Hydroponic Lettuce

hydro_scottyJune 3, 2011

Just wondering if anyone out there has some insight into elongated stems on hydroponically grown lettuce. It is happening on all the varieties with exception of butterhead. The other varieties are Romaine, New red fire and lolla rossa. These are grown indoors with 100% artificial lighting.

I keep EC, pH under tight control.

I run lights for 14 hours/day

I keep the area cool and vent the light heat out

I also chill the root zone to ensure it never exceeds 70 F

The room temp and humidity stays around 73F with 62% respectively

When the stem forms, it will develop a milky residue inside it. This tells me the head is bolting but for the life of me I can't figure out why. All conditions say they should be fine. I have had the lights up as high as 17 hours/day but the long stem was still there. It did seem longer when the light was on longer but it did not decrease much after a 3 hour/day drop in light time.

I had read an article stating that lettuce may be programmed to bolt after a certain number of total light hours. ???? Is it possible that 14 is still too long and causing this issue?

Looking for input/advise.


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Hi there - I just dealt with the same thing in my little garden. Have you grown lettuce successfully before with the light you're using now? I had to get a stronger light (I bought a T5 rather than a T12) in order to solve the long stems issue. It seems to have worked! And then just when I was looking forward to lots of salad the plants bolted. I found the same information, that said that many types of lettuce only live for so long and then they will bolt, no matter what the lighting schedule, temp etc. So I harvested the bolting lettuce and I'm starting new ones. I guess I would suggest maybe you need a stronger light for the long stems. I'm still kind of new at this however!!!

    Bookmark   June 4, 2011 at 7:27PM
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Thanks for the reply. I'm running the strongest light available at a 1000 wat metal halide. I am about 33 - 34" away fromt he plants. The light meter I have reads about 6,000 foot candles of light on average. The outdoor sun reads about 8 - 9k.

I kept thinking the light wasn't strong enough either but I'm sure I'm providing enough light. Thanks for responding though, obviously, we're both fighting a similar problem. Let me know if you figure it out and I'll do the same here.

    Bookmark   June 5, 2011 at 12:49AM
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I am having the same problem, can you keep me in the loop if you figure something out. i will do the same, but i have to say i am very new to Hydroponic.

    Bookmark   June 6, 2011 at 4:00PM
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you're sure it's metal halid and not high pressure sodium right? just asking because HPS is too red and will make them leggy.

    Bookmark   June 6, 2011 at 4:54PM
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Yep. Sure am confident about the MH because I specified 5K color temp bulbs. They are pretty blue.

I have my lights on movers and I think this may be part of the cause. The plants are getting 6,000 foot candles for a period then it drops to 0 after the lights pass over head. I may be moving my lights too far from the plants causing too much extreme conditions.

I also sent plant leaves out for analysis at the lab and just found out they are low or very deficienct in Magnesium, Calcium and Zinc. They were extremely high in the N.P.K macro elements. Not sure if excessive Macros vs. deficient micros could cause this but maybe a combo of the nutes and lights.

I'll let everyone here know what I find as I continue but ask the same from everyone. If you figure it out, let me know. This has been a difficult problem to solve and I'm about to spend $$ on lights to close the dead areas.


    Bookmark   June 6, 2011 at 9:31PM
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Keep air blowing across the bulbs. Even the flouros will bring the edge of the nearest leaves up to 85 plus which is too much. Once I added a fan to blow lengthwise on the fixtures, no bolting.

In fact, the last 3 grows have produced for over 100 days each before the bolted. This is on a 14/10 cycle under 6500k T8 bulbs.

T5 HO get warmer than you think as well. Hope that helps.

Here is a link that might be useful: My Growing Adventure

    Bookmark   June 7, 2011 at 12:19PM
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I wanted to comment back that my lights are sealed and cooled. I run inline fans to push the heat out. Pretty effective actually.

    Bookmark   June 7, 2011 at 1:18PM
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Sorry to be spamming this link out (just sent it to another lettuce post), but I'm a new grower, and I've found it very helpful. I'm also growing lettuce (they're really small, now).


This is from Cornell's growers handbook on lettuce. Cornell of course is on the cutting edge of agriculture, so I would say this is a reliable source :-)

It is interesting that the lab found a magnesium deficiency. If you check out the link, it appears that Cornell adds Epsom salt to their solution (*Magnesium* sulfate).

I'm using General Hydroponics FloraGro nutrients, and it appears they already contain Epsom salt. What do your nutes say on the bottle? Perhaps they don't contain enough magnesium?

What lab did you send the leaves to? How much does it cost?

    Bookmark   June 14, 2011 at 12:11AM
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I sent my samples to Waters Agricultural Labs in Owensboro, KY. Your targeting of the epsom salt is good. I, in fact, do use it in my nutrient mix. I use a general fertilizer mix and add epsom salt and calcium nitrate to make it a well rounded solution. What I found I had done wrong on Friday is how I mixed the three pieces together. Evidently, basic chemistry says that calcium nitrate must be dissolved separately and kept separate from the other mix until they go into the nutrient feed tank. Add one mix then the calcium nitrate and the elements will all remain available. If you try to pre-mix calcium nitrate with everything in its concentrated state, it binds and creates a slimy residue that is holding and locking out all the other nutrients.

After following the procedure, I have seen tremendous improvements in my plant appearance, including the redness of the red leaf. I guess I should have paid more attention in Chemistry :-)

The final step that appears to be helping the tall stocks is cooling the nutrient solution. Keeping it down around 65F helps the plants think it is still in the cool season and so they stay short. Cross my fingers this is going to all come together.

    Bookmark   June 14, 2011 at 9:59AM
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Hello hydro_scotty,

did you solve your problem of the long stem lettuce? If so, hows did you finally do it?

    Bookmark   February 24, 2013 at 10:37PM
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