Question: If I grow under florescent lights, how long should they be left on? How far should the plants be from the tubes?
Answer: Usually 12-13 hours each day, though the time depends somewhat upon how close the plants are kept to the lights, there being a trade-off. Personally, we keep lights on 13 hours a day, year-round, and have all lights on lamp timers to turn them on and off automatically at the same time every day. Minis and semiminiatures are grown about 10-11″ below two 40-watt T12 florescent tubes, or 1 T8 tube–measured from tubes to leaf surface. Standards are grown further away, about 18″ below the tubes. May growers will grow their violets closer to the tubes, but usually keep lights on for only 11 or 12 hours each day. Also keep in mind that there is less light towards the end of the light fixtures, as well as on the shelf edges. Older tubes also produce less light than newer ones. We should also note that this rule of thumb applies for the traditional 40-watt tube we’ve grown used too–many of those sold currently produce more or less light, more or less efficiently.
For LED lighting, there are too many options to explain. Here’s a guideline. Over a 4 foot shelf, we use a 10 watt fixture, about 18″ above plants, for 12 hours per day. As for color of the light, “grow” lights are best, but this isn’t critical.
Whatever you’ve decided, it’s always best to make adjustment based upon the performance of your violets. If growth is spindly and upright, with sparse blooms, then more light is likely needed (don’t overcrowd plants!). If growth is bunched and/or hugs the pot rim, leaves are pale, yet bloom is good, less light might be called for.