Maintaining Optimal Light for your Cannabis Plants

cannabis leaves and stem
December 08, 2020

Light is essential to growth, and everything the plants need, the sun provides. It has the full spectrum of wavelengths that promotes vegetative growth, bud development, and trichome production. Not everyone can grow outdoors, though, for various reasons. Hence, the next best option is to provide artificial lighting. In this article, you are going to learn how to create the most optimal lighting condition that enhances growth and boosts yields.

Optimizing Grow Light Conditions

Generally, the more light provided, the more buds the plants can produce, resulting in high yields. If you’re growing indoors then choosing your lights is very important. But even if you were to use high-wattage HID bulbs, there is no guarantee of an abundant harvest. You also have to address the other factors affecting optimal lighting.

These are:

  • Distance
  • Intensity
  • Color spectrum
  • Light schedule

In the outdoors, distance does not matter if you live in or near the equator. The sun is powerful, able to provide adequate light intensity and all the wavelengths needed for the plants to thrive. Daytime and nighttime are responsible for the frequency or light cycle.

Not all geographical locations are the same, though, and the farther you are from the equator, these factors change. Moreover, the season affects the climate and alters the color spectrum.

If you live in such a place where sunshine is not enough, or the climate is too harsh, then the only way to grow marijuana is indoors. Your plants would have to rely on artificial lighting. Setting up a growing space inside the house costs money. In exchange, you have absolute control over the environmental conditions. One way to boost growth and yields is by tweaking the lighting parameters.

Note: This guide uses HID lights – MH and HPS bulbs.

What Is the Ideal Distance of Light to Marijuana Plants?

If the lamp is too far, light distribution is broader but weak. Hence the plants would not maximize their productivity. They would have to reach upward, resulting in “leggy” stems. As a consequence of excessive stretching, the plant may flop over as it could no longer support its weight. Furthermore, tall plants are the last thing you need, especially when there is limited vertical space. Ideally, you want to keep them short but bushy.

On the other hand, if the light source is too close, the beam would be too narrow. The bud sites in the outer perimeter would not receive enough lighting. Moreover, the heat generated may be too excessive and cause light burn on parts of the plant closest to the lamp.

Optimal Distance of HID Lamps to the Plants:

HID Bulb Closest Distance Ideal Distance Farthest Distance
150W 5″ (13cm) 7″ (18cm) 11″ (28cm)
250W 6″ (15cm) 9″ (23cm) 13″ (33cm)
400W 8″ (20cm) 12″ (30cm) 19″ (48cm)
600W 9″ (23cm) 16″ (41cm) 25″ (64cm)
1000W 11″ (28cm) 21″ (53cm) 31″ (79cm)

The optimal distance of the light source to the plants depends on the type of bulb used. The above table is a general guideline to get you started. You would have to do some tweaking. A quick but excellent way of doing that is to place your palm on top of the upper plant canopy and feel the heat for 10 seconds. It should be warm but not too hot that it is uncomfortable.

How Do You Check and Optimize Light Intensity?

More light equals high yields, but that is not all. There is also light intensity or brightness, which you can measure in two ways using a lux meter.

  • Lumen is the measurement of the flow of light from a source. The higher the lumen, the brighter the source of light.
  • Lux is the measurement of the light’s intensity hitting a surface. Plants can only utilize light that reaches their leaves, which is why most growing guides use lux when talking about light levels.
led grow light schedule

Depending on the distance of the bulb to the plants and the type of reflector hood used, the entire upper canopy may or may not receive adequate lighting. If so, it has something to do with the beam angle. Simply put, the concentration of the light is directed towards the center mass underneath the lamp. Meanwhile, the surrounding areas towards the edge of the grow room receive less light.

Optimal Light Intensity for Marijuana Plants:

Growth Stage Minimum Ideal Maximum
Vegetative ~15,000 lux ~40,000 lux ~70,000 lux
Flowering ~35,000 lux ~60,000 lux ~85,000 lux

A portable digital light meter comes in handy. You can measure the surfaces on different spots on the upper canopy and make the necessary adjustments to stay within the ideal range.

What Is the Ideal Color Spectrum for Optimal Marijuana Growth?

The light spectrum, measured in nanometers (nm), consists of different wavelengths of energy produced by a light source. Each nm represents a band of light energy. The light we see is a part of the spectrum from 380 nm to 780 nm. You perceive each wavelength as a particular color. For example, 400 nm appears as purple.

Marijuana plants react to the color spectrum differently, depending on their growth stage.

During the vegetative stage, your plants need blue light (400 to 500 nm). MH bulb provides that, and the plants respond by growing stems and leaves. They also tend to be squat and leafy.

Once the flowering stage starts, you switch to an HPS bulb. It produces red (~660 nm) and far-red (~730 nm) light that mimics the natural sun at the end of summer. The plants tend to stay short while growing plenty of nodes. These wavelengths encourage the plants to focus on growing buds.

What Is the Best Lighting Schedule?

During the seedling stage, your plants could benefit from 24 hours of light exposure (usually using CFL). Once they enter the vegetative stage, they would need a regular cycle of light and darkness. That is why the most commonly followed light schedule is 18 hours on, 6 hours off. Optionally, you could opt for 20/4.

Once they are ready, you switch to 12/12. The plants perceive this as daytime getting shorter, signifying that their life cycle is coming to an end soon. They would ensure the survival of their species by starting the reproductive phase, producing flowers – the buds you desire. Of utmost importance is ensuring that dark stays dark – no light leak.

Maximizing Light by Preventing Wastage

Even if you have a reflector hood, not all the light reaches the plants. Some go to other directions all over the growing space. There are, of course, ways you can remedy that.

The best method would be to create reflective wall surfaces. You can order a mylar film (1 to 2 mm thick) and cover the grow room’s interior walls. With a grow tent, this is no longer an issue as they already have reflective interior walls. Instead of mylar, you can also opt for foylon, which is more durable and easier to clean. However, it is not as reflective.

Remember All the Factors Involved in Optimizing Light

As you have read, providing light is essential but needs some tweaking to help the plants attain their full potential. You have to make sure that the HID lamps are not too far or too close, and that the upper canopy ideally should receive an equal distribution of light. Moreover, you want to use the correct type of lamps – MH for the vegetative stage, and HPS for flowering. Follow the recommended light cycle specifically, and you will have boosted the growth rate and bud production. The result is a bountiful harvest of cannabinoid-rich buds.