Far-Red Light and Flowering
By this point, you might be starting to notice a trend: plants associate far-red light with shade. Thus, if you give a plant too much far-red light, they will think they are in a shady environment. Too much shade can be stressful to a plant, so it takes precautionary measures to avoid these conditions. Seeds will avoid germinating and stems will stretch out to reach more light. In response to too much shade, a plant will often start flowering. Flowers are the reproductive tissues of a plant and if a plant thinks there is a risk of death (from too much shade), it begins reproducing ASAP so that it may pass its genetics to the offspring. High amounts of far-red light accelerate flowering in many species – tomato, potato, cucumber, beans, wheat, mustard, and many ornamental flowers. In some species, far-red light also increases the number of flowers produced.
As growers, we can use this knowledge to our advantage. If we wish a plant to begin flowering (such as a stubborn cannabis plant that refuses to bud out), we can give it high amounts of far-red light. Far-red light should be applied for a short period to induce flowering, and stopped once buds begin to appear. Small amounts of far-red light applied at nighttime (~2 μmol s-1 m-2) is also effective at accelerating flowering and increasing flower number.
For most stages of plant growth, a grower should maintain a high R:FR ratio. In other words, plants should be provided with high amounts of red light (and other colors of light, like blue and green light) and low amounts of far-red light. If a grower wishes to induce flowering, they can provide a plant with high amounts of far-red light (either during the day or night) for a short period. Once flowering starts, the plants should be returned to their regular lighting conditions. When choosing a light for growing cannabis, look for a horticultural light that has high amounts of red and blue light, moderate amounts of other colors (green, yellow, and orange), and low amounts of far-red and UV light. When used for an extended period, far-red light can be detrimental to plant growth, so it should be avoided when purchasing a grow light. Far-red light causes plants to stretch out and reduces the amount of chlorophyll (which is essential for plant growth) in leaves. When used for a short period, far-red light can stimulate flowering, which can be advantageous if you have a stubborn plant that refuses to flower.