The age-old wisdom of providing your plants with different light spectrums depending on their stage of growth follows as such. A high amount of blue light is desirable for plants in the vegetative cycle as it promotes better root development and tighter internodal spacing, preventing overstretching. In flowering, however, a high amount of red light is preferable as it aids in stronger fruit production.
This is a very simplistic breakdown, but it’s enough for us to bring up the main point of this article. With the advancements in lighting technology and research undertaken in recent years, we’ve come to find that many of the things we used to believe about plants and their reaction to light are wrong. Plants do utilize green light for photosynthesis, the absence of UV and Far Red in lighting fixtures can hurt yield, and on (Nelson JA, Bugbee B, et al., 2014). As well, with the success of broad-spectrum white LED fixtures, many gardeners are only using one light spectrum that features high amounts of red for the entire growing cycle.
This article intends to revisit the age-old wisdom to see if blue light is still more important for the vegetative stage than it is for bloom. After all, the concept isn’t out of thin air. The amount of blue light that hits the planet’s surface is highest in the spring and summer, where days are long and many plants are in their vegetative cycle. As days shorten and plants start flowering, the spectral composition changes as more red and far-red photons are hitting the earth’s surface (Thorne, Helen C, et al., 2009).
Blue Light (400nm-500nm)
Compared to red and green light, the blue light spectrum appears dimmer to us due to spectral sensitivity, but when it comes to driving photosynthesis, it’s as effective. Generally defined as radiation with wavelengths falling between 400 and 500 nm, the blue light spectrum is responsible for a significant number of roles during plant growth.
Blue wavelengths mediate stem elongation, stomatal opening, and phototropic curvature by interacting with phototropins and cryptochromes. It plays a role in secondary metabolite production, helping some plants achieve their “natural” leaf color while increasing terpene production in others (Wang P, Chen S, et al., 2020).
The Advantages of High Blue Light in the Vegetative Growth Cycle
When solely given artificial lighting, blue light is essential for normal growth and healthy plants in all growth stages. However, only a small amount (varies between crop species) is required to drive all the necessary functions for fully functioning photosynthesis (Kim et al., 2005). Along with other caveats, such as efficacy and focus on flower and fruit production has pushed grow light developers to focus less on this spectrum and more towards the red spectrum.
However, this may be a grave error for many. Especially when in the vegetative stage. Here a high amount of blue light can significantly reduce plant stretching and better root development. These are critical factors for achieving optimal yield in many fruits and vegetables. Particularly, in plants where we prune to reduce the height or break apical dominance.
These advantages — along with blue light helping plants like tomatoes stay blister-free and increasing their concentrations of antioxidants and vitamins — means we can’t ignore this spectrum and focus all our attention on red.
When analyzing whether a grow light with a focus on having a stronger blue spectrum during the vegetative phase is necessary for you, it’s best to understand the spectrum’s role it has with the specific crops you’re growing.
In general, if your goal is compact plants that can better take advantage of light distribution and penetration, then higher amounts of blue during the vegetative phase are more desirable to you. If you’re new to gardening and aren’t sure if this applies to you, sticking with one lamp that favors the red spectrum over blue is best. But know that having the ability to give your plants a higher percentage of blue light in the vegetative cycle, then lowering it for flowering is going to be advantageous for many.
What Research Tells Us
Blue light plays a vital role in a number of plant functions throughout the growth cycle, yet only a low intensity is required for these functions to occur.
Now, this does not mean that low intensity is always the best move, however. Research shows that a higher amount of blue photons during the vegetative cycle can overwhelmingly create more desirable plants.
Effects of Blue Light on Plant Morphology During The Vegetative Cycle
|Impatiens, Salvia, and Tomatoes||“Partial substitution of R or green (G, 500-600 nm) light with B decreased seedling height and leaf expansion. Impatiens (Impatiens walleriana ‘SuperElfin XP Red’), tomato (Solanum lycopersicum ‘Early Girl’), and salvia (Salvia splendens ‘Vista Red’) seedlings were grown under LEDs at a PPFD of 160 mmol · m -2 · s -1 and the light quality was 100% R or R with an increasing percentage of B. Those grown under at least 25% B were shorter and had decreased leaf area compared with seedlings grown under R alone”||Wollaeger, Heidi Marie, and Erik S. Runkle. “Growth of Impatiens, Petunia, Salvia, and Tomato Seedlings under Blue, Green, and Red Light-emitting Diodes”. HortScience horts 49.6 (2014)|
|Tomatoes||“The plant height of tomato seedlings was signiﬁcantly lower under B (blue) light than under other lights. Compared with W (white) light, leaf areas under all the monochromatic lights were signiﬁcantly smaller.”||Wu, Q., Su, N., Shen, W. et al. Analyzing photosynthetic activity and growth of Solanum lycopersicum seedlings exposed to different light qualities. Acta Physiol Plant 36, 1411–1420 (2014).|
|Radish, Soybean, and Wheat||“Blue light did not affect total dry weight (DW) in any species but significantly altered plant development. Overall, the low blue light from warm white LEDs increased stem elongation and leaf expansion, whereas the high blue light from cool white LEDs resulted in more compact plants. “||Cope, Kevin R., and Bruce Bugbee. “Spectral Effects of Three Types of White Light-emitting Diodes on Plant Growth and Development: Absolute versus Relative Amounts of Blue Light”. HortScience horts 48.4 (2013).|
|Arabidopsis||“However, under low PAR, phot1 mediates a remarkable increase of green tissue fresh weight in response to blue light (Figures 2 and 3). The simultaneous optimization of chloroplast localization and stomatal opening and the maximization of effective leaf area under low PAR (Figure 6) probably contribute to increased photosynthetic rates and, consequently, accelerated growth. Furthermore, a substantial phot1-dependent growth enhancement was observed also under moderate PAR (Figure 3D) corresponding to frequently encountered light intensities in the natural environment (Vogelmann, 2002). Moreover, phot2 mediated growth responses under light-limited conditions in addition to the phot1-dependent effect, when the supplemental blue light intensity was increased (Figure 3C).”||Atsushi Takemiya, et al. “Phototropins Promote Plant Growth in Response to Blue Light in Low Light Environments.” The Plant Cell, vol. 17, no. 4, 2005, pp. 1120–1127. JSTOR|
As we can see, giving our plants an abundance of blue light during the vegetative stage can offer us significantly more compact plants, which is usually considered a good thing. One concern is blue light promoting smaller but thicker leaves. Thick leaves are great because they stand up better to fungal diseases and piercing insects, but troubling because a decrease in surface area can translate to a weaker photosynthetic rate.
We may find that minimum amounts of far-red photons are essential to fully take advantage of this promotion due to their ability to penetrate deeper into leaves and the canopy. It’s not the end of the world since far-red is very desirable in flowering. Another way around this is to find the golden ratio of B:R for the best of both worlds. Introducing more green light could also help. However, there is roaring debate over how successful green light can penetrate through leaves and the canopy compared to red and blue light.
This brings up how much blue light is ultimately preferable. We won’t touch upon this here, simply because there are too many variables, many to do with the grower’s preference.
For good measure, here’s what happens when plants get a high intensity of blue light during the flowering stage.
Effects of Blue Light on Yield During The Flowering Cycle
|Tomatoes (Nightshade), Cucumbers, Peppers, Soybean, Lettuce, and Wheat||“At a PPF of 500, increasing blue light from 11 to 28% significantly decreased dry mass in tomato, cucumber, and pepper, but there was no significant effect on soybean, lettuce and wheat. At a PPF of 200, dry mass significantly decreased only in tomato across the blue light range.”||Snowden, M. C.. “Effects of blue and green light on plant growth and development at low and high photosynthetic photon flux.” (2015).|
In flowering, we see that a high ratio of blue photons can result in lower-yielding productions. Add this to another reason why many grow lamps are lacking blue light. Because while a very high amount of red to blue might not be best for a plant’s morphology, it doesn’t potentially diminish its yield like vice versa.
However, this doesn’t discredit the minimal amount of this spectrum needed for flowering. Furthermore, there is the growing idea that reintroducing a high amount of blue light periodically in the flowering stage is beneficial for photomorphogenic outcomes and phytochemical concentrations — even at the cost of reducing yield.
Research shows that a higher percentage of blue light for the vegetative cycle is more desirable than it is for the flowering cycle. Now, while giving young plants more than the minimum amount of blue light they need for healthy growth is advantageous, this isn’t going so far as to say that it should be making up the majority of the photons they receive. Saying, “blue light for veg and red light for bloom” is too simple of an explanation that can cause more harm than good.
Now, during the vegetative cycle, one can argue that providing more than the minimum amount of blue light is better for some crops than others when it comes to the final yield. This leads to the gardener having to decide whether the additional cost of running higher blue light (spectral quality) is best for their plants or if they should go for the most economical light that delivers photons within PAR (fixture efficacy).
However, the ability to decrease or increase the intensity of blue photons at different stages of plant development offers significant advantages that can overwrite the higher cost. This is especially true if periods of intense blue light in the flowering cycle can improve crop quality so much that it overwrites any loss in quantity.
Therefore, I believe there is enough research to show that moving forward, supplementing in/out various intensities of blue light will become even more the norm across both commercial and home grows. The latest advancements in LEDs will only reinforce this.
Economic Reasons Blue Light is Desirable in The Vegetative Cycle
For both commercial operations and home growers, giving your plants more than the minimum amount of blue light required for fully functional photosynthesis during the vegetative phase can result in more compact plants with tighter internodal spacing and smaller leaves. For many crops, this opens them up to better light coverage and penetration, reduces their susceptibility to certain diseases, and slows lower leaf death that stunts photosynthetic rate. Additionally, with less pruning required, along with plants recovering faster (increasing turnover times), time and labor demands are reduced.
As we can see, the economic cost of running higher blue in the vegetative cycle can be largely erased, if not reversed.
Non-Economic Reasons Blue light is Desirable in Vegetative Cycle
Finally, while having more than the minimum amount of blue in the vegetative stage may not always be the best economic move, there are still several advantages. Not only can plants grow faster with fewer prunings, the grower will also spend less time on it. As well, the compactness and smaller leaves make it easier to see pests and diseases deep into the canopy for faster diagnosing.
More so, if you’re looking to grow multiple plants with your lights, having the ability to control their height can significantly help when crowding many plants into a smaller area. Even if more blue isn’t always economical, these advantages can more than makeup for the additional cost.
Plus, in the end, it’s just good fun experimenting with plants and the way they grow. Also, check out our article on the benefits of deep-red and far-red light for the generative/flowering cycle.
Nelson, Jacob A, and Bruce Bugbee. “Economic analysis of greenhouse lighting: light-emitting diodes vs. high-intensity discharge fixtures.” PloS one vol. 9,6 e99010. 6 Jun. 2014, doi:10.1371/journal.pone.0099010
Thorne, Helen C et al. “Daily and seasonal variation in the spectral composition of light exposure in humans.” Chronobiology international vol. 26,5 (2009): 854-66. doi:10.1080/07420520903044315
Kim, Hyeon-Hye et al. “Light-emitting diodes as an illumination source for plants: a review of research at Kennedy Space Center.” Habitation 10 2 (2005): 71-8 .
Wang, Pengjie et al. “Exploration of the Effects of Different Blue LED Light Intensities on Flavonoid and Lipid Metabolism in Tea Plants via Transcriptomics and Metabolomics.” International journal of molecular sciences vol. 21,13 4606. 29 Jun. 2020, doi:10.3390/ijms21134606
Wollaeger, Heidi Marie, and Erik S. Runkle. “Growth of Impatiens, Petunia, Salvia, and Tomato Seedlings under Blue, Green, and Red Light-emitting Diodes“. HortScience horts 49.6 (2014): 734-740.
Wu, Q., Su, N., Shen, W. et al. Analyzing photosynthetic activity and growth of Solanum lycopersicum seedlings exposed to different light qualities. Acta Physiol Plant 36, 1411–1420 (2014).
Cope, Kevin R., and Bruce Bugbee. “Spectral Effects of Three Types of White Light-emitting Diodes on Plant Growth and Development: Absolute versus Relative Amounts of Blue Light“. HortScience horts 48.4 (2013): 504-509.
Atsushi Takemiya, et al. “Phototropins Promote Plant Growth in Response to Blue Light in Low Light Environments.” The Plant Cell, vol. 17, no. 4, 2005, pp. 1120–1127. JSTOR