General consensus is to keep the lights 14- 16 hours on and 8 -10 hours off.
That is how I have been doing and the results are good.
Plants (most of them) need a rest/ sleep/darkness time to recuperate also.

Seysonn

When I used standard T12 shop lights to grow my seedlings, I left them on 24/7 from the beginning, and they seemed to grow faster and stockier that way. When I started using T5 lights a couple years ago, they seemed to do better with a six hour dark period each night.

Thanks both of you, early this morning around 3 am I turned it off for about 6 hours and will continue the process and go up to 8 and see how it works out. Now off to the garden center to get some more supplies and probably pick up some more seeds and some more mix for a backup plan just in case. My T-5 JumpStart 2ft lamp is now on its 3rd year so was just worried if it was losing its ability.

When I first started growing my tomatos from seed, the first three years I left my T5s on 24 hours / day. It really stressed the plants out to the point of where it made the leaves appear to variegate. I asked around, and reduced the lights to 12on and 12 off. They have done great ever since.

What we can do is to copy optimum natural environment.
In early to mid spring, when things like peppers and tomatoes germinate naturally, day light length is 13 to 14 hours in a lot of places . So then 13 to 14 hours sounds like optimum to me. We take it a bit too far, maybe, and give them 16 hours of light. So logically (to me) 14 hours is better than 12 or 16 hours. How much difference all this make? We don't know unless one does a lab type test and measure the differences.

Seysonn

It is not just the quantity of light (number of hours) but also the quality of light that matters. A T12 light gives much weaker light than a T5. Neither gives a plant the equivalent light energy or spectrum of the sun. I definitely saw an improvement in growth when I increased the number of hours my seedlings were exposed to T12 lights, and 24/7 produced the best results. With a better light source, like T5s, a dark period seemed to help. But I think 18 hours of T5 light was better than 14 or 16. HID and HPS lights provide much better light than any fluorescent light can, and could probably be used for a shorter light cycle. Choosing supplemental lighting that provides something close to what natural sunlight provides is more complicated than just matching the number of hours of exposure.

Yes, Ohiofem,
What you describe as "quality" in a way is also "quantity" in terms of lumen. It is also known that tomato plants don't need 12 hours ( or more) of intense sun for their photosynthesis needs.
I can tell by looking at my seedlings if they are getting enough light or not. Are they spindly, stocky ? There are watering , fertilizing and temperature factors here too. So you have to kinda play by the ear. .

Another issue here, other than the duration, Is the light spectrum choice.
For seedlings, we don't want to encourage them to flower or fruit, but to develop good roots system. For this purpose , I think that cool spectrum is the best. But if you wanted to grow under light permanently, then you would choose different spectrum range

Seysonn

Hey all, My seedlings are up about a week too, and I am using an elcheapo 4 foot 2 bulb shop light over them. Not exactly sure what bulbs are in it, probably 40W, and 16hrs on and 8 off seems to work for me. One of those timer thingys really helps too.

Hi Ed.
The T8 bulbs are 32w. I think T12s are 40 something watt. For seedlings it is not too important what type of bulbs are they. Daylight type 6500k is better.
So, when is your expected plant out date is. I know it is TOOOO cold up there now but by your plant out date things should be back to normal.

Seysonn

Come heck or high water my plant-out date will be May 1 with protection. Seems like a million years away now. Today I went out to try and chop away some of the snow pack that has captured some of my lower blueberry limbs, and I just gave up. It was like trying to chop away tundra :-(

Ed, a few warm days and all that snow and ice can melt away. This has been just a tough winter back East. Keep focused and take care of those germinates.

Seysonn

Sounds good. BTW my bulbs are 34W fwiw.

Ed, so T8 bulbs are 34w . I was off by 2w..
I am running 3 units now { 34 x 2 x 3 =204w , x 16 hrs = 3.26 kwhr per day). Cost is less than a cup of coffee at StarBuck's .

Seysonn