The more light, the more growth. If you started seeds outdoors in northern Alaska's growing season you would have over 23 hours of light.

There is an idea out there that plants require a dark period, but this really isn't true at all over the short term (such as seed starting).

So, it's up to you. If you get the itch and start things early keep the lights at 12 hours for a more modest growth rate. If you forget and start late, crank the timer up to 24 hours on 0 off to play catch up.

I generally go with 18 hours myself and adjust based upon growth rate.

JAG

Thanks. I think I'm going to extend my planting date for some vegetables a couple of weeks, since the weather has been really unstable lately, alternating between daytime temps of 70-50 and 40-23 at nights. The last frost date is supposed to be around March 27 but I'm afraid it will continue longer than that. So I guess I'll leave them on 12-12 for now and see what happens in the next couple of weeks.

Without enough light, the seedlings will get leggy. I'd go 16 at least. They won't grow like mad anyway under fluorescent lights, which put out light closer in intensity to light shade than to sunlight.

I vote for 18 hours.

The notion that plants need darkness is a myth. There is a cycle in photosynthesis referred to as the "dark cycle," but that occurs during the light too. It's just called this because it also happens when there is no like, unlike the "light cycle." More light results in more growth, so leaving your lights on for longer will result in more growth. It's extremely easy to test this for yourself, try and you'll see.

It's a slam dunk science fair project used by kids all over the world.

Ive tried everything from 24 on, down to 16 on. I have had the best luck with 20 on, 4 off. If you are using floros, and that is all I reccomend you do to root and begin veg grow with, keep them as close as you can to the tops of the plants, this will keep the stretch down. Hav efun! TiMo

i have plants coming up from seeds at this time and i think 16 hours light and 8 hours rest is the best way to go keep the lights if flourescent about 2 to 3 inches about your tops

Along with enough light proper temperature and moisture are two more requirements for healthy growth.In my experience for 8 to 12 hours of light is good enough till you start the process of hardening the small plants before transferring them in the garden soil.Now from here on till the plants produce flowers and fruits it is very important that plants receive sun light for specified time depending upon variety of the plant like minimum of 8hrs. or 12 hrs. and so on so forth.

I think the only reason to turn the lights off is to save electricity. Some might do as well on 18-20 hours of light as they will with 24 but I doubt if a few hours make any difference except for a few pennies saved.

These turn off the lights stories probably originated with professionals where costs really were significant. Then some writer incorporated them in a column not knowing why. Then the column got quoted on the net and now we all read about turning off the lights.

I do 16 on 8 off because that's the way Mother Nature does it when they are growing in the garden. ;) Worked well for me for years.

Dave

My 2 c's...

"More light results in more growth, so leaving your lights on for longer will result in more growth."

This is true; but given that, there are things to consider. Beyond the electricity usage (which is a significant consideration) there is the issue of plant size. As the seedlings grow larger, they may require potting up to larger containers - and take up more space under the lights. Unless you plan for this, you could literally run out of space, and have larger (but weaker) plants.

The recommended starting dates for transplants are generally for commercial agriculture, and usually reflect the use of ambient sunlight as a light source. If you use longer periods of artificial light, you might want to consider starting transplants a week later.

There is also the question of temperature. With 75-80 degree temps & long light periods, plants will quickly become leggy. So my recommendation is, that if using long light periods, keep the temperature to 65-70. This is the temperature at the plants, which might be warmer than room temperature, when you consider the heat given off by the lights. Check it with a thermometer, and use a gentle fan if necessary... this will have the added benefit of strengthening the stems.

Personally, I use 14 hours, because that will be the day length when I transfer the plants outside to harden off. I believe it lessens the stress on the plants.

Zeedman- why would leaving the lights on longer result in leggier plants? I've tried, and it doesn't provided you use ample light and you properly budget how much time you'll need. It does reduce the amount of time it takes to get a transplant-ready plant. You can do 14 hours of light for 60 days, or 24 hours of light for 35 days, either way the plants get the same amount of energy from the light and end up with about the same size plant and use about the same amount of electricity (everything else being equal).

So long as your plants are young and small, changes in light time will not stress them.

It sounds odd, and goes against the conventional wisdom, but try it and you'll see. I have done these light time and transplanting experiments several hundred times (with school groups). It works.

Zeedman- why would leaving the lights on longer result in leggier plants?

It won't, unless the lights are floros and the plants allowed to grow over 6" tall before being set outside. At that point the floros aren't able to adequately light anything other than the top few inches of the plants so they don't get enough light and stretch.

I definitely agree than 24 hours of light isn't going to harm the plants and will result in faster growth. A valuable tip for those who get started a little late.

PM & JAG, we are perhaps not as far apart as you seem to think... maybe you should re-read my post?

If you get beyond the "leggy" part, you will find that I was referring to the effect of _temperature_ in combination with long days... and yes, with fluorescent lighting, which is widely used (including by me). It is the only feasible means by which I can light enough shelf space to start 400-500 seedlings. Granted, "floros" are not ideal... but not everyone can afford high-intensity lighting. At the scale on which I garden, it's cost-prohibitive.

We are also in agreement that long light periods can - and should - be used for starting plants later than the recommended dates, because of the increased growth rate. I did say (if you re-read) that increased growth was a problem "...Unless you plan for this...". For those reading this who are accustomed to starting their plants on a particular date, they need to know that with long light periods, those dates might require some adjustment... or they could end up with plants larger than they planned for. I say this from experience, having made that mistake when I was still a newbe to seed starting. If your plants are too large for the lights & it's still too early to transplant, that could be a problem. Forewarned is forearmed.

The recommendations I gave for cooler temps & air movement will contribute to stockier growth, and are accepted greenhouse practice.

Perhaps for purposes of this discussion, the questions should be:
- the definition of "ample light"
- the optimal size for transplants

PM & JAG, we are perhaps not as far apart as you seem to think... maybe you should re-read my post?

No need, I understood you the first time ;-)

I agree with what you said. It's just the opposite side of the coin.

Can seedlings be grown under 24x7 light to get faster growth with no ill effect to the plants? Yes.

Are their caveats to this? Yes, and you listed them.

I turn my lights on at 6:00am and turn them off at 10-11:00pm. Works well.

I know this is an old thread but I thought I might add something. I grow a lot of Asian greens that sometimes I start indoors under lights. I've noticed over the last few years that once transplanted they have a tendency to bolt a bit too quickly. Usually I run the lights 16-19 hours a day. Would cutting back on the hours to say 12 hours per day slow the bolting down?