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Optimal Lights for Cannabis

Lighting for Cannabis Plants

Growing cannabis indoors presents some unique challenges. Like any plant, cannabis plants grow best under natural light. However, it’s perfectly possible to grow high-quality marijuana indoors provided growers are willing to invest some time and money into setting up an optimal lighting system.

Optimal lighting isn’t just a matter of providing enough light. Several other factors also influence not just how large and how fast plants grow but also the quality of buds they produce. This article will offer a comprehensive overview of everything indoor growers should consider before buying a new set of lights.

Light Intensity

Important factors to consider when setting up grow lights include the distance between lights and plants, the color spectrum of light provided, and the intensity of light provided. It’s essential that growers purchase grow lights that are powerful enough to suit their plant’s developmental needs, so intensity of light is a good initial consideration when planning an indoor grow.

Measurements of Intensity

Light intensity can be measured on two scales. Manufacturers list brightness in terms of lumens (lm) or lux (lx). Lumens are a measurement of how much brightness a source emits, while lux equals one lumen per square meter. Since plants can only photosynthesize the light that falls directly on them, most grow lights measure levels using lux, not lumens.

Growers interested in purchasing from manufacturers where the measurement is only listed in lumens can determine how much lux a light provides by using the following equation:

Ev(lx) = 10.76391 × ΦV(lm) / A(ft2)

Where the illuminance Ev in lux (lx) is equal to 10.76391 times the luminous flux ΦV in lumens (lm) divided by the surface area A in square feet (ft²).

Helpful and handy online calculators can simplify these calculations for growers wanting to establish how much lux will be covering their grow space.

Choosing the Right Lights

Determining the amount of optimal lux for a grow space involves considering the light source it aims to mimic: sunlight. The sun provides 5 – 120,000 lux depending on the density of cloud cover, the time of day, and the time of year.

Keep in mind that the movement of the earth means plants grown outdoors are exposed to different amounts of light at different points in their lifecycles, and therefore require indoor lights that mimic the shifting visual spectrum of sunlight. In their vegetative state, cannabis plants thrive when provided with anywhere between 15,000 and 70,000 lux, although 40,000 lux is ideal. During the flowering stage, cannabis plants thrive when provided with anywhere between 35,000 and 85,000 lux, with 60,000 lux being ideal. Growers can purchase two sets of lights for the two stages of growth or they can purchase lights with programmable settings.

Distance from Plants

The importance of a grow light’s brightness and coverage is equal to the adequate distancing between lights and plants. Positioning grow lights appropriately requires careful attention to detail, especially during the plants’ seedling stage. Lights that are too far away will cause the seedlings to “stretch” (grow leggy stems) that will struggle to support the weight of the plant, while placing lights too close to the plants will likely burn the leaves. To be sure, place your hand slightly above the leaves that are closest to the light—if your hand gets too hot, the light is too close to the leaves.

If there are too many plants to do that, here are a few basic guidelines growers can follow to ensure optimal results:

  • 5” to 11” for 150W lights
  • 6” to 13” for 250W lights
  • 8” to 19” for 400W lights
  • 9” to 25” for 600W lights
  • 11” to 31” for 1,000W lights

These guidelines are based on HID lights, which are popular among cannabis growers. Aim for someplace in the middle and pay attention to how the plants respond. If they’re growing abnormally long, skinny stems, move the light closer. If they’re burning, move it further away.

Color Spectrum

Contrary to popular belief, the sun is not actually yellow. Each color from the visual light spectrum is a fragment of white light. It’s in fact the earth’s atmosphere that creates the illusion of a yellow sun.

This seemingly innocuous fact is important for indoor growers because it explains the full spectrum of light outdoor plants receive from the sun, and the miraculously mimicked variable provided by indoor grow lights.

Cannabis plants in the seedling or vegetative stages are most responsive to blue spectrum light. Simply put, there is more blue-spectrum light during spring and early summer when outdoor cannabis plants are developing stems and fanleaves. MH lights are a good option for plants in the seedling and vegetative stages because they naturally provide blue-spectrum light.

When plants enter the flowering stage, they require red-spectrum light. As soon as the plant starts to show signs of flowering, like little white hairs developing at the nodes, growers will want to switch their lights to HPS lights because they provide red-spectrum light, which mimics the seasonal light shifts during late summer and autumn months.

When the sun gets lower in the sky at the end of the growing season, its rays pass through more of the Earth’s atmosphere. The result is that most of the blue light gets scattered, leaving more red light to reach the plants on its surface. HPS lights are best for this part of the season.

Some grow lights can produce light across the entire color spectrum. They can be set to focus on blue light during the earliest phases of growth then changed over to red when the plants mature. They cost more than ordinary MH and HPS lights, but they’re worth the investment since there’s no need to purchase multiple lighting arrays.

Lighting Schedule

Growers need to pay attention to the natural cycle of the sun when they determine how much light to give their plants, as well. Purchasing lights that can be set to a timer makes this much easier.

Marijuana plants can’t be kept under continuous light throughout their entire life cycles because they need around 12 hours of darkness to produce flowers. When the plants are in their seedling stage, they may benefit from consistent, round-the-clock lighting, but that changes over time.

Cannabis plants produce the hormones required to create buds in the dark, not in the light. During the vegetative stage, they need around six hours of darkness each day. This lighting schedule produces an optimal amount of growth without inhibiting the plants’ abilities to flower when the time is right.

When growers are ready to switch their plants over from vegetative to flowering, they can induce flowering by switching their lighting schedules. At this point in the plant’s development, it has reached maturity and needs a full 12 hours of darkness to produce buds and to flower. The plant will stop sending energy to its leaves and stems, so at this point, it won’t get much, if any, larger, but will instead send all its stored and absorbed nutrients to its smokable flowers.

Optimizing Lighting for Maximum Yield and Minimal Cost

One of the great things about following the lighting schedule described above is that it won’t just ensure that the plants have the energy they need to grow without negatively impacting their natural cycles, but will also save growers money. If growers have inefficient grow lights or setups, they won’t see as much of a difference in their energy bills, though.

There are a few simple things growers can do to cut back on energy waste without even considering more expensive, high-efficiency lights. First, make some adjustments to the setup in the grow room by moving the cannabis plants close to the wall and installing reflective materials on nearby surfaces. This ensures that as much as possible of the light winds up hitting the plants’ leaves instead of being absorbed into the grow room’s floor or walls.

Installing reflective materials over bare walls has a few other advantages, as well. It ensures that the light can reach all areas of the plant so that all the leaves, even the ones that would ordinarily be shaded, can photosynthesize efficiently and create more energy to fuel growth. It also helps regulate heat by keeping the lower parts of the garden warmer.

There are a few materials that fit the bill. Most growers use Mylar, a highly reflective polyester film. Those with the extra money to spare often opt for a similar product called foylon instead. Foylon is easier to clean and more durable, which means it can often be used for multiple growing seasons, but it requires a higher initial investment.

Those who don’t have the money to invest in Mylar or foylon sheets can paint the walls flat white or use a specialty paint like Kool Seal Roof Coating that is designed to reflect more light and heat. White poly plastic will also do the trick in temporary grow rooms, but it’s susceptible to heat damage.

Providing Ventilation

Indoor grow lights produce a lot of heat. That may be a benefit in cool climates, where most indoor grows take place, but it can also create an unhealthy environment for the plants that makes them prone to developing fungal and bacterial diseases like powdery mildew and bud rot. Given that these diseases can decimate a crop, most indoor growers are committed to doing everything possible to prevent them. That includes adequate ventilation.

Install both intake and exhaust fans that route to the outside of the room. There’s typically no need to keep these fans running 24 hours a day. Instead, hook them up to a thermostat and set the fans to start running when the temperature begins to climb. The optimal temperature for cannabis is between 68 – 77° F, so that’s a good limit to set fans to.

Intake and exhaust fans will also help to control humidity. When plants take in carbon dioxide from the air, energy from the sun, or in this case the grow lights, and water and nutrients from the soil, they produce two waste products: oxygen and water vapor. The water vapor is expelled through the leaves and into the air, but if the humidity in the grow room is too high, it will have no place to go and the plants will essentially suffocate.

Like light, the optimal humidity for grow rooms varies across the season. In this case, the variation is comparatively minor. In their vegetative stage, cannabis plants like a relative humidity (RH) level of around 40 – 60%. In the flowering stage, the upper limit drops to 50%. In the weeks immediately preceding harvest, RH drops to 45%. Growers should carefully monitor the heat and humidity in their grow rooms, especially while the lights are on.

The Bottom Line

Lighting is by far the most important limiting factor when it comes to growing cannabis indoors. Growers can always install better irrigation systems or change their plants’ feeding schedules as needed, even in the middle of the season, but if they start out with the wrong light array, it will cost a fortune to replace it.

When evaluating options, make sure to keep the factors listed above in mind. Look for lights that have changeable color spectrums, timers that will turn them off when the plants’ “day” is done, and adequate light intensity. It’s also important to come up with a plan for hanging the lights in advance. Some lighting arrays come with everything growers need to hang the lights, while others require a more DIY approach. Invest some time into researching each brand now to avoid investing way too much money into replacing lighting arrays later in the season.

Whichever lighting-technique growers choose, starting with high-quality seeds from a reputable online cannabis seed provider like is recommended if growers want a high level of final product from their grow operation.