Tips for Indoor Growers

Cannabis grow lighting is perhaps the most important variable to consider in planning any garden. This notion applies to indoor, greenhouse, and outdoor cultivation operations. However, designing a functional lighting system in controlled environment agriculture can prove quite challenging. Looking specifically to indoor gardening, cultivators must weigh several important variables in cannabis grow lighting design. Issues like infrastructure, budgeting, as well as plant growth must be considered in proper cannabis grow lighting design.

Cannabis Plant Physiology

Physiologically speaking, there are several reasons as to why effective horticultural lighting is so important in cannabis cultivation. To begin with, cannabis as a species has evolved over the years to thrive in arid, sunny climates. This looking to master indoor gardening are well advised to create environments that match these demands. As such, cannabis grow lighting must also mimic the ecological influences in which the DNA of cannabis responds favorably.

Secondly, cannabis is classified as “short day photoperiodic” plant species. This means that changes in sunlight patterns directly dictate the seminal phases of a cannabis plant’s lifespan. As most cultivators know, the defining photoperiodic characteristic of cannabis occurs with vegetative and flowering cycles. As related to these phases, 12 hours of uninterrupted darkness causes a cannabis plant to flower. Ecologically speaking, these light durations are indicative of late summer and fall sun cycles. Point being, serious indoor growers must accommodate for the biological demands of cannabis. This concept is especially important pertaining to the plant’s accompanying changes in growth patterns related to lighting.

cannabis grow lighting, indoor growers

Cannabis Grow Lighting Technology

In designing a lighting schematic for an indoor cannabis garden, indoor growers must consider the perks and drawbacks of a variety of technologies. Moreover, making decisions between these  technologies can be difficult and often boils down to subjective preference.

The primary grow lights on the market today are: fluorescents, metal halides (MH), single-ended high-pressure sodiums (SE HPS), double-ended high-pressure sodiums (DE HPS), and light emitting diodes (LEDs). To help understand these choices better, we put together a list of factors to consider in choosing a grow light specifically for cannabis cultivation. We have decided to base this study on the biological needs and growth patterns of the cannabis species relating to light sources.

Vegetative Stage vs Flowering Stage

Let’s look to the life-cycles of the cannabis species regarding photoperiodism. The most important factors to note here concerning grow room lighting are vegetative and flower growth. As such, each of the crucial periods of growth has specific requirements for lighting. These changing light patterns are indicative of seasonal changes in sunlight.

Lighting for Vegetative Growth

In the northern hemisphere, the vegetative growth period of cannabis plants occurs naturally in the spring and early summer months. With this notion in mind, 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 the during vegetative photoperiod.

Cannabis grow lighting 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 used for vegetative indoor cannabis cultivation. Largely because, they present predominant blue wavelengths. In recent years, LED lights have grown in popularity. The relatively new technology is trusted by indoor growers to handle vegetative growth. This is a growth phase not nearly as demanding on lighting needs as flower production.

Lighting for Flowering

Lets look over to flowering cannabis indoors. In this scenario, artificial light wavelengths seek to mirror those occurring with sunlight in the late summer and fall months. That being said, cannabis grow lighting 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. HPS lights, in both double-ended and single-ended fixtures, are revered by cannabis growers. This fact is due to their high-powered discharge and wavelengths imbued heavily with red spectrums. It’s also worth noting that HPS lights can be utilized for vegetative growth. As such, many indoor growers dual purpose them for simplicity’s sake.

cannabis grow lighting, indoor growers

Lastly, LED lights are gaining leaps and bounds with cannabis flowering applications. Because, increasingly efficient technological innovations are making the lights powerful enough to stimulate flower growth. Concerning light spectrum analysis, LEDs essentially “cut out” wasted wavelengths. They do this by focusing specifically on the blues and reds with which cannabis plants respond favorably. This cutting-edge approach ensures LEDs operate efficiently as possible. In turn, greatly reducing excess heat in grow rooms as well as utility expenses for growers.

Cannabis Plant Growth 

Cannabis plants grow rapidly under the right environmental conditions. How indoor growers design their cannabis grow lighting schematic should account for this increase in canopy size.

Looking at specific cannabis strains, indica and sativa plants present drastically different growth patterns. To this end, indicas generally grow “short and bushy” while sativas “stretch out” to extensive heights. This notion is exemplified when cannabis plants change from vegetative to flowering phase. During this time, most plants double in size. As such, indoor growers must look far beyond the “first glance” of a grow room when planning their cannabis grow lighting. Because, the canopy will doubtlessly 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 solid 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 ft from canopy
  • LED Lights:18 inches from canopy

Indoor growers should consider the space they need to work in their indoor garden as relating to light fixtures. To illustrate, 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.

Anticipating Plant Growth

A good method for planning grow room lighting relating to anticipated plant growth is to visualize a given garden space with a specific strain of cannabis in mind. Let’s say our hypothetical garden features 8 ft ceilings and the cultivator is planning on growing a sativa dominant strain like Blue Dream. The grower is still 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 5-gallon pots, to reach a height of six ft 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 3 – 5 ft minimum distance). At this point, this grower has two choices for their flower room concerning their lighting options. For starters, they can implement air-cooled SE HPS lights or LED lights, which can both be utilized in closer conjunction to a given canopy (they need 12 – 18 in 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 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 life-cycle 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 as that of considerations with infrastructure and cost.


This article originally appeared in the October / November 2018 issue of Maximum Yield Cannabis Magazine.