Depends on the light tubes type - T12, T8 or T5 - as they all put out different intensity and spectrums. I keep T12s right at the plant tops, not touching but just a hair above. T8s can be 1-1 1/2 inches above as long as you have a fan on to dissipate the heat, T-5s 2-2 1/2 inches again with the fan to control the heat.

Dave.

Same here.

Dave has it right. Never to doubt him.

I'm about 3". I have two side by side fixtures: One with T8 tubes and the second with old style, bigger diameter (F40?) . I rotate the tray to give each plant a dose of both light types. The plants look great and are growing fast. I just don't want to crisp the tops.

Does it really make them grow significantly faster when the distance is minimized?

Thanks

Harry

"Does it really make them grow significantly faster when the distance is minimized?"

No but that isn't the purpose. The point is to keep them from getting leggy and developing stretched and weakened stems.

I meant "significantly better" when the distance is minimized. So if they grow stocky, we are probably OK.

Thanks

Harry