Light Intensity and Your Canopy: What You Need to Know

When it comes to growing a nice, even canopy of delicious, resin-coated cannabis buds, there’s no greater component to master than the light itself. Your grow light, like the sun, is your life source in your grow room. But unlike the sun, you have complete control over your grow light, including its spectrum, cycle, and most importantly, when developing a solid canopy, its intensity. 

A light’s intensity can be thought of as the magnitude of light energy per unit of area. A lack of light intensity or low light will result in plants that “stretch” or reach for more light or may cause plants to slow their growth rate. On the other hand, overly intense lighting will result in a variety of problems, including light burn and leaf curling.

While it may be tempting to hang your light nice and low, directly above your plant canopy, and really soak them with tons of good light, it’s important to remember that too much of a good thing can quickly sour your operation and that more light doesn’t always equate to higher yields.

There are a few key points to keep in mind when determining the optimal intensity of light, such as the type of light, the period of time the light will stay on, and the spacing from the top of the canopy.

Types of Lights

Traditionally, indoor growing has taken place under high-intensity discharge (HID) lamps, which include metal halide (MH) and high-pressure sodium (HPS) bulbs. Additionally, growers have relied on T5 fluorescent bulbs for young plants, such as seedlings and clones. These lights have been the industry standard for decades as they reliably produce a broad spectrum of light and can be easily automated to emulate the changing seasons by using timers and switching bulbs.

In recent years, however, a large percentage of growers have begun to transition to the use of light-emitting diode (LED) grow lights in their grow room. LEDs boast several advantages over traditional HID lights, such as their ability to house various colours of diodes in a single board.

For this reason, many LED grow lights are equipped with a switch enabling the grower to toggle between a “Veg” and “Bloom” setting. Additionally, many LED grow lights are equipped with a dimmer switch, allowing the grower to adjust the brightness of their light on the fly. However, the main advantage of LED grow lights is their ability to generate large quantities of quality light without raising the temperature of the grow room drastically.

Measuring Light for Your Plants

There are two types of measurement related to light intensity and brightness, lumen and lux, both of which measure different aspects of light.

Lumen – Lumen (lm) is the measurement of visible light, or “luminous flux”, being emitted from a source. Thus, a higher lumen measurement equates to brighter light levels in your light source.

Lux – Lux is the measurement of light that actually falls upon a surface, such as a plant leaf. Lux is typically used for light measurement in grow rooms, as the light energy that falls upon the plant is of greater significance than the output of the light itself.

Unfortunately, using lumen and lux measurements alone is not adequate, as they fail to adequately perceive concentrations of blue and red light wavelengths, both of which are scarcely visible to the human eye but play a significant role in plant growth. Instead, growers rely on PAR and PPFD values to measure light for horticulture, as they more accurately measure the light which plants can use.

An understanding of each is necessary to understand how light intensity is measured.

PAR – PAR, or Photosynthetically Active Radiation, is a measure of the visible light spectrum which plants can utilize for photosynthesis, a number that falls between 400-700 nanometers (nm). PAR measurements indicate where on the visible spectrum a particular light’s output can be found.

PPFD – PPFD, or Photosynthetic Photon Flux Density, on the other hand, is a measurement of the amount of light (PAR) a plant receives over time and is measured in micromoles of photons per square meter per second (µmol/m²/s). In other words, PPFD is the measurement of how many photons of light strike the plant over the course of a second.

In order to obtain accurate PPFD measurements, it is important to average a few different readings from around your grow space. A single, centred reading will misrepresent the actual averages that your plants will experience, as most grow lights tend to focus their light output in the centre.

Light Distance from Your Canopy

It’s important to consider the conditions in which your plants will be growing when deciding on your grow light’s distance from your canopy. Height limitations, for instance, will play a determining factor not only in the height of the light but the type of light as well. HID lights produce a significant amount of heat and will need to be placed farther away from your canopy than LED grow lights. In a short grow tent, there simply won’t be room to accommodate an HID light, which requires a reflector and cooling of some kind.

Of equal importance to light, height is your light’s footprint or the amount of space it can adequately cover. This is where the importance of HID reflectors comes into play. Situated around the bulb, a reflector (sometimes called a hood) reflects light outward from its source, creating a wider footprint. Generally speaking, LED grow lights do not feature reflectors, which leads to many growers choosing to run enough lights in their space to create overlapping footprints.

Of course, more lights will also translate to higher electric costs and an increased need for cooling.

There is no steadfast rule when it comes to light distance from your canopy, as the determining factor will ultimately be the type of light you choose to run. There are, however, some guidelines that growers have developed over the years, which vary by the growth stage and the type and wattage of light.

Growth Stage – Depending on which stage your plants are in, they will require different light intensities to achieve optimal growth. Plants in the vegetative stage, for example, require between 20,000-70,000 lux, with a healthy average falling around 40,000 lux. Plants in the flowering or bloom stage will need between 35,000-85,000 lux, with a healthy average of approximately 60,000 lux. Of course, achieving these levels will depend upon your light source and your light source’s distance from your canopy.

HID Lights – HID Lights – With HID lights, it’s important to remember that the higher the wattage, the more light and heat will be produced. Thus, lower-wattage HID bulbs can be hung closer to the canopy. For the sake of a rounded standard, most measurements are based on 1000 watt HID bulbs, for, at this wattage, light intensity doubles for every fifteen centimetres closer the bulb is to the canopy. Growers will often implement multiple lower-wattage HID lights over one higher-wattage bulb to achieve a greater footprint and more uniform light distribution. Generally speaking, a 1000-watt HPS bulb will produce a footprint of 1.5 x 1.5 meters at a height of 1 meter, with a PPFD reading of 1435 µmol/m²/s.

LED Lights – The significant reduction in heat produced by LED grow lights cannot be underestimated. A quick internet search will reveal growers placing their LED lights a few centimetres from their canopy. While this is possible due to the low temperatures, there are still many considerations to consider. Generally speaking, your LED grow light should be between 60-90 cm above your canopy for seedlings and clones, 30-60 cm for vegetative growth, and 40-90 cm for flowering.

With so many variables to consider, it isn’t easy to clearly define proper distances between your canopy and grow light. It is essential to thoroughly research your grow light and take light measurements for yourself using a light meter. Often, lighting companies provide inaccurate information as to PAR and PPFD values on their grow lights, so it’s best to take these measurements for yourself.