Light Height - Does it Matter As Long As PPFD Is at Desired Level

Does it matter how close or how far your light is from the plant canopy?

Hypothetically, a 600 PPFD in mid vegetative phase with a HLG 350R light.

  1. Placing the light at the “highest” location at the very top of the tent. (approx 40” above canopy). Adjust the dial intensity up so the app shows 600 PPFD at the canopy.


  1. Lower the light to about 1 foot above the canopy and back off the dial so that the canopy reads 600 PPFD?

I wouldn’t think it would make a difference since both result in 600 PPFD (in this example) at plant canopy level. But, I figured I’d ask the lighting experts.


The only difference that you will see is coverage. More coverage, more power, higher fixture. The closer you get the more concentrated the light is in a specific area with drop off around the perimeter. You raise it up increase wattage that concentration spreads wider and usually begins to even it out. I’ll run my lights as low as I can go if it meets the numbers I’m after.


I would personally set light at 24inch for the 350r as hlg recommends. Then dim to desired ppfd. 600 is really high for veg imo. I know alot of guys go that high, 500-550 is plenty at 18 hr.


I’m with @Low . Height is less about how you get the PPF, and more about where you get the PPF.


@Low congrats on member of month brother. Well earned.



Ok thanks. But you guys lost me a little. In any case, I’ll adjust my light to about 24” and PPFD to 500-550.



It’s about the footprint. Higher is a bigger footprint, but then it takes more power because of the distance away.


@Low Congrats well deserved.
I agree with low keep the light down save some$ on the elec bill.

My plant loves the 600ppfd in mid/late veg. Some plants don’t like that.


Ok. Now I got ya!!!


Well in my case, at 600 the leafs will start to go lime color at top. Some may like it, mine do not. Not a fan of boosting mag sulfate to compensate for that level of ppfd either. I like to push mine to the limit of the fertilizer without having to modify the recipe. I also favor indica dominant plants which could be why…


That’s the part of growing there’s a base line to follow but there’s no set rules on how to get to the end. Happy Growing Enjoy


That’s been my learning curve since most folks feed their plants, and that impacts how much light they can take…as I’m soil and water only, I only push mine so far these days and will likely never need 100% of my 300w light in my 2x4.

That being said, I adjusted my light yesterday and have 3 day old seedlings getting ~400 PPFD :blush::woman_shrugging:


Thank you for the explanation in the photo diagram really helps me