Lighting Considerations for Growing Cannabis in a Greenhouse

Many growers worldwide, whether they are growing tomatoes or cannabis, prefer to grow their crops in a greenhouse. That’s because it is a terrific way to protect your plant from the environmental elements while using sunlight, saving you a lot on expensive indoor growth setups.

But if you live in a region where daylight hours tend to be short during some months of the growing season, or if you want to extend your growing season, you may have to use supplemental grow lights to compensate for the lack of sunlight.

Many people wonder if the lighting is even necessary for a greenhouse. And, then, you have to consider whether you're growing photoperiod or autoflowers. Whatever it is, you gotta understand what to do. 

In this article, learn what factors you need to take into account while fixing supplemental grow lights in your cannabis greenhouse so that you can get the best out of your cannabis plant. 

Do You Need Supplemental Lights to Grow Cannabis in a Greenhouse?

A greenhouse allows you to make the most of the natural resources while relying less on other elements; however, you may be short on sunlight at times. If you live in a usually bright location, it's fine, but not everyone enjoys the same weather. 

In such cases, you need supplemental lighting, and it is exactly what it sounds like — supplemental lights are fixed in a greenhouse to complement the natural light from the sun. 

Supplemental lights in a greenhouse offer some benefits that many growers can take advantage of, such as the following:

Longer or Year-round Growing Season

Outdoor cannabis growth depends on seasons, and in most regions, that season is adequate to take the plant from a seedling to maturity. But if your region has a shorter growing season, or if you want to extend your growing season, you must add supplemental lights.

For instance, with supplemental lighting, you can give your plant a headstart by starting its growth in early spring, when the winter has just departed. Similarly, you can extend the growing season well into late autumn (or even further), giving your plants enough time to reach their maturity. 

To Improve Yields

The second biggest benefit of supplemental lights is that they can improve your yields. Sure, your plant will grow just fine with sunlight, but more light is always better for cannabis — to an extent. 

But don’t worry, adding supplemental lights does not defeat the purpose of your greenhouse. You are merely complementing the sunlight, not investing in full-fledged grow light setups that will set you back by a thousand dollars.

And as the ol’ 1% gardening rule goes — you can increase your yield by around 1% by adding 1% more light to your greenhouse. This rule stays true to cannabis, too!

Makeup for the Greenhouse Glazing

Some greenhouse glazing may not transmit as much sunlight into the greenhouse due to dirt, material thickness, or opacity. In such cases, adding supplemental lights can help bring the light intensity to the optimal levels, protecting your plants from the lack of light.

Light Intensity and Exposure

When going about installing supplemental lights in your greenhouse, the first consideration must always be to calculate the light exposure and local light availability. This is the first step upon which the following steps rely. 

This is because, during the beginning or end of the growing season, the sunlight may not be as strong, resulting in low light exposure. This is especially true for regions with less than 4 to 5 hours of light per day.

Also, some regions are prone to more cloudy days during some months, requiring supplemental lights.

Your plant needs 18 hours of light each day during the vegetative phase and 12 hours of light during the flowering season. So, if your region’s daylight hours are short at any time, supplemental lights come into play to provide enough light for your plant to grow well. 

Measuring Sunlight in Your Greenhouse

The first thing you must do is measure the sunlight in your greenhouse. Your greenhouse probably already has a lot of obstructions that may limit sunlight, such as:

• Greenhouse glazing transmission rates — may vary from 67% in dual-polyethylene to 92% in single pane glass
• Struts, trusses, and other structural pieces
• Fans, lights, and other fixtures

Here, the glazing materials are also generally never clean, leading to lower visible transmittance (VT). 

Due to these obstructions, most greenhouse canopies only get 50% to 65% of natural sunlight. And this is something you must account for and aim to counteract with your supplemental lights.

To know this, start by reading the glazing specifications to know their VT rating and choose greenhouse-specific equipment that is placed strategically to allow as much sunlight as possible.

Daily Light Integral (DLI)

The next thing you need to focus on is DLI, which is the quantity of photosynthetic active radiation (PAR) — the photons used by plants — reaching the plant within one day. DLI is measured in moles.

DLI is essential to measure how much light you need to supplement when enough sunlight is not reaching the canopy, like during cloudy days or shorter daylight hours.

However, unlike indoor grows, where you are providing consistent light to your plant throughout the season, DLI is crucial for greenhouses because the light fluctuates constantly with climate. 

To account for this, you can search on the internet about your local DLI. In most places in Europe and America, you can find the DLI ranges easily. Take a note of the lowest and highest DLI during each month and keep them handy. 

Calculating Your Plant’s Light Needs

Once you have the DLI your local region gets, the next thing to do is calculate how much light you need to supplement. To do so, reduce natural DLI (monthly) by your greenhouse’s VT rating.

For example, if your town receives a DLI of 30 mols/sq. meter/day, and your glazing VT is 66%, you need to supplement 20% more DLI to your plant with the help of artificial grow lights to reach the target of 35 moles (flowering plant).

Here are the ideal canopy-DLI ranges you should stick to for cannabis:

• Seedling or clone stage: 15 to 20 moles/sq. meter/day
• Vegetative stage: 20 to 40 moles/sq. meter/day
• Flowering stage: 25 to 50 moles/sq. meter/day

The numbers keep increasing because cannabis requires more light the bigger it grows. More on this below.

Once you have the natural DLI range and supplemental DLI requirement, you then have to find light fixtures that offer to bridge the gap. For this, you may have to consider measuring the PPFD.

PPFD stands for photosynthetic photon flux density, which is the number of PAR photons hitting a specific surface in a second. You can easily measure the PPFD using a PPFD meter, or use the product specifications to know about it.

Measuring PPFD tells you if your canopy is receiving adequate light to grow well. For the best growth, your plant needs a PPFD of:

• During the vegetative stage: 308 to 617 μmol (18 hours)
• During the flowering stage: 462 to 926 μmol (12 hours)

And to convert, you can simply use one of the online PPFD to DLI calculators. 

So, to sum it up, first understand your natural DLI, followed by deducting it with the glazing’s VT percentage. Next, take the difference, and convert it into PPFD, which is what you have to supplement with your artificial grow lights. Lastly, use a PPFD to meter to ensure your canopy is receiving enough light.

Increasing Light Intensity for Bloom

When your plant enters the flowering stage, it will require more intense light. While doing so, you do not want to be progressive. Slowly increasing the light may not make a difference if the daylight hours are shorter — your plant will not get more light than it did during the vegetative stage.

Instead, we recommend directly starting at the recommended light intensity as soon as you decide to switch your plant from the vegetative stage to the flowering stage.

For example, if you are giving your plant 500 PPFD during the vegetative stage, switch directly to 700 PPFD when you switch the light cycle from 18/6 to 12/12. 

Creating Lighting Schedules

Creating a lighting schedule for an indoor cannabis plant is straightforward. 18 hours of light during the vegetative stage and 12 hours of the same during the flowering stage. The process gets tricky when you are growing cannabis in a greenhouse because you are supplementing the sun’s natural light, which keeps fluctuating. 

However, there is not a preset solution to this that you can use. Instead, you have to calculate the natural DLI, glazing VT, required supplemental DLI/PPFD, and daylight hours. Measuring these will give you an idea of how long, on average, you need to keep your supplemental lights on. It may either be an hour at dusk or four hours divided between dawn and dusk.

Light Spectrum

Light behaves as a wave, and it consists of spectrums of different wavelengths, i.e., visible colors. And each of these wavelengths has a specific effect on the plant, with some being beneficial while others being useless. 

Here, natural sunlight is the best for plants because it contains the necessary spectrums of light, i.e., the color of the light. But when you are supplementing sunlight with artificial grow lights, you need to pick lights with the right kind of color spectrum so your plant can grow faster and bigger.

Here are the prominent light spectrums required for cannabis during each of its growth stages:

• Seedling stage: blue-dominant light
• Vegetative stage: blue-dominant light
• Flowering stage: red to far-red dominant light

Natural sunlight has all these spectrums naturally, so no need to worry. But when using supplemental lighting, you need to pick the right lights. For example, you can follow indoor growers and use MH lights during the seedling and vegetative stages since it is blue-dominant and switch to HPS lights during bloom since they produce red-dominant light. 

But doing so is only recommended for commercial setups; for hobby growers, that is a lot of effort and investment. Instead, you can simply choose a broad-spectrum light that produces white light. White light has a good balance of red, green, and blue light spectrums, which mimic the sun’s light. 

Or you can choose LED lights that can be configured to produce a particular light spectrum easily using a controller. 

Blackout Curtains

One of the most important, yet underrated, aspect of lighting for greenhouse cannabis is the lack of it during the night hours. So, while you look at finding the best light panels for your plant, you also need to focus on blacking out the greenhouse during the night hours, so no light penetrates within. 

Blackout curtains are essential during the flowering stage to keep the plants from re-vegging or turning into hermaphrodites. This occurs when even a little bit of light falls on the plant during the dark hours, tricking it into going back into its vegetative stage.

By light proofing your greenhouse during the dark hours, you can rest assured that your plants are re-vegging.

Also, greenhouses tend to cause a lot of light pollution in the vicinity. So, even if your plant is not in its flowering stage, using blackout curtains will make you a good neighbor. 

Many growers are not ready to invest in blackout curtains, but it is well worth the effort. Blackout curtains offer many benefits apart from just protecting your plants from re-vegging or not annoying your neighbors. 

Blackout curtains can improve your greenhouse’s insulation, which reduces the heat transfer, keeping your precious plants warm during the night. Heat loss is a major problem in greenhouses in cold regions, and you can tackle this with blackout curtains. 

Other Plant Requirements w.r.t. Supplemental Light

Apart from light and darkness, your plant requires the right levels of relative humidity, temperature, water, and nutrients. 

So, when you add supplemental lights to your greenhouse, factor in your plant’s requirements for these things, too. For instance, if your plant gets more light, it will consume more nutrients and water. So, you need to rework your feeding schedule.

Next, supplemental lights can affect your greenhouse’s humidity and temperature. 

To tackle the temperature difference, keep an eye on the greenhouse temperature with a thermometer and choose lights that don’t heat up too much. Greenhouses tend to be warmer than the external climate, and you don’t want them to heat up further. 

On the other hand, your greenhouse may turn too humid due to the packed conditions and improved transpiration. Take the right measures to limit humidity fluctuations, too. 

Supplemental Light Spacing and Positioning

Supplemental lights are not the primary grow lights, so positioning them is significantly different from that of indoor grow lights. Greenhouses usually use half of the grow lights required by an indoor grow room, and often fewer. 

So, in a greenhouse, lights are best spread further apart and positioned in a way that does not disrupt the natural light. 

Supplemental grow lights, or even regular grow lights, come in a variety of shapes and sizes. You must pick the lights that are small but produce adequate light so that you can supplement enough light without blocking out the natural light. 

Depending on your grow room setup, you need to discuss with other growers and research which light panels will work best for you. By optimizing the right size, shape, and height of your light, you can offer adequate light while minimizing the light’s shadow footprint. 

Types of Supplemental Light to Grow Cannabis in a Greenhouse

The next step is deciding which type of light is the best for your greenhouse, and this depends on your DLI requirements, preferences, and budget. Here are some of the most popular light types:

Fluorescent (CFL) Lights 

These are the most common and affordable grow lights in the market, and you can buy these even at your local hardware store. They cost a few dollars and tend to last around a year, so if you are a new grower, CFL lights would work well for you.

You can even get spectrum-specific CLF lights, like the ones with 6,500K for the vegetative stage and 2,700K for bloom. 

The benefits of CFL lights are that they are inexpensive, readily available, and don’t require any specialized equipment to run. And they are fairly energy-efficient and don’t heat up a lot. 

The downside, however, is that CFL lights have a low light output and may not be enough for your plant’s DLI needs.

High-Intensity Discharge (HID) Lights

HID lights include metal halide or high-pressure sodium (HPS) lamps, and they are the most popular form of grow lights in the cannabis community. 

Here, MH lights offer a wide spectrum of light but aren’t as energy-efficient, whereas HPS lights come in all kinds of specifications to fit most growers’ needs. 

The benefits of HID lights are they are not as costly as LED lights, are easy to set up and use, and produce intense light that can easily meet your plant’s DLI needs. Plus, they are highly reliable and produce consistent output for a long time.

However, they have their own downsides. HID lights burn bright and burn your plant, so you need to take extra measures to prevent that. Plus, they require a ballast and reflector to operate. Lastly, they consume a lot of energy, so they aren’t the most energy-efficient or eco-friendly. 

Light Emitting Diodes (LED)

LED lights are the future of grow lights and for the right reasons. LED lights are capable of producing a wide spectrum of light with excellent distribution, and you can even customize their light spectrums as per your needs. 

Another benefit of LED lights is that they have an ultra-long life (of up to 11 years) and they consume 70% less energy than conventional light forms. Plus, unlike HID lights, they don’t heat up a lot or cause a light burn to your plant. 

The downside of LED lights is that high-quality panels can cost you a lot of money. Plus, there are no industry standards, so you have to do more research before choosing your ideal LED light. 

Also, LED lights do not have the same output as HID lights, the latter is superior in that regard. But thanks to the recent advancements in LED technology, the gap between the two is closing in. Within a few years, LED grow lights will be on par with HID lights in terms of output. 

LEC or CMH Lights

Lastly, you can also use ceramic metal halide (CMH) lights, which are similar to HID lamps but use a ceramic arc. CMH lights are known for producing a natural color spectrum, which is what you want from your supplemental lights. They can also produce UV-B light, which is beneficial for trichome production.

CMH lights also last longer while being more energy-efficient than MH lights. Yes, they can be a bit expensive, but they can still last twice as long as HID lights while being easy to set up and operate. 

The downside with CMH lights is that you need to wear eyes and skin protection because UV-B light is not safe for humans, and they produce a lot of heat, which can drive up your greenhouse’s temperature. Plus, they often aren’t as powerful as HID lights. 

Summary: Lighting Considerations for Growing Cannabis in a Greenhouse

According to the 2020 study by Cannabis Business Times, State of the Cannabis Lighting Market, most growers grow cannabis indoors (85%). However, more and more growers are using greenhouses to cultivate cannabis all year round, with 29% of them indicating that they use supplemental lights in their greenhouses. 

No doubt, supplemental lights offer a variety of benefits to greenhouses, and you can take advantage of those with the right strategies, as listed above. Use this guide and get the best out of your cannabis easily. 

Just remember, not a lot changes between indoor lighting to greenhouse lighting — the fundamentals remain the same — only, this time you have to factor in the sunlight and take measures to only supplement it, not replace it. 

Light is crucial for your plant in any form it gets, so why not give your plant some extra much-needed light with supplemental lights?