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Is 8 hours under lights enough?

Posted by valray Z4 (My Page) on
Mon, Apr 7, 14 at 20:06

I have a 3-tier plant stand with 2 t5 lights on each level. I have been leaving the lights on 24/7 and running two "shifts" of plants - each flat getting 12 hours on, 12 hours off. Now I'm wondering whether I could get 3 shifts in, 8 hours on, 16 hours off. (I have seedlings in flats that should be separated and potted up.) I know it's not ideal but with the fairly high intensity of light, do you think it might be okay for the next month or so? I'm growing mostly tomatoes, peppers, eggplants, leeks.

Follow-Up Postings:

RE: Is 8 hours under lights enough?

Hi valray,

You say you have two T5 lights on each level. Do you mean two fixtures or two bulbs? If you have two fixtures, how many bulbs are in each fixture. Are they 24 inches or 48 inches long? How many lumens are the bulbs rated at? Also, do you know the color temperature (e.g. - 5000K, 6500K) of the bulbs?


RE: Is 8 hours under lights enough?

One fixture, 2 bulbs per level, 24" long.

The lights are Sun Blasters T5HO bulbs 6400K full spectrum. I couldn't find a lumen rating for them.

I keep them very close (2-3") to the seedlings and have a fan on them part of the time. I understand that day length can be important for blooming, but I don't want them to bloom under the lights, just keep growing healthy leaves until they can go outdoors.

Here is a link that might be useful: Sun Blaster Lighting

RE: Is 8 hours under lights enough?

Hi again Valray,

I think your bulbs are rated at about 2000 lumens each. Two bulbs in the same fixture never equals twice the lumens, but should come close anyway. If your fixtures use reflectors, the seedling will get the benefit of maybe 80 percent of that light. Without reflectors, probably more like 40 or 50 percent. Reflectors make a huge difference.

If your fixtures have good reflectors, the seedlings should get somewhere between about 1500 and 1800 foot candles (two to three inches from the light). Without reflectors, probably half of that.

Taking tomato seedlings (grown indoors) as an example, they require 650 to 800 foot candles for 16 hours a day for good growth. If they received light for only 8 hours, they would need about 1300 to 1600 foot candles.

The lights you’re using are good, but I’m not sure if they will have enough coverage area for all the seedlings below them. If your fixtures have good reflectors, you might get by with the eight hour shifts, at least while the seedlings are small. When they get larger they’ll want more light, so at that point, it gets a little iffy :-)

Two final points … if you don’t have the reflectors for those SunBlaster fixtures, you need to get them. Also, warm ambient temperatures cause plants to grow faster. Keeping the seedlings cool (60 to 65 deg F is ideal) will help prevent leggy growth under low lighting conditions.

Hope this is helpful,


RE: Is 8 hours under lights enough?

Thank you so much art33. That's exactly the kind of info I was looking for. I do have reflectors on them so that helps. I'll have to check on the relationship between lumens and foot candles but I'll do the math now that you've given me something to go on. Thanks.

RE: Is 8 hours under lights enough?


Foot-candles is equal to lumens per square foot. When dealing with light from a point source (such as a normal round incandescent light bulb), figuring out the foot-candles is pretty easy. The light from a point source will stream out equally in all directions, forming an ever-increasing spherical wave. All you have to do is calculate the surface area (in square feet) of that sphere, where it hits your plants. The formula is Area = (4 times pi) times (the radius squared). Then, you divide the total lumens by that number of square feet to get the lumens per square foot (foot-candles).

However, for lighting that is not a point source (and especially with reflectors) the problem is not so easily solved. In the case of your lights however, I kind of cheated. I have some bulbs with exactly the same rating. I also have a couple of light meters :-)


PS - Please let us know how your plants turned out by the time you were able to get them outside. That kind of information might be used to help others in the future.

This post was edited by art33 on Tue, Apr 8, 14 at 21:49

RE: Is 8 hours under lights enough?

A light meter is a good idea. :)

Thanks for your help. I will let you know how it works out.

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