From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1899-282898
Question: My family loves the springtime lettuces/greens I grow in our
garden, but with the onset of hot weather by early June it just
doesn't last long enough. Is it possible to grow lettuce in a
"greenhouse" that could be cooled (air conditioned?) in some way, or
perhaps grow it in a container and bring it indoors on the hotter
days? I've tried using sunshades and misting during the heat of the
day, but that only works for so long before the lettuce turns bitter.
I'm determined to find a way to enjoy these wonderful, tender greens
throughout the summer!
Answer: Although it would theoretically be possible to grow greens in
an air-conditioned greenhouse or indoors under lights in the summer
time, you might want to experiment with growing some of the more heat
tolerant greens instead. Swiss chard, Malabar or New Zealand spinach,
and the old standby red oak leaf lettuce might be possibilities. You
can also extend your lettuce harvest by making successive sowings 10
days to 2 weeks apart, so you always have a supply of young greens.
Since hot weather can inhibit lettuce seed germination, during warm
weather cover the seedbed with a board or a light mulch to shade the
soil and keep it cool, removing the cover at the first sign of
germination.