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Seedlings lighting question

Posted by Gorky Chicago Suburbs (My Page) on
Tue, Apr 2, 13 at 12:57

I have a standard 4 foot long T8 light fixture hanging on chains about 6-8 inches above my two seedling trays. I run my lights 16 on and 8 off a day. I have noticed that the seedlings on the edge of the tray are bending a bit toward the center for more light.

I have heard a one 4 ft light is good for 2 seedlings trays. So is it bad to have these seedlings bend a bit toward light? Should i rotate the cells around a bit every few hours?

Follow-Up Postings:

RE: Seedlings lighting question

Hi Gorky and welcome to GardenWeb! To answer your question about the seedling bending toward the light ... no it won't hurt the seedlings, that's just their way of reaching for more light :-)

Your real problem is the distance you have your light from the plants; 6 to 8 inches is way too far away from the plants. You should have the light as close as possible without actually touching the plants. Normally, one or two inches is about right.

In regard to lighting, there's something called the "Inverse Square Law". It states that the intensity of the light is inversely proportional to the square of the distance from the source.

So, theoretically, if your light is 8 inches above the seedlings and you lower it to 2 inches above the seedlings, that's four times closer ... 4 squared is 16, so your seedlings would get 16 times more light.

Since the inverse square law generally applies when the light is radiated from a "point source" (which a fluorescent tube is not) the law would not hold completely true for your T8 fixture, but still you would get a great deal more light intensity on your plants (probably at least double).

Hope this is helpful,


This post was edited by art33 on Mon, Apr 8, 13 at 0:42

RE: Seedlings lighting question

Thanks Art. You may have just saved my seedlings, too. I have about the same setup as Gorky. Two 4' long 32 watt T8 bulbs that were about 6" above my plants. And one of my kale seedlings just started to flop over today. I think the rest of the seedlings are still small enough to benefit from more light. Actually, I was going to put the lights closer, but the clear plastic covers for my seeding trays are 6" tall, and I wanted to use them. I guess I better get some shorter plastic covers.

RE: Seedlings lighting question

Hi Miriamel and welcome to GardenWeb! Glad the post was helpful to you, don't know if it helped Gorky or not (never heard anymore from Gorky).

Anyway, yes when using fluorescent lights, they need to be very close to the seedlings. Also, unless you have some special reason for doing so, you should not be using that plastic cover over your seedlings. It's only supposed to be used to make sure the growing medium doesn't dry out while waiting for the seeds to germinate. In fact, some folks that are home most of the time and can keep an eye on the seed trays don't use the plastic dome at all. Your seedlings need good air circulation to prevent mold and disease problems. Actually, it's good to run a small fan (usually on a timer) to provide a gentle air movement in the area around the seedlings.

Removing that cover would be beneficial to your seedlings and also give you room to lower your lights into the proper position.

Hope this is helpful,


This post was edited by art33 on Thu, Apr 4, 13 at 17:24

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