Choosing the optimal lighting for cannabis cultivation can be challenging. There are many factors to consider, like light intensity, distribution, and spectrum  Spectrum can be especially complicated because there is a lot of conflicting information out there. This article is going to take a research-based approach to evaluating the effect of blue light on plant growth.

The Key Points:

Blue light…
  • Is highly absorbed by chlorophyll pigments and increases photosynthesis rate
  • Can open stomates to allow more COto enter the leaves
  • Prevents plants from stretching
  • Increases seedling size and encourages them to produce more antioxidants
  • Might impact flowering time and flower size
  • Might increase THC and cannabinoid content

Blue Light and Photosynthesis

Blue light is radiation with wavelengths between 450 – 495 nm. It corresponds to the region of peak absorption by chlorophyll and carotenoid pigments and so it strongly affects photosynthesis and vegetative growth [1]. Chlorophylls and carotenoids both absorb light and convert it into chemical energy that can be used for plant growth [2].

Even though blue wavelengths are highly absorbed by chlorophylls and carotenoids, they are actually less effective than red light for driving photosynthesis [3]. This can be explained by the absorption and action spectrums of photosynthesis (Figure 1). The absorption spectrum shows which wavelengths are absorbed by chlorophyll and other pigments. On the other hand, the action spectrum shows the photosynthesis rate for each wavelength. Although plants can absorb high amounts of blue wavelengths, not all of it is used for photosynthesis. Where does all the blue go? Well, some of it gets absorbed by lower-efficiency pigments (like inactive anthocyanin pigments) that don’t contribute to photosynthesis. However, this doesn’t mean that plants don’t need blue light. In fact, many plants can grow using just fine using only blue wavelengths [4]!

Not surprisingly, when blue wavelengths are paired with red, photosynthesis happens faster than either blue or red light alone [4, 5]. The red-blue combination increases protein concentration, chlorophyll and carotenoid concentration, leaf number, plant size, and plant weight [5, 6, 7, 8, 9].

action and absorption spectrum blue light

Figure 1: The absorption spectrum of chlorophylls and beta carotene correlates with photosynthetic output. Red light is slightly more effective than blue light for driving photosynthesis. (Modified from HyperPhysics Biology)

Blue Light and Stomatal Conductance

Blue wavelengths increase stomatal conductance, which is when gasses, like CO2, enter or exit a leaf through the stomata. It does this by increasing the number, density, and size of stomata [10]. Gas exchange is important for growers to keep in mind because it is critical for photosynthesis and leaf cooling. Furthermore, blue wavelengths are more effective than red or white at controlling changes in stomata [4, 11].

Relationship to Stem Stretching

In addition to playing an important role in photosynthesis, blue light can prevent stem stretching [12, 13]. Stretching happens when a plant doesn’t get enough light and so it grows taller to capture more. Plants grown with blue light are often shorter compared to plants grown without blue light [6, 14]. In addition to decreasing stem length, blue light has also been shown to decrease petiole length, which are the small stems connecting the leaves to the stem (Figure 2) [3]. In other words, blue wavelengths make plants more compact. Cannabis plants grown under a blue + red light had shorter internodes compared to a white light source, suggesting that the additional blue light encouraged plants to stretch less [15].

Cucumber experiment blue light

Figure 2: Effect of blue, red, green, blue + red, and blue + red + green light on cucumber plant growth. Compared to other colours of light, blue wavelengths reduce stem and petiole stretching. In cannabis, it also increases the size and antioxidant concentration of seedlings (Modified from Snowden et al, 2016).

Blue Light and Seedling Growth

Blue light also has an effect on seedling growth: it increases seedling size and the antioxidant concentration [15, 16]. Antioxidants protect plants from UV rays and harmful reactive compounds that can cause major problems for photosynthesis and flowering. Thus, increasing antioxidants in plants may be one way to offset the stresses that come along with intense photosynthesis rates. In addition, blue wavelengths increases the sprouting rate, fresh weight, and protein content compared to other colours of light [15].

Effect on Flowering

Blue light impacts flowering in two main ways: timing of flowering and flower weight. Through the action of chryptochrome (a light receptor in plants), blue wavelengths can sometimes regulate flowering time. How blue light impacts flowering depends on light intensity and whether the plant is a “long-day” or “short-day” plant. Generally, blue wavelengths cause flowering to happen earlier in long-day plants and later in short-day plants. For example, mustard is a long-day plant and exposing it to blue light causes flowering to happen 20 days earlier than it normally would [17]. In other species, blue wavelengths have no effect on flowering [18]. For some flowers, pea plants, and violets, blue light doesn’t change flowering time at all [18]!

Impact on Cannabinoids Production

Last, but not least, some recent research shows that blue light may effect cannabinoid production in cannabis. One study looked at the effect of light quality on the yield of THC and other cannabinoids in cannabis cultivation. Plants grown using LEDs (which had 6 – 16% more blue wavelengths than HPS lamps) had about 38% more THC compared to those grown under HPS lamps [12]. Cannabis plants grown under LED lights also had higher concentrations of CBD, THCV, CBG, and cannabinoids [12]. The authors suggested that UV-A and blue wavelengths might cause the plant to produce more CBG (a precursor of THC and other cannabinoids). The mechanisms underlying the effect of blue wavelengths on the cannabinoid pathways will also require further research.

Learn More

  1. Singh et al 2015. LEDs for Energy Efficient Greenhouse Lighting.
  2. Croce, Van Grondelle, Van Amerongen, and Van Stokkum. 2018. Light Harvesting in Photosynthesis.
  3. Cope, Snowden, and Bugbee. 2014. “Photobiological Interactions of Blue Light and Photosynthetic Photon Flux: Effects of Monochromatic and Broad-Spectrum Light Sources.” Photochemistry and Photobiology 90 (3): 574–84.
  4. Lu et al 2017. Uncovering LED Light Effects on Plant Growth: New Angles and Perspectives.
  5. Sabzalian et al. 2014. “High Performance of Vegetables, Flowers, and Medicinal Plants in a Red-Blue LED Incubator for Indoor Plant Production.” Agronomy for Sustainable Development 34 (4): 879–86.
  6. Chandra, S. et al. 2013. “Effects of Different Light Quality on Growth, Chlorophyll Concentration and Chlorophyll Biosynthesis Precursors of Non-Heading Chinese Cabbage (Brassica Campestris L.).” Acta Physiologiae Plantarum 35 (9): 2721–26.
  7. Hernández and Kubota. 2016. “Physiological Responses of Cucumber Seedlings under Different Blue and Red Photon Flux Ratios Using LEDs.” Environmental and Experimental Botany 121: 66–74.
  8. Naznin et al 2016. “Using Different Ratios of Red and Blue LEDs to Improve the Growth of Strawberry Plants.” Proc. of the VIII Int. Symp. on Light in Horticulture 8: 125–30.
  9. Ouzounis, T. et al. 2016. “Blue and Red LED Lighting Effects on Plant Biomass, Stomatal Conductance, and Metabolite Content in Nine Tomato Genotypes.” Proc. of the VIII Int. Symp. on Light in Horticulture 8.
  10. Kim et al 2004. “Effects of LEDs on Net Photosynthetic Rate, Growth and Leaf Stomata of Chrysanthemum Plantlets in Vitro.” Sci. Hort. 101 (1–2): 143–51.
  11. Zheng and Van Labeke. 2017. “Long-Term Effects of Red- and Blue-Light Emitting Diodes on Leaf Anatomy and Photosynthetic Efficiency of Three Ornamental Pot Plants.” Frontiers in Plant Science 8: 1–12.
  12. Hsu and Chen. 2018. “The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis Sativa L.” Med. Can. and Can. (1): 19–27.
  13. Olschowski et al. 2016. “Effects of Red, Blue, and White LED Irradiation on Root and Shoot Development of Calibrachoa Cuttings in Comparison to HPS Lamps.” Proc. of the VIII Int. Sym. on Light in Horticulture.
  14. Massa et al 2008. “Plant Productivity in Response to LED Lighting.” HortScience 43 (7): 1951–56.
  15. Livadariu et al. 2018. “Studies Regarding Treatments of LEDs  on Sprouting Hemp (Cannabis Sativa L.).” Romanian Biotechnological Letters: 1–7.
  16. Samuolienė et al. 2011. “The Impact of LED Illumination on Antioxidant Properties of Sprouted Seeds.” Open Life Sciences 6 (1): 68–74.
  17. Eskins, K. 1992. “Light-Quality Effects on Arabidopsis Development. Red, Blue and Far-Red Regulation of Flowering and Morphology.” Phys. Plant. 86 (3): 439–44.
  18. Runkle et al 2001. “Specific Functions of Red, Far Red, and Blue Light in Flowering and Stem Extension of Long-Day Plants.” J. Amer. Soc. Hort. Sci. 126 (3): 275–82.