Some of the romaine-type lettuces tend to be relatively heat-resistant.
But I don't think I'd try growing them in the summer in Florida.
Seeds of Change has a Jericho variety.
Park has Lactuca sativa Craquerelle du Midi, which they say will still
be good in June in Florida. They also have a Lettuce Summer Glory Blend,
which includes heat-tolerant varieties that are not romaines.
I'm sure there's more - just search some online catalogs for "romaine",
or maybe "heat tolerant".
Lettuce would be a winter crop in central and south Florida. You can grow
some varieties of amaranth for their leaves - which look and taste like
spinach - if you want a heat tolerant leafy green to eat fresh. I think
there is also a perennial called New Zealand spinach which has similar
properties. Otherwise, you can grow tougher greens such as collards to eat
in the summer, but most people would prefer to cook those.
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