Need Help Understanding PAR Graphs for LEDs

[MajorTom]

Can someone explain how Mfg PAR value guidelines work; and what that means for my setup?

I'm running DWC in a very small space 1.5x1.5x3.5ft with 2 autos (Blueberry and N Lights). I just added a dimmable MaxBloom x4 which says its a 400w light but only actually draws 200w from the wall (so isn't it a 200w lamp?). It supposedly comes with 2 Cree COBs at 100w each in the center; 100% red and run off of the BLOOM dimmer.

The chart shows higher PAR values in the center, yet my plants are both positioned on the sides. Plants are now only 3-4" high (see below) after 27 days since planting (seeds took about 5-6 days to sprout), and therefore in or entering the vegetative stage. So at a height of 24" am I getting enough light for my plants? If I'm reading this correctly my plants would be somewhere in the 400-800 umol/m2/s range. The light is dimmable, so I'm assuming these figures are at 100%.

How much PAR do I need? Is it saying that I've got to keep the light at 12" to maximize the PAR Values for my space? If I'm reading the chart right this would bring me to the 600-1,000 range...

I've obviously got space issues with distance from the light and width/length of the light box itself.

What can I do to maximize the energy, light/distance for this light with my setup? Any ideas? I'm thinking about lowering it from 24" and maintaining it at 18" until I start to run out of room. And setting the dimmers at 100% for both VEG and BLOOM. Does this sound like a good strategy?

This is my first grow so I'm really 'winging it' here -- any help greatly appreciated!

[DrPhoton]

Height placement and light utilization efficiency is hard to accurately guage without a quantum light meter. You typically have to use trial and error and learn the best placement for optimal growth and light efficiency.

Ideally during early vegetation, you want to bring the light fixture as close as possible with reduced brightness levels, so as to not waste light that would otherwise be dissipated from radiated losses. However without equipment to ascertain real time data, it can be difficult to guage what the light characteristics will be at a given height and power output. As such using the provided height recommendations by light manufacturers is about the best approach to safely provide light to your plants without harm. But they cannot provide data for all dimensions of growing environments and you may have to approximate what height is suitable for your particular envioronment. Based on your grow size, you may be best to start at around 24" as that would provide roughly 400-800umols. You could try bring your light source closer but be careful and watch for any signs for light damage, this can seriously hinder and slow your plant down. Once the plant has filled the canopy as much as possible, you want to balance between efficient light distribution and plant photosynthetic efficiency.

You may find my article on "Why Close Light Proximity Is Not So Important" helpful.

[gbauto]

Have to agree with DD-the only accurate method to determine if you have enough 'oomph' is to use a PAR meter. Based on the graph supplied, I'd also try the lamp @ 24" in veg and see how they respond. I tend to run approx. 400 umole in veg and then bump up to 800-1000 umole in flower. so that fixture may need to be at 12"

[MajorTom]

All very interesting, thanks!! In terms of my tiny setup and with this light would you say then that my best option for higher yield would be to grow one plant directly centered under the light and train it with topping / LST?? It seems like I’m not maximizing this light’s intensity by setting two of them on the periphery.

[DrPhoton]

To capatilize on your lights full potential, you will want to fill the entire canopy so no light is wasted through radiated losses. How you go about that with autos i will leave for others to guide you

[MajorTom]

And therein lies the challenge.. Maybe I should change my "handle" to "BonzaiGardener".. LOL

Thanks again!

[Paracelsus]

I need to ask are you vegging with lights on 24/7?
I’m using T5 so I am not speaking from experience but from exhaustive reading I’ve done on the subject. What I would suggest with your small space rather than lowering the light raise the plants, observe closely and then adjust raising or lowering as required. I don’t know how large the pots you are using but get them both as close as you can directly under the light and rotate them on a 12/12 basis is what I would suggest as well.

ps some COBs are not wired to run at maximum output, some are wired to run at Max efficiency depending on which driver was used you may be getting significantly less light than the specs advertised.

[MajorTom]

Thanks Paracelsus;
I’m vegging 20/4. With my dwc setup I can’t raise my little ladies up but I do see your point of getting them closer to the light and going 24/7. Actually leaving the lights off was causing my temps to go too cool as I have finally found the perfect exhaust/intake specs at temp/hum 75/55 with the lights on. With my tight setup I cant rotate either my light or pot. Moving the lights closer and going 24/7 sounds like a plan. Reviews on my led say they can go 12” without light burn. Does it take a long time to see the effects of light burn?? I’ve read it can take a week or more. I’ll be able to watch them for 7 continues days once I get back. Maybe I should try moving to 15” and see. Unfortunately I can’t make any changes until I see them again in about 10 days. Hopefully I’ll have some good news to report...

[Paracelsus]

I’ve seen pictures of nebula hazes grow where she raised one TFDWC next to another to keep the canopy level (2 strains) but since it is just one container I might give you another suggestion. Yes go 24/7 immediately and go to 20 inches when you return. The plants may surprise you and grow while you’re away, if you’re going to be home for 10 days in a row that should give you time to watch closely. If your light is too close you should be able to see some changes in as little as 12 hours for sure in 24. If yo see no change after that in 48 hours go to 16” and then if still no change try 14” baby steps for baby plants. Since you will be home for 10 days you will have plenty of time.
FYI DWC likes a constant temperature, 68° is ideal. 62-68°F. Above 72°F and dissolved oxygen (DO) dips too low. Below 60°F (16°C) and plants tend to slow their metabolism as they "think" the season is changing. Some growers use this to their advantage and reduce nutrient temperature towards the end of the flowering stage.

pH can drift slightly to absorb all the nutrients. The usual range (5.5 - 6.5) for pH is fine. 6.0-6.3 is optimal for vegetative growth, and 5.7-5.9 for flowering and fruiting.

Lastly,Keep EC / TDS on the low side. Try half strength (based on the manufacturer's guidelines) first or even lower when plants are young. Lower EC can result in a higher intake of water into a plant's tissue which, in turn speeds plant metabolism and increases nutrient transport.

Good luck and happy trails

[MajorTom]

Thanks Paracelsus! This is exactly the kind of info that I've been looking for...

When you say DWC temp 68 is ideal; you mean the water temp, right? I'm using one of those probe style aquarium chillers set on a timer; looking at adding a plug in thermostat controller my next go round.

On the EC/TDS since my last change used full mfg guidelines (because I didn't notice an nutrient burning); maybe staying away from them for a while during this 12 day stint could be a good thing as the backup res is only ph adjusted non-nutrient water..allowing my water to drift down a bit...? I'm such a doting parent!

Thanks again for all of your help!!

[Paracelsus]

Since you’re going to be gone I would leave my nutrient water at least half strength they need nutes. Yes I mean your reservoir nutrient temp

[MajorTom]

Sounds like a good plan; hoping they'll be ok this round... This is a bit longer than usual.... ;-/