How close can grow light be?

[MSGhero]

I currently have a SF-1000 running at 80% power, 12" above my ficus. I want to put it all in a shelving unit in my apt, but I'll have to adjust the heights.

What are the downsides if I go from 12" above plants to 6" above plants? Math tells me to turn the light power down to 20% (1/4 of original), but am I missing some practical knowledge here?

My shelving unit is only 16"x16" wide/deep, so I don't need a large coverage area for the light. I can remove one of the shelves to gain more height, but I'd rather not since it looks nice as-is.

 

[Leo in N E Illinois]

I am not familiar with the SF-1000. Why are you running at only 80%. If you are not burning leaves, you should be running at 100%.

When using T-12 shop light fluorescents, the lamps could be so close that the leaves could actually touch the lamps without damage. Closer is usually better.

2 issues, intensity and heat.
Heat:
If you have a fan, blowing a gentle stream of air over your trees, you can keep heat from building up, and have the trees closer to your lamps. Leaves should always feel slightly cooler than air temperature. Air movement is right if the leaves feel cool, even in the brightest light possible. More air movement, the closer the leaves can be to the lamps.

Intensity:
In the "bad old days", 40 years ago, it was simply not possible to have lamps so intense you could burn leaves if you had good air movement. Now a days, with modern fluorescents and LED it is possible to have too much light, though it is a serious, expensive "marijuana grade" light system that would put out this much light.

Myself, I would first make sure you had good air movement. In my lights set up I have several fans running 24 hr a day, 365 days a year. Then just slowly crank up your light towards 100%. Leave each new step for a day, and see if you get any yellowing or red pigment forming. Your are right at just a little less than what causes yellowing. Ficus can take 100% full sun outdoors, it probably will be able to take 100% even at 6 inches IF YOU HAVE GOOD AIR MOVEMENT.

Stagnant air will allow even lower intensity lights to build enough heat in the leaves to burn leaves. Put a fan in, and you can use brighter light. More light = more sugars = more growth.

 

[MSGhero]

I was burning new growth, so I turned it down to 80%. I have a fan on the tree, although it's a few feet away. The shelving unit lets me mount the fans much closer.

So it sounds like start at 20% because of math, mount the fan(s), gradually crank up the intensity since I should be able to go higher?

 

[Leo in N E Illinois]

Sounds like a plan. The more steady the breeze from the fans, the higher the light intensity will be tolerated without damage. In nature trees can take 40 mph breezes no problem, though the sound of leaves flapping on your light garden shelf might be a bit too annoying. It is a balance. Set your fans and lights. Slowly crank up the lights to the max the plants will tolerate, then dial back just a little bit. Just "off peak".

 

[HorseloverFat]

What “OL said”

I go fairly close.. but have fans ON the plants, semi-gently. Also air-“dump” vent piping outside air ALSO hitting the plants, even MORE gently, at a 40-ish degree cooperative breeze with the PREVIOUSLY mentioned fan. And a Ceiling fan unit “Sucking up” NOT GENTLY AT ALL...

I personally have not noticed problems except within 1-2”, and even THAT’S rare, for me with my ventilation/circulation design.. Certain trees that winter indoors by me, actually brush the lights with the breeze...

 

[Hack Yeah!]

I was burning new growth

Sounds unlikely, any chance there are other problems? Pest, fungus, bacteria, watering?

 

[MSGhero]

Sounds unlikely, any chance there are other problems? Pest, fungus, bacteria, watering?

Nah, it stopped when I backed off the light and started again when I turned it back up to 100%. Now maybe something else is wrong and causing that to be the case, but my watering has been good lately at least. Ficus nerifolia if it makes a difference.

 

[Hack Yeah!]

Ok, good luck with it either way. It's just highly unlikely that the light alone is your problem

 

[Hack Yeah!]

And... welcome to the site

 

[Harunobu]

Check this video for PAR:

So at your 12 inch, if it were at 100%, it gives around 700 micromoles per second per square meter. That is quite high for a ficus plant, I imagine. So no reason to put it closer.
There is actually a reason to decide to run it at 80%, namely to save energy because you don't need all the light.

Likely, with moving air you can keep it closer to the leaves. Evaporation of water helps the leaves to cool. More air movement means move evaporation. Evaporation is an extremely good way to cool down/lose energy. Your leaves are much more likely to burn because of heat than burn because of photosynthetic stress.

The daily light integral for ficus is going to be way lower than a cannabis plant. With this light, you can grow a cannabis plant in a 60cm by 60cm area. So if you have a smaller area and a way lower light requirement, it is up to you to decide whether to put the light up high, and get a nice spread, but use more energy and have most of your light not hitting any leaves. Or to run it at lower power using the dimmer. Does Leo really think one should put it 6 inch above the canopy but run it at 100%? That would be close to 1200 micromoles per second per m2. That's the level where people decide to provide additional CO2 if they are growing cannabis plants. At 6 inches, this light is basically as bright as a day get near the tropics, giving a daily light integral of about 70 if you run your light for 16 hours a day. This site says that a DLI of 14 is good for Ficus:

MechaTronix - Typical PPFD and DLI values per crop

Plants absorb radiation mostly in the 400-700 nm visible range and convert CO2 uptake and water into oxygen and glucose. The amount of absorption in each wavelength depends on the cellular structure of the plant and may differ from species to species somewhat. Another important parameter is...

www.horti-growlight.com

I guess it can take a DLI up to 30, since it says 'high' and not 'bad'.
Sadly, there is no source.


Yes, light intensity should fall off with 1 over the square of the distance. So your math is correct.

 

[LittleDingus]

Sounds unlikely, any chance there are other problems? Pest, fungus, bacteria, watering?

It is more likely than many think. My light is Phlizon, not SpiderFarm, but my light is in the same class as the SF1000 and the intensities are likely equivilant. I just stuck my intensity meter under mine to grab a measurement:



I was having trouble holding the meter and the camera each one handed but in certain spots, the meter was toping out at 175Klux. This picture is showing 158KLux. This meter tops out about 120KLux under direct sun outside on a clear day. So, not only is there on the order of 30% more lux right up against this light, at places that is 100% blue light whereas sunlight is closer to 75% blue. In short, there is a LOT more energy coming out just under these than people consider.

One thing to remember, unlike tube lights, LEDs are essentially point sources. The photon density directly under an LED can be significantly higher than the photon density extended over even a short length of tube in a tube light.

 

[TN_Jim]

a quality PAR meter would really dial this in. the sensor is important

here’s one accurate for research, I would weigh the specs of this against others

LI-190R Quantum Sensor | LI-COR Environmental

The LI-190R Quantum Sensor measures photosynthetically active radiation (PAR). The LI-190R is designed for use in horticulture, ecology, greenhouses, plant science research, and growth chamber light measurements.

www.licor.com

 

[sorce]

I have two grow lights that are so close, I now have 3 grow lights!

Sorce