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Distance from Grow Lights?

Vbkid

Getting There...
Joined
Sep 29, 2008
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832
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Madison, WI USA
Hey all,

Feels silly having been growing things for years and still have such basic questions, but really just want to make sure I'm not missing something obvious for my plants.

For reference, I just have a single grow rack, 2 shelves, with lighting above each shelf. One is a 4-bulb 4-foot T-5, the other has 2 Mars Hydro 600w fixtures above it. Currently most of my plants are in trays anywhere from maybe 12 inches to almost growing to touch the lights. This includes nepenthes, dews, heliamphora, cacti, pings...you name it. Overall, nothing is really dying, getting burned, or anything like that. But I also have some orchids next to the grow rack that seems to be doing fine with much less light.

Really, I'm just wondering what distance my plants might be happiest at?
 

bluemax

Lotsa blue
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Sep 5, 2011
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2,600
Location
Vancouver, Washington State, US
Actually, the way you phrased it that sounds like an advanced question to me. My opinion is that this is a matter of your preference. You are growing in the in-between where the plants are healthy but might do better to one side or the other. As an example - Drosera adelae can be grown in relatively low light and grown that way the leaves are very green and will develop into their largest size. Or it can be grown in much brighter light and the leaves become more narrow and contain red pigment. In both cases the plant will be healthy and do well. Of course, not all plants are as versatile but I think you get my point. I think the biggest advantage in all of this is the ability to tell when a plant is healthy, and when not. Then you can make adjustments as needed. 'Just my two bits worth.
 

collin

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Apr 13, 2015
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Location
Los Angeles, CA
Depends on the plants and depends on the lights! Curious plant has an excellent write-up of lighting requirements for CPs, you can find it here. Mark is right, it's definitely an advanced question.

Long story short, you can measure the light that is available for photosynthesis in photons per day - most people use moles of photons per day because photons per day is a very large number. Different plants have different requirements for moles per day, there's a handy table on that curious plant page. To Mark's point, most plants have a fairly wide range that makes them happy. So, for Helis, Curious plants recommends at least 26 moles per day. These numbers are their best guesses; I'd take them with a grain of salt.

You can measure the output of a grow light in terms of PPFD. Fortunately for you, your grow lights have that info published. I grabbed this image from their Amazon page.

51DlepB5eCL._AC_SY1000_.jpg

As you can see (and as is the whole point of this thread), this depends on the height of the light. It also depends on the location of the plant under the light - things are brighter in the center than they are by the edges. These numbers are measured in perfect conditions, real world values are usually lower than the published ones. If you want to measure this exactly you can buy a quantum PAR meter - but these are pretty expensive and it's totally unnecessary. Converting PPFD to moles per day is really easy: PPFD x 3,600 Seconds Per Hour x Hours Per Day / 1,000,000 = Moles/Day. So, if you put your Helis 12" under the light directly in the middle (986 PPFD) with the lights on for 12 hours per day, they will receive 42.6 moles per day, plenty of light (986*3600*12/1000000=42.6).

So, uh, it depends. :)
 
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