Experienced growers know that the lighting needs of cannabis plants change over time: a bright summer day might be great for a mature plant in vegetative state, for example, but the same amount of light on a seedling could lead to stress or even death! This effect is true not only for light intensity, but even for the light’s color before it comes into contact with cannabis leaves. To attain maximum yields and potency, growers have to adapt the color of their lighting over time.
Young plants, for example, are especially susceptible to far red; in the wild, the presence of far red indicates the seedling’s position underneath the natural canopy, since far red light penetrates the leaves of taller plants more easily than other colors. Thus in the grow room, young cannabis plants react to far red (emitted abundantly by incandescent bulbs, among others) the same way they would in the wild: by growing elongated, narrower stems. Narrow stems can limit flower production later on, so it’s important to counteract the elongating effect with supplementation of blue light. In nature, blue light would only come into abundant contact with leaves with unobstructed views of the sky above; in this situation, the young plant wisely puts its energy into sturdier, bushier growth.
But as the plant ages, its light needs change again. While the plant’s chlorophyll does a fine job of converting blue light to energy, it is more efficient at converting red light – so once plants are established, the grower gets a higher return on his energy bill with red-tinged light than with light tinted blue. And this situation changes yet again once the plant switches to flowering mode; cannabis produces its famous potent resin partly to protect its buds from UV light, so supplementing flowering plants with UVB can lead to more potent, more valuable buds (but be careful – UV may be great for testing results but it can cause skin cancer in humans!).
In the end, no one lighting plan will fit every garden; every region has its own lighting profile, and different strains can have vastly different needs. So talk to your genetic supplier, familiarize yourself with light sensors, and pay close attention to your plants. And as always, get the best possible horticulture education at Oaksterdam University; best practices begin with you!