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I am beginning to get into indoor hydroponics to grow lettuce, I haven't figured out which type of lettuce to grow yet, I am thinking of a butterhead. I have been to many hydroponics stores and seen almost all the lettuce hydroponics videos on youtube. When it comes to lighting, no one has an accurate answer. The store is trying to push a huge T5 lighting system. But I think its overkill. Now I have bought a Light Meter, to test various lights.

How many lumens do I need for lettuce to grow?

I'm trying to figure out the costs involved with the electricity of the lights.

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This isn't really an answer, but have you considered running small-scale experiments? (given that this question's two years old, perhaps you've moved on already) You might be able to assemble a cheap setup with a desklamp and a CFL, and try at least one case. – John Walthour Jan 15 '15 at 18:37
If you're going to grow lettuce, consider Bok Choy too. It flourishes under growth chamber conditions. – Wayfaring Stranger Jan 16 '15 at 15:27

Lettuces don't need a lot of light to grow. I recently read "The Winter Harvest Handbook" by Elliot Coleman and in it he describes growing lettuces in the winter in Maine (Zone 5) in the dead of winter underneath two layers of insulating row cover. They don't grow fast as in summer, but they do grow.

The nice thing about leaf crops (which lettuce is) as opposed to fruit crops (like tomatoes that have to flower to produce the part you want) is that as long as you can keep it alive it doesn't really matter how big or vigorous it grows, you can just grow more of them.

I have metal halide lights indoors and the lettuces that I plant underneath them flower and go bitter in about 8 weeks - hardly worth the effort.

My advice would be to go to home depot and get some shop lights which are 40W fluorescents or something similar. They are cheap and easy. You can spend money for the grow bulbs but I would not bother at this point. Start small and experiment, you can add more if you like.

BTW, if you are in a cold climate where the inside air in the winter is very dry you will get better results if you enclose your growing area as much as you can and do everything you can to keep the humidity up.

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I have had excellent indoor results with two T5 fluorescent tubes per 8 heads. The most important thing about the lights is that you must get 6400 Kelvin (+/- 300) wavelength lights. They are not the most common you will find.

Place the lights about 10-20 cm from the plants (only do this with fluo or LED lights, not Metal Halide or HPS bulbs)

When the plants are germinating from seed (until they get their first true leaf, most often the third one), I run the lights about 6 hours per day. Once they get their true leaves, I run the lights 10-12 hours per day.

You can also severely cut down on the amount of light (and electrcity) you need to use, by putting highly reflective material around the lights, so it bounces back on the plants.

Hope it helps a little :)


About the 6400K (how it will often be listed) lights, they are also known as "day light" or "natural", but you can not trust these descriptions. Always be sure to find a Kelvin. They can be found in your local hardwa re store if you're lucky. I found some for only 6 USD in my local store here in Berlin. Bought 11 right away.

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The reason you are not finding the answer you are looking for in regards to the lumen requirements of crops is that lumens is a measurement for humans. It is based on the level of light that we can perceive.

Plants mostly require light in wavelengths of around 430,450,650,675 nano meters (depending on the crop), with highly varied intensity requirements from crop to crop. Lettuce requires mostly the high end of the blue spectrum of light, 650-700 nm, at an intensity of 17 mols/m2/day of PAR (Photosynthetically Active Radiation). If you do some research you may be able to find the manufacturers data sheets for certain lights, which may include this information, although you may have to contact the manufacturer for more in depth answers.

The reason why lumens are a bad indicator for determining light requirements of plants is that the light plants need most is almost out of the visible spectrum for humans. Manufacturers of indoor lighting will try and target 555 nm because the photopic conversion (lumens/watt) is 683 lm/w at this wavelength, while light that plants need has a very small photopic conversion (430 nm is only 8 lm/w, 650 nm is 73 lm/w). A 100W plant light (we'll assume 100% efficiency for math purposes) could be producing only 4000 lumens, while a 100W human light would be 68300 lumens.

Financially though, you are better off using a high volume manufactured light (common bulbs) that is producing a lot of light you don't really need, but is producing a small percentage of the light you actually do need. If you use fluorescent tube or CFL lights, I have used both, you should aim for about 1500-2000+ lumens per square ft. with a Kelvin rating of 6500K. The cheapest way to do it is wire your own panel using keyless lamp holders and bulk buying the CFL's. Discard the bulbs after 6 months or less, as the spectrum of light your plants need will decrease. Use 1, 23W CFL (100W equalivant), 6500K bulb per plant.

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Fletcher, I guess you meant 400 to 450nm wavelength for Lettuce? 650 to 700nm is Red and used for flowering and fruit ripping, yes? – Nilo Bortolotti Jul 11 at 16:55

I have had success with an 80 watt compact fluorescent light (CFL) within 5 cm of the plants, you can grow at least 6 plants. Unlike metal halide and high pressure sodium lights you can position the light close to the plants without burning them. Also you don't need a ballast just a standard screw fitting for the light.

This page might be helpful under the heading: How much light should plants receive?

Incidentally using a CFL light is a great way to start seeds. It amazing how easy it is to start seeds (even temperamental varieties) with CFL light and a heating mat.

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By 80 Watt, I guess you mean an 80 Watt (incandescent) equivalent. – Niall C. Oct 5 '12 at 15:48
No its equivalent is about 400 watts (or so they say) it is an 80 watts CFL bulb - here is a a brand – stemie Oct 8 '12 at 8:05
I also use a computer fan in the grow area as air flow is important. If you want to have a bit of fun you can even add carbon dioxide. In a bottle combine yeast sugar and water make a hole in the cap, connect a pipe from the bottle to a glass of water make sure the pipe connected to the bottle is air tight. The yeast metabolises using sugar, giving off carbon dioxide which bubbles into the glass of water. – stemie Oct 8 '12 at 8:13
@BRM if you use 80 watt compact fluorescent light (CFL) you don't need a ballast just plug it in normal fitting. From personal experience I know these lights work well with lettuce and basil. In my answer there is a link, you can pick up a CFL for like $30. – stemie Oct 17 '12 at 9:16

Even with all the answers, there doesn't appear to be a mention of recommended lux/lumen for lettuce.

Approximately 80-100k will be adequate. Too much more than that (150-200k) and you'll risk having too much which if I recall correctly can cause lettuce to flower (due to the heat concentration).

A laser temperature gun can help determine if the intensity is resulting in too much heat. The leaves shouldn't be too much warmer than the ambient air.

Also a T5 is not by any means overkill. They are pretty mild lights compared to LED/MH/HPS. You will see about 80-100k at around 6 inches from a T5. Arguably perfect for lettuce.

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I use the shiny car shades to capture lost light and bounce it back onto the plants. Ideal for lighting a defined area such as a flat or many small pots.

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I'm guessing you could get by with less light than most people are recommending, and different color temperatures, too, although those they recommend might be more like ideal.

I just started growing some arugula indoors (though not hydroponically), using two regular CFLs that weren't advertised as grow lights (probably 22 watt or less, each, and probably 2500k), and the arugula (as well as my other plants around the light seem to be enjoying it, even though they don't grow toward it very fast; there's more to a light than how fast plants grow toward it. I'm guessing they grow toward blue light more than red.

The arugula is still young, and I haven't tried lettuce, yet (although I may try some Merlot lettuce soon); so I'll have to update in a while. I am, however, keeping the lights on longer than others here with their brighter lights (only giving about five hours' darkness or less). They also get some window light.

For flowering/fruiting plants, I would actually recommend using more than one color temperature (like a 2500k one and a 5000-7000k one, if not more than two). However, for lettuce, you probably want mostly the higher color temperatures which should help the leaves grow most.

The color of your lettuce may impact your light requirements. I'm guessing since Merlot lettuce is dark red, it would prefer a higher color temperature even more than regular lettuce, since lower color temperatures have more red, and the lettuce would reflect more red (rather than absorb it), because it is red.

Anyway, I'll probably get more and different lights in February, and update this post, but probably not before then.

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