Cacti are genetically adapted to a high level of light intensity. They have evolved to desert conditions with a bright sun. The fluorescents you have are not very bright, you can put them close right away. The top part of the plant should be within 3" of the tubes. I would also supplement with sunlight from the window or add another light.

For a single plant, I think the 25 watt CMH with parabolic bulb is best; for this light, keep a distance 10", about 4300 fc, probably even closer. This light retails for about $100 with fixture.

I would set the time to 20 hours/day of light on. Do not move the plant, as this will interrupt root development of cacti.

Lermer still spamming the group? Please don't shell out $100 for a bulb that performs worse as a plant light than a $2 spiral from HD.

How long are your fluorescent tubes? And hence how powerful? Long straight tubes are not well suited to lighting a single barrel cactus. They're 2'-4' long and the barrel is a little round blob :) What kind of reflector do you have? I'm guessing just the standard batten that came with the shoplight? You can place that almost touching the cactus. Of course now you have nicely lighted the top of the cactus but the sides are in the dark.

I am going to make a radical suggestion, but if you ask around a bit amongst cactus growers you will find it makes sense. Turn off the lights because you can't grow an adult barrel cactus under lights without a whole lot more light. Inadequate light will produce a mis-shapen, pale, unhealthy plant with weak spines, you won't like it. Stick it somewhere cool but mostly frost free, don't water it, and forget about it until spring. It will not grow so the level of light is not important (although the brighter the better so that your cactus isn't shocked when you move it to the sun), but it will be raring to go as soon as it gets some real sunlight.

If you have no access to sun at any time of year then my first advice would be to start growing something else. But if you're dead set on the cactus, you need to get some more powerful lights.

Information stays on the internet for ever for everyone to search. This guy will ruin his business forever by developing a bad reputation for what he is trying to do.

dcarch