What Color Light Is Best for Plant Growth?

Rows of leafy greens growing under large multi-colored grow lights indoors.

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Whether your home is lacking some natural light or you are just looking to provide your plants with a little boost, a grow light is a great addition to any indoor plant setup. However, there are a few things to consider before you run out and buy the first grow light that you see. One of the most important things to consider is the color of light that the grow light emits. 

The Visible Light Spectrum

If your knowledge of the electromagnetic spectrum is a bit rusty, here’s a little science class refresher for you. The visible light spectrum is a segment of the larger electromagnetic spectrum containing the light that is visible to the human eye. This includes wavelengths from about 380 to 750 nanometers. As you might imagine, the visible light spectrum is also what drives organism growth and photosynthesis. The range of visible light used by plants to drive photosynthesis ranges from about 400 to 700 nanometers and is referred to as Photosynthetically Active Radiation (PAR). PAR includes blue light (400 to 520 nanometers) and red light (630 to 700 nanometers) and everything in between. While blue and red light have been recognized as particularly significant to plant growth and the photosynthesis process, it’s important to know that the entire PAR spectrum (including green and yellow light) is essential to supporting balanced, healthy plant growth. 

Red vs. Blue Light

While the entire PAR spectrum is used during photosynthesis, red and blue light make up the majority of light used by plants. Each type of light supports plant growth and development in a unique way. 

Red light primarily supports the growth of stems and expansion of leaves and regulates flowering, germination, and dormancy. On the other hand, blue light is responsible for chlorophyll production, root growth, and leaf thickness. The importance of red versus blue light is sometimes simplified to a difference in promoting flowering versus vegetative growth, but the role of each type of light shouldn’t be simplified so easily. In the end, both red and blue light are essential for plant growth and development and no plant can survive long-term without one or the other.

The Best Light For Plant Growth

So what type of light is best for encouraging plant growth? The short answer is that there isn’t one color of light that is better than the other, as they are all essential. That being said, there are times when growers will utilize grow lights that are heavier in one color of light. For example, in large commercial applications, growers are usually pickier with the type of light that they expose their plants to as they are trying to achieve specific outcomes and large yields. So they will cycle through lights that are heavier in blue light or red light depending on where their plants are in the growing cycle. However, for most small-scale, residential applications (like houseplants!), this kind of thing isn’t necessary. A grow light that provides the entire PAR spectrum is ideal.

Choosing the Right Grow Light

So all of that being said, what should you look out for when purchasing a grow light for your houseplants? First, while there are a few different kinds of grow lights available on the market right now, LED is usually the best choice for homeowners and small-scale applications. Not only is it more cost-effective than other kinds of grow lights, LED lights are also the most energy-efficient. Plus they are widely available. Then, make sure you go for a full-spectrum light: one that covers the full PAR spectrum (400 to 700 nanometers) and includes plenty of red and blue light. 

Another thing to watch out for, although it’s not a standard unit of measurement with most grow light manufacturers, is the PPFD (Photosynthetic Photon Flux Density) value, which indicates the amount of light emitted by a grow light. More specifically, PPFD measures the number of photons in the PAR range per unit of time on a unit surface. The ideal value for indoor plant growth will fall in the 500 to 700 µmol/m2 range. However, if you don’t see this value reported don’t be alarmed. While it is not the most effective way to measure light output for grow lights, manufacturers usually report light output in watts or lumens. In these cases, aim for a grow light that covers about 500 lumens per square foot, or about 20-25 watts per square foot.