[Rob the rake]

It never used to be a thing, I'm sure, but in recent years the mini-cos lettuce (like Little Gem) that I've grown have quickly gone bitter in the summertime. So much so that I gave up growing them altogether in favour of salad bowl types. I miss the crunch, however, so I'm hoping I've missed something obvious which a wiser head can point out. Growing in partial shade alongside a row of peas, perhaps?

[murphy99]

i find lack of water can make them bitter

[Hampshire Hog]

I’ve found that virtually all lettuces I have grown get bitter when too old.
Perhaps down to the watering or just age??

[Snow]

They definitely get bitter as they go to seed. I've given up on growing lettuce for hearts, always find a slug or two in them. Instead I just pick leaves off as I need them, they take longer to go to seed as well so you need to sow less

[Rob the rake]

Good advice, thanks all. It's possible that it's under-watering, considering that the climate in this area seems to be getting drier overall. I intend to grow them in partial shade, too, which may also help.