I grow enough lettuce that I am pretty much sick of it by June (heck, late May!). But I have found that many of the normal types will perform ok if you start them inside under lights and then move them outdoors to finish them. I grow in large deep aluminum trays and space the seeds about five inches apart. Then, after they have their first true set of leaves I move that tray outdoors in bright shade and start a new tray inside. The seed doesn't germinate well when the temps are over 70 degrees (which is why I do the inside thing). If the outside temps are going to be really hot then I move the tray back inside until things cool off a bit. After a couple of weeks of the shuffling dance I give up on lettuce and eat store bought. This same technique will work for growing Cilantro for summer salsa (Cilantro has its signature flavor only when grown in cool weather and its seeds rarely sprout if the temps are over 70 also).

I have good luck with lettuces over the summer as long as they are shaded, and I don't try for heads but just keep picking the outside leaves as they get big enough. Some varieties work better than others, but I have found that most of the reds, Black-seeded Simpson and a variety called 'Freckles' seem to work very well for me, as have several packagers "summer mixes". I made shade covers - 4 legs on a frame of shade cloth, that fit over my slightly raised beds, which seem to work well, and plant in blocks. I do need to redo the frames this summer as the legs are beginning to fall off the frames after several years.

tj - is that soil temp or air temp of 70?

Have you considered growing lettuce in a container? I am going to try that this summer. With the container you can move it into more shade as needed and it helps control pest problems as well.

I'm growing lettuce in hanging baskets this year due to taking apart my raised bed last fall. I think I'm going to add some of those water crystals to the next baskets I do.

I meant to say - air temps above 70 degrees. You can get some germination at higher temps, but the best is around 70. Thats why winter sown lettuce can take a while to sprout (it waits for a warm spell). A lot of that family of plants do the same thing - parsley, dill, fennel..etc.

I buy those large aluminum foil pans used by pro caterers at Sam's club. You get like 10 of them for less than $10 I think. I punch holes in the bottom of one of them and set it inside another one without the holes. I fill with sterile media and sow seeds. I water from the bottom by prying the edge of the bottom pan out a bit. I try to keep the surface dry to inhibit fungus gnats. The trays are about 4 inches deep so you can just grow the lettuce in them without having to transplant. But after you move it outside you will have to watch it closely. If it gets really warm the lettuce will bolt while still small - which ruins the taste. Sometimes you only have a week or so outside before you have to harvest the entire crop. This is why I give up and just buy lettuce during the heat of summer. Cilantro can be a bit harder to manage - it seems pickier. But you have to do it if you want Cilantro with your salsa.