Preface: I don't really know what I'm doing.

So I decided to grow some kweik lettuce in my ebb & flow hydroponic setup. In addition to that, I'm growing a variety of chili peppers, mostly superhots.

It's been about 40 days, and I've had good growth so far, but I tasted some today and it was possibly the most bitter thing I've ever tasted.

It's growing in coco coir in an ebb/flow system, with a nutrient solution at a pH of ~5.8 and a TDS of ~800PPM, flooding twice a day at 7am and 1pm. Lights are currently on 18 / 6 cycle, running a 600 watt HPS lamp, with some thermal insulation around it that keeps the ambient temperature in the tub about 23 degrees C.

My suspicion is that it's too hot for the lettuce and that's causing the bitterness, but I also suspect that it may be a nutrient thing as well.

Can anyone shed any light on whether my suspicion is correct, and if there's a way I can save it before it's at maturity?

  • That's a pretty low pH... I actually wanna know what is the makeup of your nutrient solution? The actual makeup and percentages, not the npk rating
    – J. Musser
    Commented Nov 29, 2015 at 19:47
  • 3
    Kwiek lettuce is suited to cold, unheated greenhouses, for overwintering. This type of lettuce is often bitter if grown in summer. I suspect your greenhouse may be too warm - given you can also grow super-hot peppers. Look for a completely different lettuce variety, bred for warm weather. Lettuce bred for tropical climates would probably do well. Also get a variety of lettuces to see which works for you. There are alternative warm-weather greens that are good too. Purslane (goldberg) and vegetable amaranths (lots of types) are favorites of mine. Commented Nov 29, 2015 at 20:11
  • Thanks for the responses. I'd be hard pressed to tell you that, honestly. I'm using CocoTek Bloom A&B with Aussie Tonic at the recommended concentrations. I realize that's kind of a stupid nutrient choice, but it's primarily for the chilis - the lettuce was more of an experiment to see what would happen. Now I know, I suppose. Assuming that the heat is the primary contributing factor, will reducing the temp for the next little while lessen the bitterness? Commented Nov 29, 2015 at 23:34
  • Unless I was growing lettuce in an unheated greenhouse, I would use a different variety. There are many heat-tolerant varieties out there. I've seen people growing them successfully in warm greenhouses (at least I think they were successful - you can't taste with the eyes!). I don't think nutrients would be the cause, but I could be wrong. From what I read the PH is right on - as it's much different than soil PH. Commented Nov 30, 2015 at 17:34

1 Answer 1


The bitterness is caused by heat. Lettuce likes cool temps, where they don't often get over 75F, if you don't want them to get bitter. When the heat becomes more consistent, certain chemicals are created for flowering, and producing seed. This is what causes the bitterness.

You cannot fix this batch of lettuce but you can grow another batch and keep temps cooler this time.

A recording thermometer, $37usd, is very handy for these issues. I have this one and it's one of the best deals I could find for the number of datapoints it stores. It also tracks humidity.

  • I have the same problem with lettuce indoors at 69 degrees. The warmest it might get is ~73. That hasn't happened with this batch yet, so I don't think temperature is only possible cause.
    – user39636
    Commented May 30, 2022 at 0:20

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