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Distance from Grow Lights?

Vbkid

Getting There...
Joined
Sep 29, 2008
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853
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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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Joined
Sep 5, 2011
Messages
2,746
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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Joined
Apr 13, 2015
Messages
141
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. :)
 
Joined
Jan 19, 2022
Messages
6
Rite. I have been using flourecents and leds for several years. Some plants are much happier with less light but can handle more. They just grow better with less light. Nepenthes can vary greatly in how much light they need/like. I have noticed most plants do better at 24to 30 inches from my led grow light. Some do good at 24-18 inches but by the time they hit 12 inches or closer they start acting funny or even get bleached out leaves and litterally grow white colorless deformed leaves. But thatvwas only one type of plant. Flourecents are best in the close range since the usable light doesnt travel as far as with leds. I have always had tube or cfl bulbs at the 6 inch from top of plants range and let them grow to almost touching the bulbs then reset to 6 -8 inches so they stretch out a tad reachingvfor the light but get close enough again before they get leggy. Most plants can handle 6 inches from flourecents at 14 -18 hours a day. I am actually building a super insulated waterproof closet now. Its mostly done. Just need finishing touches and to buy some new light fixtures. I am thinking of going with an led that has a dim switch. So I can adjust it depending on plants. And I am gonna go with an all white light this time . no pinky purple. Actually it will be 2700kelvin and 5000-6500 kelvin. So it will have some of the orange ish hue of a soft white bulb but also lots of clean white looking light in the 5000kelvin and above range of the spectrum. I may buy sepperate strip style grow lights at specific kelvin and stagger them next to each other. I enjoy research and development of grow light systems though, I been in the testing phase for 8 years now testing cfl and tube flourecents, then maybe 4-5 yrs ago I got a mars hydro led and have grown everything under it too. Mainly fig tree cuttings the past 3 yrs but other stuff too. Small aloe plants of crazy varieties, they get strong colors under they led. I do find that all my plants do like some additional 5000-6500kelvin light when under the led to help with vegitative growth , if not they get short-to- very short internodal spacing which is fine but I do like the look of a more natural node space and the shade of green lightens up some to a more natural color on my fig trees with additional 5000 kelvin light. Most leds designed for plants Ive noticed get harsh if my plants get closer than 24-30 inches, at least for most plants. Anyway. If they look happy then :) if they look like they are thirsty all the time but have plenty of water try moving those specific plants to the outer edges of your lights . my one nepenthes does well at 30inches from 2 fifty actual watt 2700kelvin leds in a east facing window. But am investing in some veitchii next week now that I have over 2 yrs experience with nepenthes and mine is extremely happy all the time. So am gonna finish my water tight grow closet specific for nepenthes and other carni plants. Pics bellow you can see every inch is insulated with foil bubble wrap and the floor with black heavy rubber exercise mat. All glues and sealed in place. I will install some white bamboo flooring or something waterproof over the insulation so its all flat white and not shiny metal. Flat white from all my research reflects the actual useable light for the plants better than gloss paint. Looks better too at least for my taste. The shiny walls are cool for now but I preffer a clean all flat white lab kind of look. Thats what I am going for actually as a finished look. Like a nepenthes lab with a microprop area and small flow hood for sterile work. I will make the flow hood as small and portableas possible so I can use it anywhere.
 

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