In other words, even if the light appears the same to the eye, it may be missing certain wavelengths (colours) that plants require for healthy growth. Because LEDs tend to emit light in a very narrow colour spectrum, LED grow lights are usually outfitted as “full-spectrum” setups. They consist of a number of different-coloured LEDs that together cover most of the necessary spectrum for cannabis plants. These full-spectrum LEDs are comprised of different reds and blues, often mixed with additional white LEDs. Other, newer LEDs, such as COB lights, emit a light spectrum that more or less approximates natural sunlight; there's no "gap" in the colour spectrum.
WHERE DOES KELVIN COME IN WHEN CHOOSING A GROW LIGHT? For vegging your cannabis plants, go with a cool light, one that emits a "daylight" colour with a high Kelvin of 6,000–6,500K. For flowering, a warm light with a reddish tone, about 2,800K, is optimal. You can also find grow lights with a “best of both worlds” colour temperature of about 3,500K, which you can use for both vegging and flowering. Every organism living on Earth needs information what is going on around them to react to environmental changes, and ideally, get a slight advantage over other members of their species regarding natural selection and evolution. Interestingly, cannabis plants receive a lot of their information from the light they’re exposed to, and almost instantly react to different bands of wave lengths - a complex topic to fill books with, but let us focus on the basics first. Vegetative Stage – “Blue” light for healthy leaves (range: 400-500nm; ideal: 460nm) During the vegetative stage it is recommended to aim for as many leaves as possible, and to make sure plants stay rather compact, don’t stretch too much, and develop strong stems.
Indoor growers tend to use metal halide bulbs, compact fluorescent lamps (CFL’s), or T5/T8 lighting fixtures with a blue band of light for the first few weeks to achieve these goals. When cannabis grows in nature, the high angle of the sun in spring and summer allows more “blue” wave lengths to penetrate through the atmosphere, a signal for cannabis plants to grow strong, large and healthy leaves. Flowering Period – “Red” light for giant buds (range: 620-780nm; ideal: 660nm) When cannabis plants enter the flowering period, highest yields can be achieved by exposing them to a light spectrum that contains lots of “red” wave lengths to promote budding. The rate of photosynthesis peaks when plants are subjected to “red” wave lengths of 660nm although latest NASA findings suggest that even “green” wave lengths, which are not associated as a major factor in photosynthesis, can also have an impact on how plants grow. Seeing a cannabis plant as simple photosynthesis factory is consequently a little hasty. But for now, choosing a lighting solution with a high degree of “red” in its spectrum remains the best way for growers to imitate the shallow angle of the sun in late summer and autumn. INCREASING THC LEVELS WITH UVB LIGHT – MYTH OR REALITY? Have you ever wondered why potent cannabis strains often originate from landraces that naturally grow in high altitude regions? There are experts who suspect ultraviolet light, especially a high exposure to UVB wave lengths (280-315nm), to be responsible for an increased THC production. The theory is based on the fact that a high elevation means lesser atmosphere between cannabis plants and the sun, leading to a higher exposure to UV rays. These ultraviolet wave lenghts knowingly damage our skin, and the human body reacts by producing melanin as protection – a cannabis plant assumingly does something similar – it produces more resin and THC as a form of natural sunscreen. It is too early to say if we are dealing with a theory or a cost-effective method to grow better cannabis but the concept seems plausible enough for hands-on experiments. UVB bulbs for reptiles only cost a few bucks; perhaps we should give it a try. The Ultimate Lighting Guide for Cannabis Cultivation. A Science-Based Guide for Understanding Grow Light Terminology and Lighting Options. When growers do not have the option of growing cannabis outside, they will have to decide on the best cannabis grow lighting for indoor growing. Growers have a challenging task when deciding what grow light to use for cannabis indoors. Grow lights can also be needed for growing cannabis in greenhouses. Greenhouse grow lights for cannabis will have very different functions than grow lights for indoor cannabis cultivation. Aside from picking the right type of light, there is an abundance of grow light terminology that can be confusing. Understanding grow light terminology is essential for creating the best indoor cannabis grow. Cannabis sativa is a short day plant that possesses great medicinal qualities. Short day plants change from the vegetative growth stage to the floral growth stage when they are exposed to critical short day lengths. For cannabis this is 12 hours of light to trigger flowering. Most growers will keep cannabis is 16 to 18 hours of light during the vegetative growth stage. Another way to reduce energy use during the vegetative growth stage of cannabis is to use a gas lantern routine. The gas lantern routine runs lights on a schedule where they are on for 12 hours, off for 5.5 hours, on for one hour, then off for 5.5 hours again. This cycle repeats and maintains vegetative growth by tricking the plant with the one hour of lighting between off periods. Instead of using 16 or more hours of light, only a total of 13 hours is used with the gas lantern routine.
Cannabis grown outdoors will move through growth stage changes as conditions naturally change. When grown in a greenhouse or grow room, growers can take advantage of this trait, called photoperiodism, to shorten or extend cropping time. Lighting specifically for inducing growth stage responses in day length sensitive plants is referred to as photoperiod lighting. Photoperiod neutral plants are not sensitive to day length and will flower based on other cues. Autoflowering cannabis plants flower based on the plant’s age. These unique cannabis plants have this adaptation because they are native to areas near the equator that do not experience short days soon enough in the natural growing season, and to areas like Siberia where day lengths can be shorter than 12 hours before the plant has reached sufficient vegetative growth to move on to flowering.
Daylight Extension Lighting vs Supplemental Lighting. Long day plants like spinach and lettuce also exist. These plants need a critical long day length to initiate flowering.