Blue Light for Veg Stage, Red Light for Flowering Stage
Of equal significance to the photoperiod is the spectrum of light provided for plants as they transition from veg to bloom. During long summer days, the sun is more directly overhead in the sky, allowing for more penetration of cooler, blue light through the atmosphere. Plants interpret this cool, blue light as a signal to begin photosynthesis and produce chlorophyll, generating lots of dense foliage in preparation for the coming flower cycle.
As the days shorten and the sun dips lower in the sky, the angle of the sun’s rays allow more of this cool blue light to be reflected away by the atmosphere, resulting in more warm-coloured red wavelengths reaching plants. During the autumn months, plants interpret this increase in light from the red spectrum as a signal to begin flowering, as it is nearing the end of its lifespan, and the plant hopes to be pollinated for the sake of generating seeds and reproducing.
This transition from one light spectrum (cool blue) for veg to another (warm red) for bloom is nearly as significant a factor in bringing about flowering as photoperiod (the duration for which your grow light will stay on). A grower can easily change their light fixture’s photoperiod, but if the grow light doesn’t contain the proper light spectrum to correspond, plants may react slowly or produce little in the way of a harvest. This is where a bit of homework will come in handy.
Types of Lights and Their Spectra
Traditionally, indoor cannabis horticulture has a few different types of lights, such as compact fluorescent (CFL), metal halide (MH) and high-pressure sodium (HPS) bulbs, though the LED revolution of the last decade or so has begun to shift the norm. Understanding the benefits and drawbacks of various types of lighting can be beneficial when selecting a grow lamp for your indoor plants.
Compact Fluorescent (CFL) – Compact fluorescent bulbs produce a spectrum that is heavy in cool, blue light, with some bulbs even reaching slightly into the UV range. CFLs have traditionally been used for germination and the development of seedlings and clones, though because of their cool blue spectrum, growers have used larger fixtures for vegging their plants as well.
Metal Halide (MH) – Like CFLs, the spectrum produced by metal halide lights is heavy on blue light, making them a classic choice for vegetative growth. One type of high-intensity discharge (HID) lamps, metal halide lights, produce significant light intensity, making them a staple in the cannabis cultivation community. Their performance lacks significantly in the warm, red end of the light spectrum, though, so growers most often switch bulbs or supplement MH lighting with high-pressure sodium lights for the flowering stage. Metal halide lights produce significant amounts of heat, also, and thus usually require an air-cooled reflector, which can drive up electricity costs.
High-Pressure Sodium (HPS) – High-pressure sodium lights give off a warm, red-orange glow that makes them the perfect companion to metal halide for the bloom phase. Another high power HID lamp, HPS bulbs have been used for years in indoor grow environments for their ability to produce far-red light, but their significant heat output also requires cooling. For this reason, many growers have begun turning to LED lights, even for commercial grows.
Light-Emitting Diode (LED) – LED lighting has quickly taken the cannabis world by storm. The diodes for which LEDs are named come in various colours, and grow lights equipped with them can be fine-tuned to provide a nearly full spectrum of light. Moreover, many full spectrum LED grow lights are equipped with a variety of different colours out-of-box, enabling the grower to switch between cooler blue and white LEDs for the veg phase and warmer red and infrared LEDs for the flowering stage. In addition, LED grow lights do not produce nearly the amount of heat as their HID counterparts, easily allowing the grower to daisy chain many lights together.
Whichever grow light you ultimately select, it is vital to consider the light spectrum produced by it. Different bulbs may be used for different phases of the grow, or in the case of LED grow lights, one light may suffice from beginning to end. The main concern is that your grow lamps provide ample amounts of cool, blue light during veg and ample amounts of red and infrared light during the bloom phase.