How the Light Spectrum Affects Your Cannabis Grow

Along with water and food, light is one of your marijuana plant’s primary sources of life. Your light source provides marijuana plants with the light photons necessary to carry out various metabolic processes, such as cellular respiration and photosynthesis. 

These processes working together generate magnificent plants with potent buds and increased yields, so it’s essential to understand how light and the light spectrum affect your cannabis plants and their various grow phases.

What is the Light Spectrum?

The light spectrum is the portion of the electromagnetic wave spectrum that is visible to the human eye. The electromagnetic spectrum covers all types of active radiation, from short-frequency radio waves all the way up to high-frequency gamma waves, most of which we cannot see. The visible light spectrum, measured in nanometers (or nm), is simply electromagnetic radiation occurring between the range of 380 and 760 nm, though the range of visible pigments varies slightly between individuals.

The lower end of the visible spectrum contains far-red light, and beyond lies infrared (IR) light. Just beyond the upper range lies ultraviolet or UV light; both IR and UV light are typically invisible to the naked eye, though each can prove beneficial in its own right when growing cannabis.

PPF is the measurement of how many photons of light are emitted by a light source, though it measures only visible light. Grow light output is measured in PPFD (photosynthetic photon flux density), which also takes into consideration light outside the visible spectrum.

Measured as µmol/(m2s) (micromoles per square meter), PPFD values tell the grower how many photons of light which actually contribute to photosynthesis are available to the plant. Familiarizing yourself with these variables (and investing in a good light meter) will help ensure your plants can maximize their light absorption.

Colour Temperature and the Kelvin Scale

Artificial light is often described in terms of its “colour temperature,” a reference to the appearance of light provided by a particular lamp. A bulb’s colour temperature is measured in Kelvin (K), and most fall within the range of 2,700-6,500K.

Not a reference to the actual temperature of the light itself, colour temperature refers instead to the way light appears to the eye, with lower colour temperatures corresponding to warmer (or “redder”) light, and higher colour temperatures corresponding with cooler, “bluer” light. The cooler, blueish light at the upper end of the Kelvin scale is most similar to natural sunlight and measures between 6,000-6,5000K.

As marijuana plants thrive in full, natural sun, it makes sense to attempt to emulate natural lighting conditions indoors, too.

Emulating Nature with Lighting

When you see photos of three-meter-high, monster cannabis plants, they’re always grown outdoors under full sun. These plants are able to achieve their massive proportions because they grow along with the changing seasons. Like many other consumer crops, outdoor cannabis is typically planted in the late winter or early spring and allowed to grow all year round, with flowering and harvest occurring naturally in the fall.

As summer transitions into fall, day lengths shorten, and the sun lowers in the sky, resulting in cannabis plants ending their life cycle by flowering. It is this shortened photoperiod that growers attempt to emulate by switching their plants’ light cycle from 18/6 for vegetative growth to 12/12 for flowering. But the shortened days aren’t the only factor that forces plants into flowering.

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.