Growing Weed With CFLs

Sophia Delphi May 12, 2022 - 8 min read
Fact Checked
Image of CFL lights growing weeds

When you decide to set up your first indoor grow room, the most intimidating purchase facing you is grow lights.

Most of the accessories you’ll need are pretty easy to deal with. Pots, tables, fans, heaters, air filters, racks, shears, and timers are all items you’re familiar with. Even grow tents (if you’re going to use one), hygrometers, and humidifiers (or dehumidifiers) are pretty easy to figure out.

Grow lights are a different matter. LEDs? HIDs? T5s? You need to learn a new language before you can begin to consider the alternatives.

For most beginners, the right answer is CFLs. That stands for compact fluorescent lights, which are inexpensive, widely available, and easy to use. CFLs may not be the best choice for commercial crops, but they’re ideal for home growers.

Let’s shed some light on the subject with a CFL grow guide.

Types of Grow Lights

You can’t use household incandescent bulbs to grow weed. The light they produce isn’t conducive to growth and they put out too much heat. Household fluorescent lights are better, but not by much; they simply don’t put enough light.

That means you need to grow lights for the plants you grow indoors, and CFLs are the easiest choice. The best way to understand the advantages of using compact fluorescent lights in a grow room is to look at the other possibilities.


High-intensity discharge lights are commonly seen in commercial grow rooms. There are several different types, including light-emitting ceramic (LEC), ceramic metal halide (CMH), metal halide (MH), and high-pressure sodium (HPS).

HIDs all run very hot, they require additional equipment, they’re complicated to install and operate, and they’re expensive.


These are fluorescent lights often used in grow rooms. (T5 refers to their size).

T5s don’t produce much heat, they’re energy-efficient and they’re easier to use, but they’re large and difficult to use in small spaces. You also need to change the color of the bulbs regularly as plants progress through their growth stages.


You’re familiar with LEDs (light-emitting diodes), of course. When they’re used in grow lights, LEDs generate light that plants love, they run very cool, and they’re easy to use. LEDs are also valued for their ability to produce higher-quality and more bountiful crops.

However, LEDs need to be placed more than a foot away from plants, and that’s not easy to do in a small grow room. They’re also an expensive option.

That leaves compact fluorescent lights — CFLs.

All About CFLs

Compact fluorescents lights create ultraviolet rays when electricity is run through mercury vapor and argon gas. When those rays hit the coating on the inside of a CFL, the bulb generates visible light that is in the proper spectrum to grow marijuana plants.

You’d recognize CFLs if you saw them. They’re the weird-looking bulbs that have a small circle of fluorescent tubing running out of a regular light bulb socket; they were a big deal in the 1990s and early in the 2000s.

CFLs are — no insult intended to anyone — virtually idiot-proof. You screw them into any regular lamp that takes normal incandescent light bulbs, and they work. You can buy them in hardware stores, you can use them in small grow rooms, and they’re available in different colors for different plant growth stages.

And while CFLs use somewhat more electricity than LEDs, they’re much, much cheaper to buy.

One caution, though. CFLs are eventually going to disappear. They’ve been largely replaced by LEDs in general usage, some states have banned them and many companies have stopped making them. If you rely on them for your grow room, stock up now.

How to Use CFLs in a Grow Room

The primary considerations when using CFLs are the size of the plants and the wattage and color of the bulbs.

CFLs and Plant Size

First things first: the effective range of light from CFLs is about 8-12 inches. That means you have to place them close to your plants, so they’re best used for plants that don’t grow higher than three or four feet tall. They’re perfect for most indica strains and indica-dominant hybrids, but you should train or trim your sativas to grow out instead of up if you use CFLs.

How close should you place these grow lights? If possible, be as close to the plants as possible without burning them. 2-4 inches is a good range in most grow rooms. After positioning your lights, put your hand between the light and the plants; if it feels warm but not hot, you’ve found the right spot.

There’s one other consideration, though. Since weed plants grow quickly, the position of your lights must constantly be changed. If you don’t visit the grow room at least once every couple of days, keep the lights further away so the plants don’t “grow into” the lights and get burned.

CFLs and Wattage

You have to be careful when buying CFLs because there are two different wattages listed on the package. These bulbs are energy-efficient, so a package may be labeled both “13 watts” and “60-watt equivalent.” That means the bulb only uses 13 watts of power but emits as much light as a 60-watt incandescent bulb. The latter number is the one that matters.

During the vegetative stage, each plant in your grow should receive 80-100 watts of light, which means using two 40-watt equivalents or one 100-watt equivalent bulb.

During flowering, the more light a plant gets during the day, the higher its yield. So adding another 100 watts of light per plant is the best way to coax the most flower from your grow. (You should, of course, be giving them 18 hours of light per day until the flowering stage, and reducing it to 12 hours from that point on.)

How do you add light? The easiest approach is to use splitters, which are essentially cube taps for lights. They let you screw two or four bulbs into each socket and aim them in the desired direction.

CFLs and Color

You can purchase CFLs that emit two “colors” of light, daylight (sometimes labeled daytime or warm white) and soft white. If you’ve used these bulbs in your home, you’ve probably chosen the soft white ones because their light isn’t as harsh.

The best approach in a grow room is to use both colors but at different plant growth stages. Cannabis plants love sunlight during the vegetative stage, so daylight CFLs are the right choice until it’s time for the plants to flower. At that point, the daylight CFLs should be replaced with cool white ones, which come closer to replicating late summer and autumn light conditions.

Changing bulb color isn’t mandatory, but it’s definitely a good idea. CFLs are known to produce smaller harvests than any other type of grow room light, so it makes sense to give your plants all the help you can.

Most home growers eventually “grow out” of CFLs, deciding to take the financial plunge and move up to LEDs or HIDs. As long as you can still find CFLs, though, they’re a very good, cost-effective option for growing a small weed crop indoors.

CFL Grow Guide: FAQ

Q: Aren’t they taking CFLs off the market because they’re dangerous?
A: That’s one of the supposed reasons, but it’s overstated. CFLs contain a little mercury, so they could theoretically be hazardous if they break, and they require special disposal (you can’t just throw them in the trash). However, the same could be said for all fluorescent bulbs; if they break, be careful. The real reasons they’re disappearing are that they’re much less energy-efficient than LEDs — and that when they were pushed as replacements for standard incandescent bulbs, the market overwhelmingly rejected them.

Q: Doesn’t having grow lights so close to cannabis plants harm them?
A: Not unless they’re so close that the leaves or bud are burnt. The bigger danger is not providing them with enough light. It’s important to watch the plants as they grow larger, to make sure there aren’t “dead” spots in your lighting setup. If there are areas in your grow that are dark or more than 6-8 inches away from a CFL, it’s time to add more lights.


Kondro, W. (2007). Mercury disposal sole health concern with fluorescent lights. [1] [2]