New to lights- lettuce question

OSgardenerJune 5, 2011

I am new to growing under lights, and I have some questions about my setup. I am trying to grow lettuce, as lettuce appears to be simple to grow under a light. I have spent hours of reading trying to figure out if I have the correct setup, but cannot seem to figure it out on my own.

I am using a shop light (the round kind that have clips) with a Sylvania CFL 23 watt daylight bulb. It is 6500K and has a light output of 1,450 lumens. From what I understand, for growing lettuce one needs a light at the blue end of the spectrum. Will this bulb work for growing lettuce, or did I buy the wrong bulb? If so, what bulb would you recommend? I am trying to keep things simple/cheap, hence starting with lettuce.

Any help is greatly appreciated.

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My raft system is about 30" wide and I use 4 40-watt daylight bulbs. Seems to be working fine.


    Bookmark   June 5, 2011 at 4:59PM
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Mine setup seems to be working fine as well. Although my lettuce has yet to sprout, I have two tomatoe seedlings and an eggplant under the light and they seem to be growing quite well. My lettuce is in a round container, which is why I am using a round light. All seems well, I just wanted to double check I was doing things correct. Thank You for your reply.

    Bookmark   June 5, 2011 at 6:35PM
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I also have another question I forgot to include in my original post. How many lumens are ideal for growing lettuce?

    Bookmark   June 5, 2011 at 11:10PM
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Not to be flippant, but lumens are like points in Drew Carey's "Who's Line Is It?" program. What counts is the intensity (measured in Foot Candles or Lux) of the light hitting the plant. I just measured mine and I'm getting 6500 lux or ~604 FC.

A very broad rule of thumb is 1 lumen is equal to 1 FC if the light is one foot away. But that is for a point-light source - in other words, all the light is aimed at a point. My bulbs are 2800 lumens and I have four of them inside a white reflector. The light is 3-4" above the tops of the plants.


    Bookmark   June 6, 2011 at 4:37PM
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So, instead of focusing on lumens I should be looking at lux (which is 1 lumen/square meter, correct?)? Is there a formula for figuring out lux? Sorry for all of the questions, but all of these light terms are throwing me for a loop.

    Bookmark   June 6, 2011 at 5:01PM
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Yes, 1 lux is 1 lumen hitting a square meter. And yes, there is rough formula for calculating lux (or Foot Candles) but it is anything but precise. It presumes that a bulb rated at 1000 lumens will deliver 10,700 lux or 1,000 FC if the bulb is one foot away. In reality, the light will diffuse, some will be blocked by the bulb itself (the light coming from the top of the tube, for instance and bulbs seldom provide the lumens they are rated at, unless they are brand new and have a perfect reflector. Then there is the inverse square law that says that as the distance from the bulb to the surface is cut in half, the intensity is increased 4 times.

Lumens are a good way to compare one bulb to another, if they are the same type and color temp. But alone, they are like engine horsepower. A 1949 Cub tractor with 12 HP will pull a 12" moldboard plow. But that same plow behind a 20 HP Cub lawn tractor and it will sit there.

If you plan on doing any serious indoor growing, I highly recommend a light meter and reading about Daily Light Integral by Jim Faust. Of course, I hope to grow at least 144 heads of Romaine lettuce at a time after my outdoor season ends this fall.


    Bookmark   June 7, 2011 at 5:44PM
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Something many growers here haven't discovered is that lumen specs are weighted around human vision, which sees best, or is most receptive, at green wavelengths. A "totally green" light might rate high in lumens (your plants would be light starved) while a "blue-red only" light would rate low in lumens (your plants would do well).

However, as Wordwiz correctly stated, "Lumens are a good way to compare one bulb to another..." if the bulbs have similar spectra, as he inferred. I always found that just comparing spectral energy (SPD graphs) to be my favorite method.

To add a bit more info, color temperature (CCT) is NOT a reliable indicator of light content. While CCT is a good indicator of the color content of the standard blackbody definition, it fails to convey the true output of artificial lights. In that case, the CCT merely indicates the average center of the sum of the wavelengths emitted by the lamp source. For example, you could have a lamp which only emits infrared and UV. By simply varying the level of the two , you could achieve ANY color temperature rating between maybe 1700K and 30,000K. How about that - no useful plant energy there at all.

If you have the time to read, it would be educational to look up the specific meanings of Color Temperature, Metamerism, Blackbody, Lumens and Spectral Power Density(SPD). If you compare the smooth blackbody spectrum to the "jagged and uneven" spectrum of fluorescent and HID lamps, you will begin to understand. Google and Wikipedia might help.

I am sure that some of the regulars here really do know that, but nobody seems to try to convey that information. Good luck reading!


    Bookmark   June 7, 2011 at 10:08PM
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A couple of other crumbs to nibble on:

If you want to see a spectral graph of the light you are using, it's cheap and easy. The simple way is to hold a CD or DVD at a 45 degree angle under the light with the bottom side facing the light.

A more precise way: You need something like a shoe box and the CD or DVD. Poke a small hole in the top of the box. At the other end, cut out part of the lid or side. Insert the CD or DVD so the bottom is facing the hole in the top of box and tape it so it is at a 45 degree angle. Place the box under your light and view the percentage of each spectra of light.

When it comes to useful light, here is a comparison among some common types of lights and how useful their type of light is to plants. The figure is the amount of mols (an hourly accumulation of PAR light) plants receive. For simplicity sake, the amount is based on 1,000 Footcandles.

Sunlight: .713
HPS: .473
MH: .546
Cool-white fluro: .524

Veggies such as tomatoes, peppers and cukes need at least 20 mols per day (multiply mols per hour by hours of light) for decent production. I have not seen the figure for lettuce but IME, it is probably 12-14 mols/day.


    Bookmark   June 8, 2011 at 5:38PM
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Thank you guys for your replies. I still have a bunch more reading ahead of me, but I have since started. After some research I am wondering if it would be beneficial to switch to the shop style grow lights. It seems to me that this is the prefered setup. My lettuce has since sprouted, however, I am a wondering if it will do well under my current light setup. I went to the hardware store today to price some shop lights, and they seem pretty cheap. I have spent more on my round shop lights/ bulbs than I would have spent on a shop light. I am looking at purchasing a 4 bulb shop light, maybe one that is around 36" long. I do have some questions about such a light.

I believe the shop light I looked at uses T8 bulbs. What bulb would you recommend I purchase for growing lettuce? I am still so new to this that I do not even know what to look for as far as bulbs go. I believe (correct me if I am wrong) that I should be looking for something around 6500K for growing lettuce. Could such a light be found at Lowe's, or would I be better off purchasing the bulbs online?

Thank you for your help.

    Bookmark   June 8, 2011 at 8:10PM
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Wanna know a secret? You cannot go wrong with the most inexpensive "cool-white" bulbs. I have used more exotic bulbs at different times, but as flourescents go, cheap cool-whites have always done me well. I am pretty sure that they currently have a two-layer phosphor: a halophosphor on the glass surface and a second coating of "rare earth" triphosphor blend on top of that. The old ugly "cool-white" tubes from years ago were really great for plants. Now they have been officially mandated to be more efficient and have better CRI(color rendering index). That is why they now contain the triphosphor blends that are in flourescents sold currently.

Lamps that are labeled 7xx(eg 730/735/741) have the halo-triphosphor combination I mentioned above, and are in the 70% CRI range. Lamps that are labeled 8xx(eg 830/835/841) have only triphosphor blends in them and are in the 80% CRI range. CRI is a rating for human color accuracy. Plant just don't have eyes, yet, so don't worry about CRI ratings. (We cannot predict what future genetic manipulation may bring, like... plants that can see you coming, but right now CRI is meaningless to them).

Lucky for plant growers, both the old halophosphor and the current blue-phosphor component in the current triphosphor blends cover a broad spectrum in the blue wavelengths that just happen to center on both the human eye's blue cones AND the blue-absorbing chlorophylls.


    Bookmark   June 8, 2011 at 10:45PM
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I forgot to note that the ugly older "cool-white" tubes that I mentioned were halophosphors only. They had a bland, almost greenish hue, and did not have much of a red component. That didn't seem to matter. They did a great job on plants. I once painted a small coal bin in my basement from top-to-bottom with flat white paint. I had 30 cool-white lamps across the top of the ceiling. It was startlingly bright. My plants loved it.


    Bookmark   June 9, 2011 at 12:16AM
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I like the daylight (6000-6500K) bulbs better than the Cool White (4000K) but they both will work.

What size you need (2 or 4 bulbs, 24, 36 or 48 inch length) depends on how many heads and what type of lettuce you might need. A four foot, four bulb shop light will easily support 21, maybe 24 plants in a 3x4' DWC hydroponic raft system. If you figure 60 days from transplant to eating, you could figure on having two large heads of lettuce per week. Enough to feed a family of rabbits! Or, you could take a couple of spaces and grow chard, spinach or a number of other greens. If you start the plants in something like Oasis Horticubes or Rockwool, it's easy to raise 12 plants to transplant size under a 23-watt CFL, cutting the time until you have harvestable size plants to 40 days or less.


    Bookmark   June 9, 2011 at 12:21AM
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Thank you guys for all of your help. I ended up going to Lowe's and checked out their selection of shop lights. Interestingly enough they had one 48" 4 bulb light left, so I bought it. However they only had two light bulbs (go figure). I have since purchased four 6500K bulbs for my lettuce. I am keeping the round shop lights over some seedlings, but the majority of my lettuce is under the new setup.

    Bookmark   June 13, 2011 at 5:24PM
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khyberkitsune(Redlands, CA)

"For example, you could have a lamp which only emits infrared and UV. By simply varying the level of the two , you could achieve ANY color temperature rating between maybe 1700K and 30,000K. How about that - no useful plant energy there at all. "

Except there's a Pfr and Pfb reaction that IR and UV encompass in plant physiology, so yes, there IS usable energy there. Not a very accurate statement.

Photon flux density is the proper measurement used in photobiology. Not lumens (or lux) or candles, raw photon count. In a targeted-light application you would call it photosynthetic photon flux.

Spectral graph + photon flux density (which most lamp manufacturers will not list) is the best way to obtain the info you need.

Pick up a $200 cheapo quantum meter (yes, that's for the cheap ones with a decent degree of accuracy) and get to your nearest hardware supplier with it to test out the lamps.

    Bookmark   August 17, 2011 at 6:19PM
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Hey everyone.

This post has been helpful and I was hoping you could help me.

I have a 2x2 foot box and with winter coming I need to move indoors and equip with lighting. Inside it will get roughly 1-2 hours of sunlight per day (if there is a sun out) I am looking to grow spinach, lettuce, swiss chard, and beets.

At the moment I am looking to purchase a 2 foot, single bulb, T5 24watt fixture with a 6400K bulb (2480 lumens)

Do you think this would be enough for growth?


    Bookmark   September 18, 2011 at 8:54PM
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