Light Requirements for Successful Indoor Grows

Growing cannabis indoors has become increasingly popular, especially as legalization movements gain traction across the world. Cultivating cannabis indoors offers growers more control over the plant’s environment, which can result in higher yields and more potent flowers. One of the most crucial factors in indoor growing is light. This article will delve into the light requirements for successful indoor cannabis cultivation.

Understanding Photosynthesis and Its Significance

Cannabis, like all green plants, relies on photosynthesis to generate the energy it needs for growth. Photosynthesis is a process in which plants convert light energy, usually from the sun, into chemical energy in the form of glucose. This glucose fuels the plant’s growth and development. Thus, the type, intensity, and duration of light a cannabis plant receives directly influences its health, growth rate, and yield.

The Light Spectrum

Visible light is composed of a spectrum of colors, each with its unique wavelength. These wavelengths are categorized as follows:

  • Ultraviolet (UV): Short wavelengths that can increase THC production but may damage the plant if too intense.
  • Blue: Supports vegetative growth, leading to shorter, bushier plants with dense foliage.
  • Green: Mostly reflected by the plant but can penetrate deeper into the canopy than other wavelengths.
  • Red: Boosts flowering and fruiting, promoting longer stems and larger buds.
  • Far-red: Can influence the plant’s photoperiod, potentially accelerating flowering in some strains.

Types of Grow Lights

Over the years, various types of grow lights have been developed, each with its advantages and drawbacks.

High-Intensity Discharge Lights

HID lights have been a standard in the cannabis growing community for years. They are efficient and produce a lot of light. There are two main types of HID lamps:

  • Metal Halide (MH): Emits a blue-white light suitable for vegetative growth.
  • High Pressure Sodium (HPS): Produces an amber-red light, optimal for the flowering stage.

Compact Fluorescent Lights

CFLs are energy-efficient and good for small-scale grows or as supplementary lighting. They don’t produce as much heat as HIDs, making them suitable for tight spaces. However, they are not as intense, meaning they need to be placed closer to plants.

Light Emitting Diodes

LEDs have become increasingly popular due to their energy efficiency, long lifespan, and broad spectrum capabilities. Advanced LED systems can be tailored to emit specific wavelengths, optimizing growth at various stages. They also produce less heat than HIDs.

T5 Grow Lights

These are a type of fluorescent light that’s more efficient than CFLs. They’re best used for seedlings, clones, and early vegetative growth.

Light Duration and Photoperiodism

Cannabis plants’ growth and flowering stages are influenced by photoperiods, the duration of light and darkness they receive.

Vegetative Stage

During this stage, cannabis plants require longer light periods to grow and develop. The typical light cycle is 18 hours of light followed by 6 hours of darkness.

Flowering Stage

To induce flowering in photoperiod-sensitive cannabis strains, the light cycle is adjusted to 12 hours of light and 12 hours of darkness. This mimics the natural change in day length that signals the plant to flower.

Auto-flowering Strains

Some cannabis strains, known as auto-flowering strains, are not photoperiod sensitive. They will flower based on age, not light cycle changes. These strains typically require more consistent light, often around 18-20 hours per day throughout their life cycle.

Light Intensity and Plant Health

Light intensity is as crucial as the duration. Too little light, and the plant may become “leggy” as it stretches to find a stronger light source. Too much light can burn or bleach the plant, stunting growth.

It’s essential to maintain an optimal light intensity, usually measured in Photosynthetically Active Radiation (PAR). PAR refers to the light spectrum (400nm to 700nm) that plants use for photosynthesis. A PAR meter can help growers ensure their plants are receiving adequate light without causing damage.

Ensuring Uniform Light Distribution

Uniform light distribution ensures all parts of the plant receive equal light, promoting even growth. Reflective walls or using light movers can help spread light more evenly across the canopy. Regularly rotating plants also ensures all sides get equal exposure.


Light plays an integral role in cannabis cultivation. By understanding the plant’s needs at various growth stages and selecting the right type of light, growers can optimize yields and potency. As indoor cultivation technology continues to advance, growers are afforded even more control over their crops, but understanding the basics of light requirements will always remain foundational to success.