24 hr grow lights for seedlings?

[smallfarmer]

Question: Is there a problem using 24 hr grow lights on my greenhouse seedlings. It really speeds their growth. Vague in my mind is something about what plants do in the dark, cell building of woody tissues??, that mine would be missing. Help me with the science. They are under 400 watts of sodium with some florescent , the plants are about 2 1/2 feet from the lights.

[sliphorn]

The science I don't know about, but it just makes sense to me to give them a bit of a rest during any given 24 hour period. 18 hours should be plenty.

[Glenn 50]

I use Halogen. I don't know about the science either but 3500 watts spread over 7 floodlights gets the growth going.

[Fusion_power]

I've grown plants with as little as 8 hours of light per day and as much as 24 hours. Tomato plants don't seem to suffer any adverse affects from the 24 hours per day treatment. Keep in mind that 24 hour light will significantly speed up growth which could make a seedling get too large for the pot its in and/or be ready to plant outside well before the weather is agreeable.

Fusion

[JerryL]

Like the others I don’t know the science of it but I have always tried to simulate Mama Nature. 12 – 16 hours of light, the rest darkness.

A larger concern I have with your set up is the distance the lights are from your plants. Being that far away I would think the plants would get very leggy. If it isn’t natural light (outside, natural light in a greenhouse, etc) most folks try to keep the light within a couple of inches of the tops of the plants. They seem to be stockier that way. But hey, if what you have works for you then it is the right way to do it.

Jerry

[bcday]

My plants start to take on a grayish washed-out appearance and fold their leaves a little downward away from the light after about 18 hours, so I take the hint and turn the lights off then.

Jerry, I think sodium lights need to be kept farther away from plants than fluorescent ones to keep from burning the leaves. But I only use fluorescent myself, so someone else could probably explain it better.

[Tomaddict]

Too much light is not helpful. During periods of adequate light, most of a plant’s energy is focused on manufacture of carbohydrates necessary to fuel cell growth; there is a little, but not a great deal, of cell division and growth during light periods. Those accumulated carbs are what fuels growth of the plant. During dark periods, plants devote the great majority of their energy expenditure to cell division and cell growth. An endless supply of light will eventually disrupt the balance between carbohydrate accumulation and utilization, as there is a limit to how much carbohydrate a young plant can accumulate.

The relative length of day and night is also a key determinant for the beginning of the fruiting stage for many plants, but I don’t believe this is true for tomatoes – fruiting (beginning with flower formation) appears to begin when plants have developed a sufficient carbohydrate supply, or sometimes when they undergo certain stresses (such as irregular water supply).

And remember, plants are adapted over many generations to what mother nature provides. It might be helpful to extend the period of light modestly, but i'm confident that it will be better to give them several hours of dark to grow and reset for the next photoperiod than to give them endless light.