What happens if I leave the lights on 24/7 ?

tomatoes4ever(7 Fairfax VA (DC))April 15, 2007

I read in the FAQ's that the lights should be left on the seedlings for 16-18 hours a day. What happens if I leave them on all the time?

Will they grow faster?

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I'm sure a thorough Internet search would reveal a complete answer. However, I do believe that tomatoes don't need a dark period. Thus intuitively, longer light exposure should increase the rate of growth, at least to a point.


    Bookmark   April 15, 2007 at 11:58PM
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mine have been under 24hr/day light for 2 full days now since i took them off the north facing window sill and put them under 3 dual tube 40 watt shop light fixtures, and they are growing great, the stems are thicker, the first set of true leaves have almost doubled and they grew about 3/4 inch in height in 2 days(from what they were) under the cool whites, also the stems and parts of the leaves are covered in these tiny white hairs,what are these HAIRS and are they harmful?

    Bookmark   April 16, 2007 at 12:12AM
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tomatoes4ever(7 Fairfax VA (DC))

Thanks seedboy! That's all I needed to know. I just wanted to make sure I wouldn't be harming them.

Also, I have halogen lights. Any idea how close the seedlings should be to the lights?


    Bookmark   April 16, 2007 at 12:19AM
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Halogens put out a lot of heat, so be careful! I definitely wouldn't let the leaves touch the lights. I would also monitor the temperature with a thermometer. Also, I would run a small box fan near the seedlings to help dissipate heat.

    Bookmark   April 16, 2007 at 1:25AM
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tomatoes4ever(7 Fairfax VA (DC))

They aren't especially close to the lights, so I think I'm OK. But I'll describe it anyway and you can tell me if I'm being an idiot.

The plants are sitting on top of my stove. The halogens are the 2 lights that came in my new Wolf Hood. The plants are at least 2.5 feet from the lights. Actually, I didn't think they were getting eough heat, so I just used boxes to get them closer. They are still at least 1 foot away from the lights. So I don't think it's giving the plants anything other than a warm & cozy feeling.

    Bookmark   April 16, 2007 at 1:30AM
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if i am not mistaken halogens run a little warm i would keep them 4-6 inches away, but i am only a novice, i would think as long as the folige is not warm to the touch they should be ok, mine are under 240 watts of florescent light about 2 inches from bulbs.

Here is a link that might be useful:

    Bookmark   April 16, 2007 at 1:33AM
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It sounds like you'll be ok with your setup.

    Bookmark   April 16, 2007 at 1:45AM
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korney19(z6a Buffalo, NY)

What happens if you stay up 24/7?

    Bookmark   April 16, 2007 at 10:33PM
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tomatoes4ever(7 Fairfax VA (DC))

"What happens if you stay up 24/7?"

I become the life of the party.

    Bookmark   April 16, 2007 at 11:58PM
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korney19(z6a Buffalo, NY)

"Because it's difficult to reproduce the sun's intensity indoors, an artificial system has to be left on longer to compensate for its reduced brilliance, 12 to 18 hours a day is usually required. (Except under special conditions and for particular plants leaving the lights on 24 hours a day is not recommended. Most plants require some darkness everyday for proper growth.) "

Here is a link that might be useful: FLUORESCENT PLANT LIGHTING

    Bookmark   April 17, 2007 at 8:24AM
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korney19(z6a Buffalo, NY)

"Plants NEED Darkness
Plants need dark periods. Light (called photo-periods) and dark periods and their relative lengths have an effect on plant maturity. Recent studies have conclusively proven that it is not just the length of the day which affects growth, but the duration of the dark period which follows. The dark period of each day affects flowering and seeding of most plants. Although many plants can grow under continuous light, nearly all plants prefer a dark period each day for normal growth. All plants need some darkness to grow well or to trigger flowering. The ideal photoperiods of plants vary, some preferring long days and short nights; other the reverse; and some do best when the length of the night and day periods are equal."

Here is a link that might be useful: Buying The Right Indoor Light for your Growing Room

    Bookmark   April 17, 2007 at 8:29AM
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korney19(z6a Buffalo, NY)

"Light duration or photoperiod refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photoperiod."


    Bookmark   April 17, 2007 at 8:40AM
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tomatobob_va7(z7 VA)

Despite the above quotations, I think we still have an open question here. In two cases that Korney19 cites, the researcher seems to be growing plants to maturity, or at least to flowering. One of the studies is on Venus flytraps. But we're wondering about growth BEFORE blossoming. We want them to put their growth into roots and stem. So if the dark period is necessary or conducive to flowering, perhaps 24/7 light is the way to go.

    Bookmark   April 17, 2007 at 10:07AM
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korney19(z6a Buffalo, NY)

Personally, I say save on electricity. If you're only lighting one 80w fixture it wouldn't matter much but my stand has three 4-light fixtures, and 2 tubes are overdriven, so it can make a big difference in electric bills.

Anyway, here's from a properly conducted scientific study done in the 1950's:


    Bookmark   April 17, 2007 at 1:07PM
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migarden(z5 MI US)

This is my third year growing tomatoes from seed and I always leave the light on 24/7. I always have healthy plants with a good productive crop. Now I'm asking myself if they would be even better with a period of darkness??? We need some definitive information on this! Is it true that tomatoes don't need a dark period?

    Bookmark   April 17, 2007 at 3:16PM
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korney19(z6a Buffalo, NY)

Here's some of the report I posted the pic from, don't know how well it will format here, it's from a pdf file.

In the course of an experiment in which plants
were grown under various alternations of light and
darkness, it was found that some plants grown under
conditions which deviated too greatly from a 24 hour
cycle of 12 hours light and 12 hours darkness were
inhibited in their growth.
Sunflowers, peas, and tomatoes were grown from
seed under cycles of 6 hours of light-6 hours of darkness,
12 hours of light-12 hours of darkness, and 24
hours of light-24 hours of darkness. The plants were
grown in vermiculite at 23'C and about 1000 fc of
artificial light from warm white fluorescent lamps
supplemented with light from incandescent bulbs. In
a 48 hour period all plants received the same quality
and quantity of light.
The most striking effects were those on the growth
of the tomato plants (fig 1). The peas showed some
inhibition of growth in the 6-6 and the 24-24 hour
regimes as compared to the 12-12 hour treatment, but
the sunflowers were not significantly affected. Garner
and Allard (3) have shown that short alternations of
light and dark inhibit the growth of many plants.
Arthur, Guthrie, and Newell (1) found that tomato
plants were extremely sensitive to long day length and
will not survive under continuous illumination. The
leaf injury symptoms reported by these authors are
similar to those observed here-small, stiff, yellow
leaves with dark necrotic spots.

...during one phase (the photophile) of
the endogenous daily rhythm, light promotes the assimilatory
activities of the plant, such as those that
affect flowering. At the other extreme of the endogenous
daily rhythm, there exists a phase (the scotophile)
in which light has no promoting effect or is
even inhibitory.
The effect which light-dark periodicity has on the
growth of tomato plants might well be explained on
the basis of Bunning's hypothesis. Thus plants grown
in the 6-6 and the 24-24 hour regimes would receive
light during all or some portion of the scotophile
phase, if the tomato has an endogenous daily rhythm
of 24 hours (fig 2). That the tomato does have such
a rhythm is shown by such a phenomenon as diurnal
rhythms in bleeding (4). Continuous light should
have the same effect as the alternation of light and
dark periods in the 6-6 and the 24-24 hour regimes,
since any light period in excess of about 16 to 18 hours
would expose the plants to light during advanced
stages of the scotophile phase.
If Buinning's hypothesis is applicable to the growth
of some plants as well as to such a phenomenon as
flowering, a light interruption during the normal dark
period (the scotophile phase) should be inhibitory.

An experiment was designed to examine this point.
Tomato plants were grown at a constant temperature
of 23C, at high intensity greenhouse light for 8 hours,
from 8 A.M. to 4 P.M., followed by supplementary,
artificial light of 1000 fc from 4:00 P.M. to midnight.
After initial growth rates were determined, half
of the plants were given their supplementary light
from 4 P.M. to 10 P.M. and from 2 A.M. to 4 A.M.,
thus interrupting the dark period with 2 hours of
light. Both the control and the treated plants continued
to receive a total of 16 hours of light of the
same quality and intensity, with the night interruption
being the only variable.
The results of this experiment are illustrated in
figure 3. Dry and fresh weights fully confirm the
inhibition shown by height measurements. Although
highly significant statistically, the inhibition is not as
great as expected. This confirms the observations of
Arthur et al that plants grown in the greenhouse
during the day are more resistant to the inhibitory
effect of artificial light given during the night than
are plants grown entirely in artificial light. A repetition
of the dark interruption experiment with the
plants growing entirely in artificial light of about
1400 fc and a constant temperature of 23°C shows a
far greater inhibition of growth.
Biinning's hypothesis, although it gives us no
mechanism to test, might be applicable to growth of
plants such as tomatoes, which are photoperiodically
indeterminate with respect to flowering, as well as to
flowering of photoperiodically sensitive plants.

    Bookmark   April 17, 2007 at 3:51PM
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Lots of info here, but I have a different problem but on the same line. Due to a volume problem I wasn't able to keep all my plants under the lights. Had to put some trays in front of the windows.

the observation is that all of the plants in front of the windows are doing OUTSTANDING. But the ones under the lights(on 24/7) are stunted, dry leaves and wilted. I haven't done anything differently to either set of toms. I water them all the same and at the same time. I have different sizes of cups but all sizes are at both locations.

I try to water according to the size of the cup. But the cups always seem to be dried out, at least on top.

So my question is, could it be the lights or am I over watering?


    Bookmark   April 19, 2007 at 1:14PM
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Bill, with natural sunlight, I would expect much faster growth. Artificial lighting offers a doable alternative, but still, they may not require the same watering schedule. On the other hand, it could be your lights.

Tomatoes4ever, mine are under 4 fluorescents, 24/7 again, and are doing fine. Only because I have no timer, and dont remember to turn them on and off on a reliable schedule otherwise.

    Bookmark   April 19, 2007 at 10:29PM
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Just to be scientific, a publication from nearly 60 years ago is not generally accepted as strong evidence for a particular study unless it has been varified by many other scientists. Anyways I have a little indoor growing experience if you know what I mean? I am retired from that for some time now but never remember the plants suffering from 24/7 lighting. Maybe Tomatoes are different? Cheers all!


    Bookmark   April 20, 2007 at 1:50AM
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korney19(z6a Buffalo, NY)

Vince, the document can be found at the following link, in pdf format, from plantphysiology.org. Take it as you will, for whatever it's worth.

    Bookmark   April 20, 2007 at 2:01AM
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I would imagine the data is just as valid today as it was 60 years ago. Im not sure how it relates to our question though, (as all groups had periods of dark / none were grown with uninterrupted light) other than to suggest that there is an optimal amount for tomatoes.

    Bookmark   April 20, 2007 at 1:02PM
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I'm using 4' shop light with T2 lights, I believe. Have them right down on the plants. I going to test and see what happens when I move all the plants to in front of a window.

So I was wondering, if once the plants reach a certain size should they be moved from out under the shop light?


    Bookmark   April 20, 2007 at 7:59PM
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korney19(z6a Buffalo, NY)


"Arthur, Guthrie, and Newell (1) found that tomato
plants were extremely sensitive to long day length and
will not survive under continuous illumination. "

    Bookmark   April 20, 2007 at 8:15PM
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Thanks Korney, I must have read right over that somehow. Seedlings seem to be pretty forgiving, at least during the early growth stage, although I suspected this might not be optimal. Ive been meaning to pick up a timer, just havent gotten around to it.

    Bookmark   April 20, 2007 at 9:56PM
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I've read of some that light 24/7 for the first 7-14 days and claims it works. I think it is another thing if a person wants to know he will have to do a comparison planting and just see under their conditions how it works. I have never tried it myself. I go to 16 hours after transplanting out of the plug trays at one and two days. Works well for me. Jay

    Bookmark   April 20, 2007 at 10:27PM
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This article seems to be implying that some plants do require a rest...that be said they also indicated that exposure to certain colours of light could shorten the requirement. I won't lie and say I understood it all...maybe you guys can take a look and fill me in! :D

    Bookmark   July 31, 2007 at 2:07PM
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