When planning any cannabis grow operation — indoor, greenhouse, or outdoor — light is perhaps the most important variable to consider. Designing a functional and efficient lighting system in controlled environment agriculture can prove challenging. When considering indoor gardening, cultivators must weigh several important variables relating to infrastructure, budgeting, and plant growth when choosing the proper lighting plan for a successful cannabis grow.

Physiologically speaking, there are several reasons why effective lighting is so important in growing cannabis. To begin with, cannabis has evolved to thrive in arid, sunny climates. Indoor growers are well advised to create environments and lighting schematics that match the ecological influences in which the DNA of cannabis responds favorably. Secondly, cannabis is classified as short-day photoperiodic plant species, meaning changes in sunlight patterns directly dictate the seminal phases of a plant’s lifespan. As most cultivators know, the defining photoperiodic characteristic of cannabis occurs with vegetative and flowering cycles, in which 12 hours of uninterrupted darkness causes a plant to flower. These light durations are indicative of late summer and fall sun cycles. Serious indoor gardeners must accommodate for the biological demands of cannabis and the plant’s accompanying changes in growth patterns related to lighting.

In designing a lighting schematic for an indoor cannabis garden, growers must consider the advantages and drawbacks of a variety of technologies — making decisions between these can be difficult and often boils down to subjective preference. The primary grow lights on the market today are: fluorescent, metal halide (MH), single-ended high-pressure sodium (SE HPS), double-ended high-pressure sodium (DE HPS), and light emitting diodes (LEDs). Rather than focus solely on grow room infrastructure or the technical specs of specific lights, the following information is based on biological needs and growth patterns of the cannabis species relating to light sources.

Vegetative and Flowering Stages

Looking at the lifecycles of the cannabis species regarding photoperiodism, the most important factor to note concerning grow room lighting has to do with changing light requirements of both vegetative and flower growth. Each of the crucial periods of growth has specific requirements for lighting. These changing light patterns are indicative of seasonal changes in sunlight patterns, spectrums, and intensity.

In the northern hemisphere, the vegetative growth period of cannabis plants occurs naturally in the spring and early summer months. Studies show that light wavelengths in this time of year present predominant blue spectrums. Under the influences of sunlight imbued with blue light wavelengths, cannabis plants naturally grow stout and strong during the vegetative photoperiod. As a result, grow lights designed specifically for vegetative growth seek to mimic the spectrums occurring naturally in the spring and early summer months. Traditionally, fluorescents and metal halides have been the grow lights of choice for vegetative indoor cannabis cultivation because they present predominant blue wavelengths. In recent years, LED lights have grown in popularity as the relatively new technology is trusted by growers to handle vegetative growth, which is not nearly as demanding as seen with flower production.

Looking at flowering cannabis indoors, artificial light wavelengths seek to mirror those occurring with sunlight in the late summer and fall months. Light spectrums during these seasons see a significant shift towards red colors. In the past, HPS lights have been far-and-away the most popular choice for cannabis flowering. High pressure sodium lights, in both double-ended and single-ended fixtures, are revered by cannabis growers due to their high-powered discharge and wavelengths imbued heavily with red spectrums. It’s also worth noting HPS lights can be utilized for vegetative growth — many hobbyist growers dual purpose them for simplicity’s sake. Lastly, LED lights are gaining leaps and bounds with cannabis flowering applications, as increasingly efficient technological innovations are making the lights powerful enough to stimulate flower growth. Concerning light spectrum analysis, LEDs essentially reduce wasted wavelengths and focus specifically on the blues and reds with which cannabis plants respond favorably. This cutting-edge approach ensures LEDs operate efficiently, greatly reducing excess heat in growrooms as well as utility expenses for growers.

Plant Growth

Cannabis plants grow rapidly under the right environmental conditions, so how you design your lighting schematic should account for this increase in canopy size. Indica and sativa plants present drastically different growth patterns, with indicas generally growing short and bushy and sativas stretching out to extensive heights. This notion is exemplified when cannabis plants change from vegetative to flowering phase, where most plants double in size. Growers must look far beyond the first glance of a grow room when planning their lighting, as the canopy will look much different when plants are in full-flower phase — this anticipatory planning is especially important regarding ceiling height. If curious about the ideal distances of certain lights to a cannabis garden canopy, here are some figures to consider:

  • Air-cooled SE HPS/MH lights: 12-18 inches from canopy

  • Fluorescent lights: 6 inches from canopy

  • DE HPS lights: 3-5 feet from the canopy

  • LED Lights: 18 inches from canopy

Finally, growers should consider the space they need to work within their indoor garden as relating to light fixtures, as massive hoods crammed into a small room can easily render pathways impassible without a hard hat. As most experienced indoor gardeners know, hitting your head on the corner of a hood is never a fun experience.

A good method for planning grow room lighting relating to anticipated plant growth is to visualize a garden space with a specific strain of cannabis in mind. Let’s say our hypothetical garden features eight-foot ceilings and the cultivator is planning on growing a sativa dominant strain like Blue Dream. The grower is unsure of their best lighting choice and needs some more information to help them make a sound decision. It is not unusual for Blue Dream plants, even grown in smaller five-gallon pots, to reach a height of six feet in full flower. Therefore, in this hypothetical scenario, DE HPS lights are not an option as they run extremely hot and can easily burn foliage if set too close to the canopy (they need three to five feet minimum distance). At this point, this grower has two choices for their room concerning their lighting options. They can implement air-cooled SE HPS lights or LED lights, which can both be utilized in close conjunction to a given canopy (they need 12-18 inches minimum distance). Otherwise, this grower could choose to grow a stalky, indica-dominant strain like Grand Daddy Purple and implement DE HPS lights.

Cannabis loves the light and indoor growers seek to mimic the environmental conditions in which this species thrives. When balanced correctly, grow lights are used to trigger vital lifecycle changes in cannabis plants as well as stimulate exceptional growth. When planning the lighting design of an indoor garden, these plant processes should garner as much attention infrastructure and cost.