artificial indoor grow light intensity/distance from spouts

[denemante]

Hey all,

I have two 48 inch T5 grow light bulbs. I've rigged them to hang next to each other. I just moved many spouts to 4 inch pots. Some are still in the 72-cell trays. So my grow table under the light has just gotten more crowded.

Many have said to put the bulbs about 2 inches from the spouts. That's how I had it.

Before, I had the lights close together and they fit perfectly over 2 of the 72-cells longwise.

But now, I've moved the lights about 10 inches from each other.

The light kit with a SINGLE 42 inch bulb is marketed to fit over 72 cell tray. At 2 inches over the sprouts - it's much dimmer on the outside of the tray. Doesn't seem right. With my two lights, it seems perfect.

Of course, I have a new problem because now my plants are more spread out.

Is the two-inch rule true? So I just need to get more lights?

Or as sprouts grow, could the light be farther away? Just trying to get a feel as to whether there is good benefit even if the light is farther from plants on the outside of my table, or whether any light farther than about 2 inches from the leaves is useless.

[rainbowgardener]

Well, you could stretch it to 3". But it is true that the lights need to be quite close to your seedlings. Light diffuses by the inverse square law. So if your lights are 2" away and you move them to 8" away (four times as far), your plants are then getting 1/16 th as much light intensity as they were.

But you are in Atlanta, it has to be well in to spring there. If you have things in 4" pots, why not start taking them outside to harden off?