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 spectrum and is part of a series exploring the effect of spectrum on plants. This post will focus on our knowledge of red light and its effect on plant growth and reproduction (flowering). Red (R) light is radiation with wavelengths between 620 and 750 nm. These wavelengths are within the visible spectrum and red light has a pronounced effect on both photosynthesis and flowering.
Red light fits with the absorption peak of chlorophylls, which do photosynthesis to produce sugars and carbons. Sugars and carbons are the building blocks for plant cells. Because red wavelengths are highly absorbed by chlorophyll, they also increase photosynthesis rate and plant size . Furthermore, red radiation at 690 nm may be more effective than 660 nm for increasing plant size . Although a plant can be grown using just red light, it’s not a good idea. Most plants will have faster growth with a full-spectrum light source. Plants grown with red wavelengths alone often get skinny, stretched stems (“etiolation”) with fewer leaves .
Ideally, red wavelengths should be provided in combination with blue (B) light and other colours as well. The R+B combination results in a faster rate of photosynthesis compared to R or B light alone. Compared to just red radiation, the red-blue combination increases plant size, leaf number, leaf size, and chlorophyll content [4, 5]. How much red and blue light is needed for cannabis plants? One way to answer this question is to test what ratio of R:B light is ideal. Depending on the species the ideal R:B ratio varies. A higher R:B ratio increases biomass in tomato, strawberry, and marigold [4 – 6]. Early research in cannabis suggests that a higher R:B ratio may increase plant height . On the other hand, higher levels of blue light compared to red are documented to increase biomass in pea .
The addition of other colours of light, especially green (G), further benefits plant growth. For example, a combination of R+B+G light increases plant size and height, and leaf size compared to a R+B alone .
Some research has shown that red light can also impact root number and length, however this is still a relatively unexplored area. Grape plants grown with red radiation produce a greater number of roots compared to plants grown under far-red or blue light . Tomato plants grown with red LED light alone produce more and longer roots compared to white, far-red, or blue light .
Red wavelengths also impacts the timing of flowering, the duration of flowering, the weight and number of flowers produced, and possibly THC and CBD content.
Flowering time is in part regulated by the amount of red and far-red (750 – 780 nm) light available to the plant . The effect of red radiation on flowering time is species-specific. For example, red wavelengths accelerates flowering in cranberry, wheat, and strawberry but delays flowering in mustard plants [12 – 15].
Red light can also impact the number and weight of flowers produced. Marigold plants produce five times more flowers when grown with fluorescent light supplemented with red light . Early research in cannabis suggests that plants grown with high amounts of red (HPS) light produce more flowers . In that same study, cannabis plants receiving high amounts of red wavelengths also produced less THC, CBD, THCV, CBG, and cannabinoids compared to plants grown under spectrums with more blue light .
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