Grow light distance vs brightness

[UrbanHillbilly]

I have an LED quantum board light in my little tent with adjustable brightness. I just moved my seedlings into the tent and want help understanding if it makes any difference how far the light is as long as I have it adjusted to the proper brightness at plant level. They're tiny seedlings right now and I know seedlings need less intense light, so I used the PPFD app on my phone to adjust it to about 200 as measured at the top of the plants, but the light is a few feet above them.

 

[greenmachine29]

I have an LED quantum board light in my little tent with adjustable brightness. I just moved my seedlings into the tent and want help understanding if it makes any difference how far the light is as long as I have it adjusted to the proper brightness at plant level. They're tiny seedlings right now and I know seedlings need less intense light, so I used the PPFD app on my phone to adjust it to about 200 as measured at the top of the plants, but the light is a few feet above them.

Sounds about right. But don’t adjust it to far down. The seedling will tell you right away if they are light hungry. If they begin to stretch then amp it up.
Most the time I put seedlings under intense led its always way over light needs no matter what I do. The plants stay almost to short I’ll have 10 nodes in under 6 inches sometimes.

 

[Psychobilly]

Yeah, there's a picture of me being a sissy with my light, where I have plastic forks holding up the ones that stretched too far, but, it's better than having the light too far down where the heat cooks them haha. That's the main thing to watch for; if the Light is too close to the plants, it'll generate that heat, and sometimes cook them a bit.

 

[OldG]

If the light is too far away they will stretch VERY long and flop and die.

my lights are all at 90 % and hang em high... i use a bucket, or a tote to adjust height to light. And you bend over less. If you have a meter it makes it easy.

and the plants are pretty resilient within reason.

 

[UrbanHillbilly]

If the light is too far away they will stretch VERY long and flop and die.

my lights are all at 90 % and hang em high... i use a bucket, or a tote to adjust height to light. And you bend over less. If you have a meter it makes it easy.

and the plants are pretty resilient within reason.

I don't understand how it's a problem if the light levels still measure appropriately at plant level. I mean, I could lower the light fixture some and turn down the light intensity a bit to get it closer, but I don't understand how that would make any difference to the plant as long as the PPFD measures the same.

 

[RookieBuds]

My advice to help avoid stretch, put the light much closer to the seedlings and dim it accordingly, and aim for more like 300 ish PPFD. Having the LED feet away, as you mentioned, can cause seedling stretch which isn't bad, just mostly annoying. Or, put the seedlings on a "booster seat" to get them closer to the dimmed light, that's often what I do because boosting the seedlings to me is easier than screwing around with my light height.

 

[Smokieokie]

I don't understand how it's a problem if the light levels still measure appropriately at plant level. I mean, I could lower the light fixture some and turn down the light intensity a bit to get it closer, but I don't understand how that would make any difference to the plant as long as the PPFD measures the same.

It's not a problem if all parameters are met no matter light height. The plant doesn't care how far away the light is as long as it's the correct amount at plant height. The difference is the amount of electricity you use to get there.

 

[UrbanHillbilly]

My advice to help avoid stretch, put the light much closer to the seedlings and dim it accordingly, and aim for more like 300 ish PPFD. Having the LED feet away, as you mentioned, can cause seedling stretch which isn't bad, just mostly annoying. Or, put the seedlings on a "booster seat" to get them closer to the dimmed light, that's often what I do because boosting the seedlings to me is easier than screwing around with my light height.

I'm a little less, umm, impaired than I was when I read this last night. In my case, stretch could cause problems down the road because this tent is so little, only 2x2x4. The auto strain I selected is rated to only grow to 80-100 cm because of that height limitation, but if it stretches there could be problems. I'll be cranking up that light a touch tonight.