Lettuce seed can become dormant at high temperatures.
I suggest you try oriental - type salad leaves, mustards, pak choi, mizuna etc ... in my experience they germinate just fine at higher temperatures and if sown after mid summer (and we have reached that time of year) they are less likely to bolt than those sown in the first half of the year.
“I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh
I agree we even had trouble with Little gem earlier in the year for some odd reason. Sow in the late evening & keep in a cooler shady spot, until they germinate.
I recommend organic gardening cat. Spicy greens mix - keeps going long after most salad leaves have bolted. You are having my weather at the moment, which is death to lettuce!
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
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I suggest you try oriental - type salad leaves, mustards, pak choi, mizuna etc ... in my experience they germinate just fine at higher temperatures and if sown after mid summer (and we have reached that time of year) they are less likely to bolt than those sown in the first half of the year.