
Be critical about videos comparing one LED light to another with a PAR meter. Many PAR meters on the market incorrectly read LED lights. If you’re really interested about this, check out Apogee Instrument’s superb presentation [Scroll down to “In-depth Look at PAR-Quantum Meters” click play and skip to
15:00]. If you’re a grower, you should understand the concept of how light is measured. And hey, pick up a quality light meter!
Truth: PAR is the correct light measurement for plants, and not lumens.
Myth #7: A High Center-Point PAR Reading Means the Brightest Light!
Most grow lights are only good at shining light directly below the fixture in a “spotlight”, so the most attractive light reading an LED company can advertise is the highest reading they can measure — the one right in the center of the light.
Plants easily burn with this type of focused LED light, which is one of the reasons you may want to pay attention to this reading: if a light is putting out over 800 umol.m .2 s 1 of PAR in the center, that is in the danger zone for burning your plants. You’ll have to raise the light to the point where the center isn’t so “hot”, which actually decreases the brightness for the rest of your grow area. Counterintuitive, huh?
Look for LED grow lights that cover the entire grow area like the our lighting systems. These systems help eliminate plant burn while keeping your entire grow area at a uniform light level.
7 Myths LED Grow Light Companies Tell You So you’re interested in buying the best LED grow light for indoor plants? You have come to the right place. To make an informed decision, you should