If I were you, I would mount two 26 watt CFL bulbs per shelf, horizontally to increase usable vertical distance. A 26 watt CFL bulb costs as little as $1 and I guess a horizontally mountable socket would cost around $5. Also, paint the shelf ceilings white and cover the sides with aluminized mylar.

Stuwwelpeter, I'm not sure I understand the set up you are suggesting. Do you mean 1 twin light fixture t-12 horizontally under the shelf?

Do you mean 1 twin light fixture t-12 horizontally under the shelf?

No, get these:

and cover the ends of the shelf structure with thin (3/8"?) plywood and mount the sockets on the plywood so that the bulb is about 0.25" from shelf ceiling. That bulb is 2.25" in diameter so it will only use 2.5" of vertical space.

Don't use those unless you plan on growing one plant per bulb. They might be cheaper, but definitely not more efficient. The reason is fluorescent light has little effect on plant growth past 6-8". You cover more surface area with strip lights. Especially when starting seedlings in a 10x20" tray. I would buy 2 sets of 2'- 2 strip T-5 fluorescents for each seed tray. That's a total of 4 tubes. You could get away with one set(2 tubes), it depends how healthy you want your plants to be. The T-5's put off more lumens per watt. They cost $25 for a 2 strip light or $17 for a single strip light and sell at Home Depot. When you compare it to the cheapest 2' 4-light system at $120, this seems pretty reasonable. If you want to get fancy, buy a 2' piece of shiny metal ducting, for a reflector. It also sells at Home Depot for around $3. Hope this helps.

urban,

I used four of those bulbs in a box and they were enough to grow 3 1020 nursery flats of tomato and pepper seedlings (54 plants in 3" containers).

Mike

CFL's have their place, and I use them occasionally, but they will never be as efficient as linear fluorescents. If you're growing seedlings on a shelf, that's a perfect situation for linear fluorescents. T5's would be best as urban suggested, but they tend to be expensive and I've never seen them at my local Home Depot. I'd go with a T8 fixture with some good reflectors that wrap around the bulbs, so that you're directing most of the light down where it's needed.

They might be cheaper, but definitely not more efficient.

The CFL in my picture is 1650 lumen/23 watts = 71.7 lumen/watt and costs $1 whereas the T5 here (S20852) is 1750/24 = 72.9 lumen/watt and cost $9.75. However, although I might be wrong, I believe the CFL rating includes the power consumption of the ballast, whereas the power rating of the T5 excludes it. If I assume the ballast uses 10% of the power and include it in the calculation, then that T5 has 1750/(1.1*24) = 66.3 lumen/watt which is less efficient than my CFL.

To each their own, I guess. I used 105-watt CFLs last year in my grow chamber and over a bench and was able to cover 12 sq. ft. with each bulb.

Mike

Well it may not be the definitive comparison, but here's the bulb I use along with the T8 I suggested and a comparable CFL bulb (same brand and color temperature).

4' 54W 6500K T5 that I use: 92.6 lumen/watt
2' 17W 5000K T8: 82.4 lumen/watt
42W 6500K CFL: 66.7 lumen/watt (74.1 if you subtract 10% ballast wattage)

I suggested T8's because they are cheap, widely available, efficient, and would cover the area very evenly. CFL's are typically a bit less efficient, and the bulb shape results in additional reflective losses. I like them for spotlights and small spaces, but I wouldn't use them to light a shelf.

I like them for spotlights and small spaces, but I wouldn't use them to light a shelf.

A 2 X 1.5 ft. shelf is a small space.

Jamsey40,

If you can get the sockets cheap enough, you should install more sockets than you intend to use. That way you can just screw in more light bulbs in case you need more light later.

Furthermore, retrofit LED bulbs are beginning to become available that are twice as efficient as CFL. Maybe, when these become cheap, you will want to screw in such LED bulbs.

Thanks all for your input. I definitely have some options to consider.

I agree with penfold2. The T-8's are cheaper and will work just fine for propagation.

Keep in mind that no light is very effective or efficient unless it has a reflector to direct the light. I realize that you are dealing with a space issue, but if that were not an issue then the 4' florescents should be a much better option. Two fixtures (built in reflectors) $20. Four bulbs $10. The CFL lights would be fine for one or two plants, but if you are planting a seed tray then IMO they would not be what you need. They would be more difficult to mount, need reflectors (more money), not designed to cover a square area, difficult to raise/lower, more costly for initial setup, etc. The florescent setups provide plenty of light for seed starting and more wattage is not necessary.