I get this question a lot.
Everybody wants to know how many CFL bulbs are needed per plant.
But how can you answer that without a little more info?
I mean, how large is the plant?
What is the wattage on the bulbs?
In order to accurately tell you how many CFLs for one plant, I need to know those two variables at least.
That said, I can still help, even without them.
You’ll need to do a little (very simple) math on your own, but together, we can figure out exactly how many CFL bulbs you need to light your garden.
Let’s begin by figuring out what you’ll need to light one plant, since everyone always asks about the lighting needs per plant.
How Many CFLs For One Plant
As mentioned above, plants vary greatly in size.
For that reason, we need to use a standard measurement to determine lighting needs. The easiest to use is the square foot.
When using CFL bulbs to grow a plant with a high light requirement (like cannabis), you want a minimum of 60 watts per square foot.
Ideally, you want 100 watts per square foot.
And it doesn’t hurt to go higher (though there’s not much point going over 150w). You really can’t give your plants too much CFL light.
Those numbers are for the flowering stage. For the vegging stage (or for seedlings or clones), you could cut that in half. I’d shoot for around 60 watts though. The same goes for plants with a lower light requirement.
Actual Watts Or Equivalent Watts?
Compact fluorescent lamps usually have two wattages listed on the packaging: one is the actual wattage that the bulb consumes and the other is the incandescent equivalent.
For example, a CFL that is equivalent to a 100 watt incandescent bulb will actually use around 23 to 28 watts of power.
When I say you want to aim for 100 watts per square foot, I am referring to the actual wattage.
So, to continue with the bulb from the example: you would want 4 of these bulbs, not one of them. The actual wattage counts.
If you have two larger plants that each take up 3 square feet of space, you would want 300 watts per plant, or 600 watts total. That would mean 24 of those 23-28 watt bulbs from the example.
Note: once you have more than a plant or two, it makes more sense to get a T5 fixture or a cheap LED light. CFLs are great for a small grow, but once you get up to 10 (or even 5, in my opinion) bulbs, it just becomes too much work to keep readjusting the lights, due to the rapid plant growth.
Where Do I Put The Compact Fluorescent Bulbs?
Ideally you want to surround your plant(s) with CFL bulbs, so that they get light from every side. Most people asking these questions are looking to grow marijuana, so let’s focus on that plant for a bit.
First make sure you plant grows the right way
You can choose how your plant grows. Weed plants grow upward into a Christmas tree shape naturally, but that is not good for us. This results in one large bud at the top and several smaller buds below.
You want to force your plant to grow short and fat. That way they are more flat on top and can grow multiple large buds. You can get them to grow this way by bending and tying down various stems to get them to grow outward.
If you force the main stem to grow horizontally from the beginning, other stems will grow upward from it, creating a shorter and fatter plant naturally. You’ll find much more on training your plants to grow the way you want here.
Then put your bulbs in the right spots
If you have a short and fat plant, there is no need to hang bulbs below the canopy. Hang them all above, with some off to the side and a bit lower, so that they are next to the top of the plant.
Anywhere you see a shadow area, put a bulb there. If you do not have enough bulbs to fill all the shadows, that means you need more bulbs.
CFL bulbs can generally hang fairly close to your plants, but this does depend on their wattage. A 20 to 30 watt bulb can hang within a few inches.
You can always use the “back of the hand test” to see if your bulb is too close. Hold you hand where the plant is, with the back of your hand facing the bulb. Keep it there for 30 seconds. If your hand gets too hot, then it will also be too hot for the plant. Back the bulb off a bit.
Weed plants grow rapidly, so keep a close watch to make sure they don’t grow into the bulbs and get burned. They can grow several inches in a day, so if you won’t be around to keep an eye on them, make sure to position the bulbs a bit further away.
If you have a plant that is growing tall, you may want to get some light on it from beneath as well, or at least from the sides. This will depend on the plant, however. If you are growing cannabis, you will get a much better yield by not growing tall plants, so I highly recommend keeping them short and fat.
Do I Need Special Grow Bulbs?
No, you do not.
In fact, I would encourage you not to buy special CFL grow bulbs. They are generally much more powerful, so you would only have one or two above the plant, ideally in a reflector.
The problem is that fluorescent bulbs do not have great penetration. That is why you hang them so close to the pants. With fewer powerful bulbs, you have to hang them further away and are more likely to get shadows. More bulbs that are less powerful allow you to hang them close and to completely bathe your plant in light.
In addition, bulbs that are specifically labeled as grow lights are almost always overpriced. Regular old fluorescent bulbs are every bit as good and do exactly the same thing, but they cost far less.
The main thing to look out for is the color temperature. There are cool white bulbs that have a color temperature around 6000K to 7000K (‘K’ stands for Kelvin), warm white bulbs with a color temperature around 2000K to 3000K, and neutral bulbs with a color temperature in the middle, around 3500K to 5500K.
The cool bulbs (with a higher color temperature) are better for vegging, since they contain more blue light in their spectrum and the warmer bulbs are better for flowering, since they contain more red.
You best bet would be to have a ratio of 2 cool to 1 warm for vegging, then add a few more warm ones for flowering. That way you also add more wattage for flowering and plants need more wattage then than they do during vegging anyway.
Where do I get CFL bulbs?
You can buy CFL bulbs pretty much anywhere. If you want to get them online, Amazon has some good options. Their value packs are especially good deals. You can get all the bulbs you need in one pack.
If you don’t have light fixtures for these bulbs (they fit in any standard lamp fixture), you can simply buy cords with a socket on the end like these.
They are extremely flexible and let you mount the lamps anywhere you like and in any position you like. Alternatively, you could put them in a reflector (or just rig up your own), to direct more of the light in the direction you want, i.e. at your plants.