I live in a Mediterranean country and we are still in full summer weather (average. temp. of 27 Deg C / 80 Deg F). I planted some seeds a couple of weeks ago (speedy salads, rucola & parsley) but only a couple of speedy salad seeds have germinated. In winter my germination success rate is almost 100%. Does anyone else experience the same problem in hot weather? We have a very long hot summer and I haven't planted anything since last winter. So I'm itching to get things going again! 2 weeks ago temperatures went down slightly, but now they've gone up again.
I tend to have the same problem here in our hot Texas summers. As Diana suggests, I suspect my problem is that in the heat, I just can not keep them damp enough for germination. It is possible that they do not germinate well at high temps.
I do know that when they fail to germinate at the end of summer, that pot rarely has anything come up later when the temps are milder.
I was very aware that there were minimum temperature for germination, but this is the first time I have seen a chart with max temperatures. So nice to have both in one place. Printing this out and gluing it to the front of my garden notebook.
Lettuce germination is slower or non-existent for many varieties above 75 F = 24 C. Maybe you need a "summer" lettuce variety instead of "speedy" varieties.
I found it very helpful to set up an "irrigation" system with 1/2" and 1/4" hoses and some mini-jet sprayers. (12 mm and 6 mm) They moisten the surface uniformly without disturbing the surface.
I had it on a "wind-up" timer so I could set it for 10-15 minutes and walk away. For a few days, i did that twice per day to keep the surface moist. Even so, it had dried out by the time I came home form work, and only two varieties out 6 germinated well.