Josh, mesclun mix or any kind of lettuce prefers cool temperatures. I don't know where you are located in PA, but here in zone 6, we've been having hot, hot weather! Lettuce will rapidly go to seed and become bitter in those kinds of temps.

As for the requirements, lettuce prefers loose, fertile, well-drained soil. (As does most everything else. LOL).

Plus, I think that some of those lettuces are naturally bitter, much like their close relative....wild lettuce. The bitterness just hasn't been bred out of them, like we've done to the others! So maybe it's a combination of genetics plus temperatures?

you know, that would make sense
as it has been wildly hot this summer

i am considering growing some indoors just to have on hand. along with maybe some broccoli. it would be nicer than going to the grocery.

thanks for the input, both of you

yours / josh