Use good reflectors and 15-18" will be fine for the plants you are growing. A VFT needs more light but if you're just using it on overcast days it should work out.

Just be sure that they are bulbs designed for the purpose. If buying from a hydroponics store they should be OK, but the compact flourescents sold for home lighting can be nearly useless for plants.

Just be sure that they are bulbs designed for the purpose. If buying from a hydroponics store they should be OK, but the compact flourescents sold for home lighting can be nearly useless for plants.

That couldn't be more false. Standard fluorescents not only put out the same frequency of light but in many cases more of it. The only difference is standard lights have far red, red, blue, yellow. green and orange phosphors where most grow lights only have far red, red and blue. Personally I'd rather see my plants in natural light so I can tell if there's a problem.

lycopus you are very misinformed. Grow lights are over priced and a gimmick. Many "experienced" CP growers are using standard fluorescents. Please DO NOT post misinformation here.

Taz, you are the one who is uninformed. The CRI of fluorescent lights varies widely. I use standard cool white fluorescents in my T12 and T8 fixtures and yes they do work, but this thread is not about standard fluorescent bulbs. I have purchased commercial CFLs that have extremely poor CRIs that failed to yield growth despite putting out upwards of 500 PPFD. Note that I said CAN BE, not that all have the same low CRI. I know that Sylvania makes some bulbs with 90+ CRI, but one should not assume automatically that is what they are getting. Assuming that bright light = high PFF is the reason some people have watched their plants waste away under mercury vapor lights as they scratch their heads wondering why such bright lights do not work.

I will add that as far as I can tell that I am one of the few, if any, people who post here who successfully grows VFTs under artificial lights. Since I began posting here I have been shouted at regarding the very notion of doing so, but I will continue to add my two cents for the benefit of the people who come here looking for guidance. It is certainly needed since half of the responses people get here is "google it", and most of the information that is disseminated is just regurgitated information from other websites.

You might as well be talking to a wall. I have a friend that uses the bulbs the OP is talking about to grow many different plants.

I have purchased commercial CFLs that have extremely poor CRIs that failed to yield growth despite putting out upwards of 500 PPFD.

CRI has nothing to do with plant growth. The higher the rating the more the human eye sees it as natural light. That's all the CRI rating means
Lux or Foot Candles is what matters and lumen is a figure based on Foot Candles at a foot away.

BTW please put things in layman's terms so most people can understand them. Most people don't know what PPFD stands for. Yes moles is another way of measuring the light a plant receives but lighting companies don't put PPFD, lux or foot candles on packages.

Most people don't know what a foot candle is any more than what PPFD is. I wasn't referring to what is written on a light bulb, I was referring to the measured light intensity on a bulb that I tested for comparison. A PPFD of 500 should be enough to grow just about any type of plant, though it might not be enough for some to flower. But if the bulk of the light produced is between 500 and 600 nm it will be of virtually no value for growing plants. Try growing something under a green light and see how it goes. Won't matter if it's putting out 30,000 lumens.

Now that's great if you can figure out the brand and model of the bulb that OP is planning on purchasing based solely on the form and wattage. My experience has been that not all CFLs are equal. Given the amount of electricity that such a bulb will use over its life I always do a little research before buying.

AFAIK there is no fluorescent bulb that doesn't have enough far red, red and blue in it that is between 4100k and 6500k.

Doesn't matter what brand! My friend has tried all the 105w medium base bulbs available and there is NO noticeable difference. Yes it's all hearsay and I've never tried them but his plants look great.