Although a plant can be grown using just blue light, most plants have faster growth with a broader spectrum of light. Blue light is typically used in conjunction with red light, since photosynthetic pigments more efficiently absorb red light than other regions of the spectrum. The blue-red light combination allows for a faster rate of photosynthesis than either red or blue light alone. Compared to blue light alone, the red-blue light combination increases photosynthesis rate, leaf number, and plant size in many species, like tomato, cucumber, peppers, lettuce, and strawberry. What about adding other colors of light too? The addition of green light to red-blue light further increases plant height! At this time, the ideal ratio of red, blue, and green light (as well as other colors of light) is not yet known for many species. For one tomato variety, the ideal ratio is documented to be 1:2:1 for R:B:G. Likely, the ideal spectrum for certain plants vegetative growth will be strain-dependent. For this reason, it’s important to choose a “full-spectrum” horticultural light that has high amounts of blue and red light and moderate amounts of other colors of light.
In addition to playing an important role in photosynthesis, blue light is well known to suppresses stem elongation (“stretching” or etiolation). Stem elongation occurs when a plant is receiving insufficient light and it stretches up to capture more light. Plants grown with blue light are usually shorter and have thicker, and denser leaves compared to plants grown without blue light. This is attributed to the action of chryptochrome, which responds to blue light to control several aspects of plant growth and development, including stem elongation.
It’s clear that blue light is important for vegetative growth. The addition of red light and other colors of light further increase photosynthesis rate and plant size. At this time, the ideal ratio of each of these colors of light is unknown. Likely, the ideal spectrum for vegetative growth will be species- and strain-dependent. For this reason, it’s important to choose a “full-spectrum” horticultural light that has high amounts of blue and red light and moderate amounts of other colors of light.
Figure 1: California Lightworks SolarSystem 550 VEG | specially designed for vegging plants: Seedlings, Cuttings, Motherplants
Blue Light and Flowering
When it comes to growing certain plants, many cultivators are most interested in the quality of light used for the flowering stage. Blue light can regulate flowering in a number of ways, such as increasing bud size, and regulating the timing of flowering. The flowering process is resource-intensive, and there is a strong positive correlation between plant size (i.e., vegetative growth) and bud size. A plant with high photosynthetic rates will accumulate more resources that later allow it to produce large, dense flowers. Thus, in order to have a high yield, it is essential that a some plant are provided with high amounts of PAR during the vegetative stage. As described above, blue light is essential for achieving high rates of photosynthesis that translates to high yields.
In addition to controlling bud size, a cultivator may wish to control flowering time. Through the action of chryptochrome, blue light is a strong regulator of flowering time. In long-day plants, blue light can promote flowering, and in short-day plants, it can inhibit flowering. For example, in mustard, a long-day plant, blue light accelerates flower initiation by 20 days compared to plants grown without blue light. At the time of writing this article, the effect of blue light on certain plants flowering is not yet know. Some plants are long-day plants and increased amounts of blue light are predicted to induce flowering. Until additional studies are performed, it is recommended to use a “full-spectrum” horticultural light that includes high amounts of blue light, as well as other colors of light.
Blue light is essential for both the vegetative and flowering stages of plant growth. Blue light should be used in conjunction with red light to increase photosynthesis and plant size. The addition of other colors of light, such as green, has been shown to have further benefits for plant growth. For this reason, “full-spectrum” lights usually produce better results that mono-colored or dual-colored lights. Blue light is a strong regulator of plant height and flowering. Generally, blue light makes plants more compact by making stems shorter and thicker. In long-day plants, like