Can You Grow Cannabis With Fluorescent Lights?

Compact fluorescent lights have been around for a while now and were a popular choice for cultivation before the rise of HID and LED lighting. Nowadays we have so many different types of light on the market it can be a bit confusing to know which one suits your cannabis best. In the end, it all comes down to your setup, budget and goals.

You might have seen CFLs being used for cannabis growing, especially during the cloning and vegetative stages. Fluorescent lights help to keep energy costs down, while providing your plants with the right spectrum for healthy growth. This article shows you how CFLs can be a worthwhile addition to your cultivation practice.

What Are Compact Fluorescent Lights (CFLs)?

Compact fluorescent light bulbs can be found in most hardware stores and are commonly used in households as a standard form of lighting. They can also be seen labelled as compact fluorescent lamps, tubes or energy-saving lights. For growing, many CFLs are designed to have a high output, making them extremely useful for small indoor spaces.

CFL lights are usually tubular, and contain mercury which heats up when current passes through it. This reaction emits an ultraviolet light which is converted into visible light via a layer of phosphorescent substance on the inside of the tubing. Normally they have a small built in ballast which turns the electrical current into a usable form of energy.

One of the most common types of fluorescent light used in cannabis cultivation is a long tube that comes in different diameters. The size is normally stated on the end of the bulb using the letter 'T' followed by a number, which indicates the diameter of the tube, in inches.

For example, you might have heard of the widely used T5 fluorescent tube, which has a diameter of 5/8 inches, or 0.625 inches. High output T-series bulbs are available for purchase, and are labelled with 'HO'. These tubes can range from between 24 - 80 watts, which is enough to grow with comfortably.

Bulb-type CFLs are also well suited to growing cannabis. These have been specifically designed for indoor growing and can work extremely well when they are set up correctly. The bulbs are fairly big and normally have a power consumption of up to 250w.

Understanding Colour Temperature

CFLs emit a certain colour temperature, which indicates how warm or cool the shade of light it produces is. The scale used to measure colour temperature is referred to as Kelvin (K). A low Kelvin gives off a warmer, yellow/orange colour and a high reading means the emission has a cooler, blueish tint.

If you're thinking about growing cannabis with CFLs, you'll need a cool light mainly for vegetation and a warm light for flowering. For the vegetative stage it is recommended to use a CFL with a Kelvin rating of around 5500-6700K. More blue light helps to prevent plants from stretching too much.

Adding lights with a lower rating of 2700K is necessary for flowering. At this stage most growers tend to switch their lights to a more powerful, high intensity light such as an HPS (high-pressure sodium), but using CFLs can be done and will still produce great harvests provided their output is high enough.

Below is a chart indicating how different Kelvin ratings can be described in relation to the colour they emit.

Pros and Cons of CFLs

Apart from being affordable and easy to use, CFLs are a great option for beginners looking to have a go at cannabis cultivation. Fluorescent lights provide plenty of advantages when it comes to setting up a grow, but there are some negatives involved which also need to be considered. Before we get into how cannabis can be grown using CFLs, let's take a quick look at some of the pros and cons.

Pros:

• Cheap compared to LED or HID lighting.
• Produce low heat.
• Lights can be placed close to the canopy area.
• Use less electricity, saving you some costs.
• Easy to use for beginners.

Cons:

• They contain mercury which is toxic, making CFLs tricky to dispose of.
• Low lumen output.
• Yields tend to be lower than other types of high output grow lighting.
• Short life span.

CFL grow by pessak from GrowDiaries.

Growing Cannabis With CFLs

Some lights are indeed better than others, and their efficiency depends a lot on what you choose to invest in. However, how you maintain your grow is just as important. Make sure you are using the right light for each stage of growth.

Fluorescent grow lights are predominantly used for clones, seedlings and vegetative plants. Because they are cool and can be placed close to the tops of the plants, the light is used more efficiently and less is wasted. Most CFLs produce blue wavelengths, which imitate the spring or early summer conditions, making them ideal for the earlier stages.

Warmer bulbs can be used for flowering as a way to mimic the reddish light produced at the end of summer/beginning of autumn. The red spectrum promotes flower development and resin production. These can be especially useful if you live in a very hot country and have difficulty controlling temperatures or humidity. 

Growers will often replace their blue lights with red lights, and use the blue lights to give extra exposure where it is needed.

Being able to have the lights so close to the canopy is great for your top buds, but less is able to penetrate it to reach the bottom areas of the plant. This can be solved by placing more lights lower down that point sideways or upwards. 

Royal dwarf / Big easy autos

• Clones - Cuttings perform well under fluorescent lights. T5 tube lights work well because they can be placed very close to the plants. An HID or LED light can be too strong for clones so a cooler option is recommended.
• Seedlings - 125W CFLs bulbs (6500K) can be placed 4-5 inches away from the tops of young plants to prevent them stretching too much. Be careful not to place lights too close as they could still burn your plants.
• Vegetative - Replacing your bulbs with blue 250w fluorescent bulbs can encourage growth while controlling plant height. Your red lights can be used to provide extra lumens if necessary but make sure your are providing plenty of blue light.
• Flowering - For blooming your lights should be emitting mostly orange/red (2700k). They need this spectrum in order to keep producing flowers until they reach full maturity. After flipping your light schedule, use the highest output CFL you can afford and place it as close to the canopy as you can without burning your plants. You may need to add extra CFLs surrounding the plant to make sure the undersides of the canopy also receive light. 

Although you will probably not be able to yield as much using CFLs as you would with a high output light, they can still produce decent harvests. A yield of 0.25-0.3 per watt can be achieved with the correct conditions, which is not bad at all considering the costs.

Tip: If you're planning to grow using CFLs, consider applying stress techniques to help you even out the canopy. This means plants can be placed closer to the lights, allowing them to make better use of the energy emitted. Less light is wasted and plants don't spend energy trying to grow flowers in areas that aren't receiving enough light. 

Conclusion

Compact fluorescents are a perfect way to start off your first grow. You'll find using them can be very beneficial in the early stages as they provide exactly the light your plants need to stay happy. For all those that were doubtful, it is totally possible to use fluorescent lights for growing cannabis, all the way from seed to harvest.

Have you grown using CFLs before? Share your tips with the GrowDiaries community down in the comments section below!

External References

The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.. Medical Cannabis and Cannabinoids. - Magagnini, Gianmaria & Grassi, Gianpaolo & Kotiranta, Stiina. (2018).

Compact fluorescents light the way for energy efficiency. - Fischbach, A.F.. (2004).

This article was updated October 2020.