Shoplights are designed to operate at 77F, that is the temperature of the tube. At lower temperatures they are less efficient and give off less light. At very low temperatures they won't start at all. Different ballasts and tubes will start at different minimum temperatures, some stop working at around freezing, some are specially designed to start at 0F or lower. Most shoplights will start at freezing or a little below, I haven't really tried them out much colder than that. If you run for long periods and have a fairly enclosed reflector, the temperature of the tube will warm up to a reasonable level.

Avoid T5's they are designed to run warmer than 77F. Compact fluorescents on the other hand I have found to run quite well below freezing.

pietrasa,

The thing to do is just try them and see. I have been amazed in the past at how well some of mine with shoplights did in an unheated garage when I built a plastic tent around the shelves.

John

Standard shoplight you say? What do you mean by standard?

Rapid Start Magnetic Ballasts will usually not start below about 45F. And ABSOLUTELY DO NOT use those 34W tubes, they do not function the best in temeratures below 70F. I have placed 32W T8's in them with great success...nice and bright. However T8's will burn out magnetic ballasts that are the residential 6" long models. Use the standard 9" for the T8's.

I do recommend using Electronic T8 ballasts. Most of them start up at 0F or lower. You can find cheap ones at home depot for about eight bucks.

Yes, avoid T5's...I believe they burn out faster in cold weather.

CF's are good in cold environments.

-j-

WHAT! Avoid T5Âs? They burn out faster in colder temperatures?

This is a very backwards interpretation of the facts of optimum operating temperatures. And to boot  T5Âs are designed to start in extremely low temperatures!

To begin with, the operating temperature is determined by the COLDEST part of the tube while the lamp is operating. If the coldest part of the glass tube is at 55ºF, then that is the determining temperature of the brightness output curve.

Most all families of fluorescent lamps - T12, T8, HO, VHO, with the exception of the T5Âs, are designed to reach their optimum brightness at 77ºF (25ºC). The T5Âs, on the other hand, reach optimum brightness at 95ºF(35ºC). Above or below the optimum design temperature, the light level decreases.

In most fixtures, the lamp eventually heats up until it is above, often WELL above, that optimum temperature. Luminaire manufacturers even began designing their lamp holders with a 3± degree tilt, so as to promote better air flow to CORRECT the problem, which is very common.

The good thing about cold temperatures is that the lamp will usually NOT heat up above the optimum, and will be nice and bright. This is an even BETTER situation in which to use T5Âs, which do get very hot if it is in too enclosed a space. I have seen this demonstrated several times as my light meter shows a peak brightness, then backs off slightly as the lamp goes beyond its optimum temp  both with T8Âs and T5Âs.

It is the T12Âs that have the larger exposed glass surface which have a more difficult time heating to full brightness in cold temperatures.

Also, if you will carefully observe the PRINTED end of like a 54wT5HO, you will notice about an inch of extra glass extention that does NOT contain the tube contents. This is a design element specifically meant to allow you to place THAT end of the lamp at the COLDEST end of the fixture. That end, or the COLD end, is the operating temperature of the T5 lamp. It is there to help keep the T5 from burning as hot as it would  too hot for optimum output.

Another consideration going for T5Âs is that they use mercury amalgam technology instead of liquid mercury. This has allowed manufactures to make smaller diameter lamps with more expensive phosphors, and has widened the temperature range of optimum full output.

The whole problem is that fluorescent lamps universally tend to exceed the optimum brightness temperature in a warm indoor location. Placing lamps in a cooler location PROMOTES optimum output.

You should see my garage when it is cold. Every fluorescent lamp, except my old magnetic ballasted T12, is exceptionally bright!

Johnva has it right.

Zink

The amount of light you'll need is another factor. Fluorescent tube lights work best for seedlings and very small plants, since the light more than a few inches away isn't enough to be useful to light-loving plants, like rosemary. If your plants are beyond the seedling stage and you want to supplement outdoor light, you might try an HID fixture, which cost a couple of hundred dollars but are actually more efficient than fluorescents.

Ask nearby growers what they do for rosemary, since it goes dormant in the winter and may not need supplemental light; venting your greenhouse to keep temperatures below 40-45° F on warm winter days might be better for your plants and budget.