CFL, or compact Fluorescent Lamps, have reputation for being very energy efficient and having long service lives. In some cases they can be used effectively in an indoor growing situation, although you do need to be conscious of the fact that they do not cover the red-spectrum of light very well.
Metal halide, or HID lamps are a different story – they are more expensive to run (and buy), but produce a much more powerful light that is excellent for growth to maturity. Metal halide lumens per watt are quite high, but keep in mind that lumens really is only a measure of light we perceive as human beings. Using cfl and metal halide (HID) lamps in conjunction can be a great way to lower overall indoor growing costs.
Here is a basic break down of cfl lumens per watt from common lamps that you can use to work out how many lamps you will need for your purposes.
9–13 watt cfl lamp = approx 440 lumens
13–15 watt cfl lamp = approx 750 lumens
18–25 watt cfl lamp = approx 1,050 lumens
23–30 watt cfl lamp = approx 1,500 lumens
30–52 watt cfl lamp = approx 2,500 lumens
The HID or metal halide lumens per watt range is between 60 and 110. Therefore a few common lamp values would be:
100 watt HID = 6000 – 11,000 lumens
250 watt HID = 15,000 – 27,500 lumens
600 watt HID = 36,000 – 66,000 lumens
In theory, a single 9w cfl should then provide enough energy to grow about 1sq foot of small plants, or one medium sized plant. In practice however, you will find that although some leaf growth occurs, stems will remain thin, and buds, flowers, and fruits will not form very well. A good compromise can be to use cfl’s for early growth, where the cfl lumens per watt ratio gives you an advantage, and then switch to an incandescent or small HID lamp as the plant begins to mature.
As you can see the kind of light that you get from HID systems is enormous using the 1/60-110 figure of metal halide lumens per watt – in fact this can be way to much for small setups. Unfortunately HID lamps don’t often come in sizes smaller than 150 watt, so incandescent may be a better alternative.
And hey – if cfl is all you have, then just use them. They are cheap and safe to run, so can be installed almost anywhere. Of course LED grow lights are quite possibly the best low cost-to-run option, but you are looking at a much higher initial purchase cost which probably is not worthwhile when you are only growing a few dollars worth of herbs or small vegetables.
Generally I find lettuce and Italian parsley grow really well under cfl only light, and other greens like chard can also be raised to salad dish size.
Here are a few tips to get the most cfl lumens per watt from your low cost indoor salad or herb growing system:
1. use a reflector. This saves that wasted light, concentrating it on the plant(s)
2. Hang the cfl low over the plant. The low heat output means you will not scorch leaves, and by reducing distance you effectively increase your cfl lumens per watt even further. Anything down to 2 inches clearance is OK.
Use a single high-watt rated bulb rather than several lower ones. A single 50w cfl will provide more penetrating light than 5x 10w bulbs.