Pat, you are correct, some plants need full darkness for a certain number of hours in order to bloom. For example, poinsettia growers have to use black cloth to insure longer nights to get them to bloom for Christmas by simulating shorter days than naturally occur at that time.
I have only ever heard of this being an issue to get them to bloom, and not to grow. Many daylily growers using artificial lights and timers give their plants extra long days to get more growth and more compact growth in seedlings.
Some plants don't use day length to determine when to bloom in season, but wait on soil/air temps to reach a certain point for a length of time, or cold weather for a certain period of time. This is how many spring flowering trees 'know' when to bloom. If you read a quality fruit nursery catalogue, you'll see 'chill hours' given for peaches. This tells how many 'hours' below a certain temperature they need to bloom out. If you plant a northern peach in the south, it will never get enough chill hours and won't bloom properly.
Bottom line...different plants have different ways of deciding when to bloom. To the best of my knowledge, daylilies bloom when they reach a certain maturity level and the soil and air temps are consistently warm. This is why they bloom early in the south and progressively later in the north as the season gets warmer.
We have a dawn-to-dusk street light across from the house and it has had no effect on bloom. I think your seedlings will do fine, even with a light on them at night.
Other opinions?
