Led light height

Start at 24 n work your way down as plant gets older. If u c stress move light up

 

There is no good rule of thumb for all led lights. Light distance can vary wildly from several factors. How powerful is the light? What model diodes are they using? How many actual watts is it? How large is the space? How many watts per square foot? How old/large is the plant? What strain?

An led light under 150 actual watts or so can get as close as 12" from mature plants but if that same plant is under a 500 actual watt led light it will need more like 18-24". That also depends on the plant. Some strains can tolerate much closer light distances then other. The same two lights under a group of seedlings may require up to 3feet or more of light distance to not stress them out.

The best way to manage light distance effectively is to get good at watching the plants for signs of light stress. When the light is too close your uppermost leaves will upcurl at the tips of the serrations causing a sawtooth or taco leaf appearance.

Watch for that and if you see it move the light up a few inches. Place the back of your hand slightly above the plant and see if it feels hot to you.

This is the leaf condition you watch out for.


Looking at a plant and knowing what it needs is one of the biggest skills you can learn as a grower. The plant talks to you with it's leaves, color, and posture to tell you what it needs. It's your job to learn how to listen.

 

If I had to give you a safe number 18-24" is pretty safe and won't ever burn your plants unless you have a super high output light.

 

is that light stress or heat stress.?

I am not familiar with LED's, but with HID lights there is NO such thing as too much light only heat issues.!

I am learning a little about LED's,
from what I am seeing the over all light is not an issue, it's the added UV and IR portions of the light causing stress...????
Too much UV and IR ain't good for us and can't be good for the girls.!

 

Light and heat stress will both give you that leaf condition. I use white light leds. Not really much if any added UV compared to HPS. I'm pretty sure they have less IR. It's quite easy to light burn a plant with HPS or led. You always do need a proper light distance especially when you use more powerful lights.

 

I strongly disagree....
"proper light distance" for a 400w HPS is 12-18 inches, but i have grown within 6 inches of an open bulb with no glass and had NO diverse effects with enough ventilation, but if the temp gets high they would show signs of stress.

I have also grown outdoors in full sun and have NEVER seen any light burn (can't get more light then that),
in all my 15 yrs of past experience, "light burn" is actually heat stress, which i have had inside and out.
inside heat stress shows on the tops, outside it shows up in the middle as well as the tops.! The ground seems to help cool the bottom of outside plants, and the sunlight is just as strong on the bottom as it is the top and middle.

raise your light to 3 feet above the plants and let the temp get to 120 deg fer., they will show "light stress" on the whole plant.! LOL

 

Light burn or bleaching is a real thing. No way you can grow with my grow lights at 6". I get that leaf stress indicator in temps as low as 78 degrees with good air circulation if I run the light too close. Some strains I have had will allow me close to 12-14" but that's as close as I can get to my bloom lights. It starts singeing the very top pistils right off when the buds get too close. You can see them all shriveled. It's not because the room is too hot. It never gets much above 80.

If you google it you can find pictures of white buds that were bleached from leds too close.

In my experience with led 82 is ideal temp for most rapid growth. I target that.

Light stress and heat stress can be caused by different things but both cause the same leaf condition because both elevate the surface leaf temps too much.

 

humidity is directly related to the amount of heat a plant can take, 90 deg. fer. can damage a plant with 30% RH,
but 90 deg. is not too bad if the Relative Humidity is 70-80%.!

as always, it is not just one factor, but the combination of all factors.!!

 

"both elevate the surface leaf temps too much".....

EXACTLY.!!!! temperature, not light.!
if the light intensity is burning your plants you have the wrong spectrum of light, a light NOT meant for plants.!
change lights and the problem will disappear and plants will grow better.!!!

 

where are you measuring the temps?

if you place your thermometer at the tops of the plants the reading will be MUCH higher then 78.!!!
AND disreguard any claims that a thermometer should be shaded from the light, they do not read light, only heat.!

 

I always keep my thermometer at the top of the canopy.

I don't see why you're doubting that you can run a light too close. That's just common knowledge. You can't go closer then 12" on any grow light with over 500 watts of output if it's any good. You'll almost definitely burn the plant between 6-12".

The surface leaf temps and the air temps are not the same reading. The air circulating around the leds isn't much warmer until it has something to strike/penetrate. The surface leaf temps are also impacted differently depending on the lighting type and mix of spectrum. HPS which are heavy in IR compared to led when run open bulb usually need more light distance and lower temps for optimal growth. 78 being the target for an HPS room.

I'm only growing one lady right now but she's doing pretty well. Almost 7 weeks since flip to flower light cycle. About 5 1/2-6 weeks flowering. Wonder Woman DWC bucket. Running about 16-18" light distance. 570 watts of Horticultural Lighting Group's qb260 kit's both with qb304 boards. The left in 3k and the right one in 3500k.

 

I agree

there is no doubt that a light can be too close, not for light intensity, but heat.... like you keep posting.!
bleaching and leaf margins curling up are both signs of heat stress,
both of which you will see outdoors on a very hot, dry, windless day, but will not see on a very hot, humid, and windy day.
leaf surface temp is the determining factor, not light intensity. Not with good light anyway.

if I had the room and legalities i could prove this with a 1000w HPS growing within 6 inches of the bulb.
it takes constant cool air flowing across the tops to do it, but it is very possible.!

try blowing 60 deg. air between the LED's and the tops, you can get them 8 inches above the plant.!!!
grow tents do have their limitations, and this is one of them,
circulating fans do just that, they do not provide cooler air where needed.

in my box the air flows evenly and constantly across the grow chamber.....
I need to patent this thing.! lol

 

sorry Dondokken, this is not answering your question,
I will leave your thread and let you get some helpful info from some GC members!

Tbone seems to be a great GC member, helpful, knowledgeable, and experienced.!
I would accept his advice anytime.!!!

 

Hey @tbone mind a few weeks back i bumped into you talking about this. I pointed out that i was running 90w per square foot with no issues......
Well about 2 days after i posted that she started foxtailin lol.
Was definately too much. Lesson learned tho haha

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I had a closet, 24"x42", with a 400w HPS and a 400w MH in it (open hoods), 114 watts per square foot,
pretty low efficiency, but awesome quality, harvesting the dankest buds 10-20 grams each when dried.!
with such a small footprint to cover I was able to keep both lights 6-12 inches above the plants,
and when temperature and relative humidity were kept in check there was no burning or stress.

I suggest 50-75 for optimal overall performance and efficiency.!!
my current box (empty) has much higher efficiency at 80 w/s.f. and preliminary testing shows that the tops will safely be within 6" of the hood's glass.!! It has my old 400 HPS and two 25w CFL's (6400k for some blue light).

 

Need nutes from the get go in hydro mate. I start off at 100ppm for first 2 weeks then up it by roughly 150 each week. If the ppm drops you add more nutes. If its rising its too strong so add more water. Try to keep the ppm dropping slightly and you shouldnt ever get any nutrient issues.
What nutes you got and do you have hydroguard, sm90 or something similar to keep the res clean?

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