Quote Originally Posted by amckean
I'm using a 400w HPS and I red in a grow book by Mel Frank that the light should be 18" to 24" in early stage of vegative growth and I have mine at 18" and they seem to be growing fin with no stretching but at 24" it seemed to slow down growth for a week until I lowered it again.
Hope this help
Ali
Mel (the ass hole) Frank wrote his book about 30 years ago and has sense disavowed everything he wrote. Most of his conclusions have been proven inaccurate anyway. You should pick up a newer book with better information.


There are two factors to consider. Temperature is first; monitor the temperature at the average height of your plants. To close and you burn the tips to far away and you give up luminance penetration (PAR). Second is Luminance loss. Most experts are in agreement that light intensity diminishes exponentially fast.

For example:
A 400-watt MH from 0? to 12? is about the same luminance say 55000
Same light at 24? the luminance is now about 30000

Still good

Same light at 36? the luminance is now about 1200

Fan leafs block most of the light so nothing below sees enough of the light of day to do any good.

About 16? is average if your plants get too tall just bend them over (if you bend them over the buds get more light and grow bigger). You could crop them to keep them short and bushy you crop between the 4th & 5th node early in the vegetation state for best results.

I hope this helps