When we talk about colors of light, we’re really referring to their wavelength; it’s the wavelength of a light source that makes it appear blue, or red, or green. The visible light spectrum spans from violet/blue light which have short wavelengths (ranging from 400-500 nanometers) to red light, which has the longest visible wavelength of around 700 nanometers.
Because of its short, high frequency wavelength, blue light carries a lot of energy relative to other colors. In fact, if its wavelength was any shorter, it’d fall outside the visible light spectrum into the territory of powerful ultraviolet light.
How Blue Light Effects Cannabis
Blue light is critical for cannabis. If blue light levels are low, cannabis plants experience a shade-avoidance syndrome that causes them to stretch. That’s because, to the plants, a lack of blue light signals that they’re in the shade of other, taller foliage.
More specifically, it’s the green-to-blue ratio that signals shade. Green light penetrates into the canopy while blue light is mostly absorbed. So, when the green:blue ratio is high, the plants think they’re shaded. And that’s why it's important to include blue whenever you’re providing plants with high-intensity broad-spectrum light that includes lots of green.
When cannabis isn’t getting enough blue light, it’ll compete for exposure by growing longer shoots and eventually exhibit leaf hyponasty (an upward curl of the leaf surface). That’s not a good thing — at least not in the opinion of indoor cultivators who want short, manageable plants. Without any blue light, plants get unruly, tall, and poorly adapted for flowering. Because they’re busy stretching for light, they fail to develop the root system that’ll support their nutrition when it’s time to bloom.
Blue light also regulates the opening and closing of stomata. Stomata are the tiny pores that allow gas exchange and CO2 intake. This gas exchange is critical for bringing in the CO2 needed for photosynthesis, and also for cooling the leaves through the release of moisture. For that reason, most sole-source lighting fixtures include at least some blue light. Even high-pressure sodium (HPS) lights, which are strongly red-dominant, have a little blue.
Which Grow Lights Have Blue Light?
During the vegetative growth phase — when plants are establishing the foliage that’ll drive photosynthesis over the course of their life cycles — indoor growers make sure to give their plants lots of blue light. In the past, that meant using metal halide (MH) lamps that were rich in blue wavebands. Now, growers can select an optimized LED spectrum or an LED fixture with tunable wavelengths. Spectrum tunability allows cultivators to increase — or decrease — the amount of blue light they’re providing their plants as the production cycle changes.
But when we look at horticultural lights, pure blue light won’t seem as bright as other colors. Our eyes are less sensitive to blue and more sensitive to mid-spectrum wavelengths like green. That’s why a light with lots of blue will seem less powerful than it actually is. If a light were to produce only blue light, it’ll appear dimmer than a light producing only green or amber light. Yet blue light matters to plants, even though we can’t see it that well.
Why Does Cannabis Love Blue Light?
It’s important to give your plants the blue light they need — especially in veg. With blue light — either from LED or MH lights — plants show robust lateral growth without extension. What’s more, plants grown with a spectrum that’s rich in blue have thicker, darker leaves than those grown without blue light, and that prepares them for a robust flowering period.
But why does blue light matter in the vegetative phase? And why does the reddish HPS spectrum work better for flowering? The answer comes from the natural seasons.
During the peak of summer, when plants are in veg, there’s a lot of blue light. As fall rolls around and it becomes time for reproductive growth, the light has less blue light and more red. That’s because the blue light gets filtered out when the Earth tilts away from the sun and the light has to pass through more of the atmosphere. The atmosphere diffracts the blue, so there’s less for plants.
Grower Trick: Blue Light for Higher Cannabinoids
Some growers have discovered a strange spectral trick to get higher terpene and cannabinoid levels. It goes against the natural seasonal light shift, but it can increase terpenes by a reported 50%.
At the end of flowering, in the final 3-7 days, growers return to a spectrum with lots of blue light. Sometimes, they switch bulbs from HPS back to MH (if their ballasts are compatible). Or — if they’re running tunable LEDs — they can simply change the settings on their controller.
It’s unclear why the change in spectrum — which would be unnatural outdoors — produces more terpenes. Some growers think that it’s a stress response because blue light has more energy than other wavelengths. Other growers wonder if the plants “think” they’re back in the vegetative phase and they push more growth as a response. It’s as though the blue light sparks growth, except the growth goes into the buds and overall terpene levels instead of foliage.
Either way, switching back to a bluish, vegetative-phase spectrum increases the quality of many varieties of cannabis. With more terpenes, the crop has a higher market value — and the grower has a higher reputation for producing high-quality product.