How much lighting do I need for growing lettuce using indoor hydroponics?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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 :)

Update:

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

For most leafy greens use a red:blue ratio of 5:2, this will be sufficient to promote good growth. While it would initially appear you want more blue light, that is not how photosynthesis works when you get down to the nuts and bolts. Just use a red:blue ratio of 5:2 and you'll find that works best. Today 2017, many of your indoor vertical farms are playing with the r:b ratio to affect taste, color and texture.

Answer 9

According to my experience, 5-15 klux is ok for lettuce (I use Lux Light Meter app on my smartphone). If more than 15 kLx, leaves become purple, I think it`s not good.

Answer 10

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

The reason no one is saying a real true number is as was pointed out by one other person in thread lux/lumen and foot candle have nothing to do with what the plants need to accurately give a number you need to talk in plant light requirements which means the need for par (Photosynthetically Active Radiation), or ppfd amount of light photons that hit a surface (your plant canopy) in one second. It’s measured in units called micromoles (μmols) for light wavelength that is usable by your plant. and these are converted to DLI Daily light integral (DLI) describes the number of photosynthetically active photons (individual particles of light in the 400-700 nm range) that are delivered to a specific area over a 24-hour period.

If you happen to have an iPhone, there is an app call KORONA - cut a small piece of 3/4 pvc about 1/2 inch long and super glue a piece of bright white 20lb paper over it to act as a filter. place this over the iphone front camera lens and use korona to measure light and korona has options to show ppfd - dli - and if curious lux plus can upgrade app to also show kelvin scale

So its not simple thing pop a couple bulbs out throw a lumen number at them and say ok plants will grow, yes maybe they will grow but not at the best or optimum for the plant. so now if that has you all confused its not that hard to go by ....

Lettuce needs 12-14 mol/m-2/day which equals if you have a light system putting out 280 ppfd the DLI will be about equal to the number of hours the lights are on 12 hr = 12.1 dli , 14 hrs = 14.11 dli .

My current LED lights are 2:1 ratio of 3000K/5000K running dual meanwell 100 watt drivers with a real Watt draw at kill-a-watt of 209Watt, produce at 9.5 inches from canopy 295 ppfd @ 12 hrs a DLI of 12.7 and not that it has any cross correlation to plant because it refers to visible light 16300 lux meter/sq ( SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time ) and not all light plants use is visible, hence the difference in the unit of measurement to judge light for plants.

But a grow light the gives actual numbers as stated in lux and ppfd as tested in sphere and watch out for lights that tell you " yes replaces lights of XXX watt much lower watt but dont give you a hard data sheet of the light wave length, and actual wall watt draw. i wouldnt swap out my led for any t5 cfl or metal halide out there for power ppfd/watt light efficiency.