I'm sure you'd be good as long as you don't get below 20. On especially frigid nights you could cover them of course, with a blanket or something of the sort. I have no reference on heat tolerance, but if you gave them some shade during the hot weather they may survive quite long. You could buy some salad mix packets, sow those successively through the summer under some cardboard to keep the soil cool, and harvest while very young of course as well, before the heat can do much damage. There are many ways of beating the heat that I've seen many people do.

My main lettuce bed gets shade from 1:30 until sunset.

I will try around mid month. I’d go earlier, but the forecast highs aren't supposed to get out of the 40s for at least 10 days.

Probably too cold to do anything now.

Yeah, but it’s not like it’s going to grow at these temps, is it?

I’m going to be sowing some lettuce under floating row cover as soon as the soil drys out a bit.

Speaking of dryness, we had a wet February, but are still technically in drought here.

The 7-day forecast has a nice “rain hole” right over me.

We were in a pretty good drought for a while there, but these recent rains have backed it off well. Still feels more on the dry side though.

Have you considered starting lettuce indoors? I use a low tunnel rather than the usual grow light to start mine early :)

I've always found direct planted seeds grow too slow in cool April to get them to ready to pick before June, but it depends on the weather. I just find that May Lettuce is the best, before the longer days and heat, but I'm picky. So I'm for transplants.

Low tunnel is the way to go if you want early lettuce. Plant as soon as the soil can be worked. I have tons of full grown lettuce by May 1 in zone 5

Tom,

What do you use to cover your tunnel?

Hairmetal- I use a layer of agribond-19 and 6mm greenhouse plastic. For support you can use pvc tubing bent into a semi circle. To hold the bent pvc in place pound one foot pieces of rebar on each side and slide the tubing over the rebar and push the tubing a few inches into the ground. Then secure the plastic to the ground . Set up can be used multiple years.

It is still very wet here and snowing right now.

I also use a tunnel or cold frame for spring lettuce. In the fall I'll let some lettuce go to seed. When the pods look ready I'll shake them around the area. Generally by January they will start growing in clumps. Lettuce can handle much colder temperature than 20F at least if it germinated in winter. Here in eastern Colorado winter lows are generally 0 to -10F. Sometime in February I'll dig up the clumps and separate and replant. Works for me.