For those in warmer climates, like the OP, I would highly recommend Chayote, also known as Vegetable Pear or Merliton. They were first cultivated by the Aztecs and subsequently moved all around the gulf of Mexico, which is where you're more likely to find them now. They can be found in many southern supermarkets and most markets specializing in Mexican fare. You plant the whole squash and it grows vines at astonishing speeds. Both the vines and squashes are edible and very healthy. The best thing is they are cheap to plant. I think chayote might be my chickens' favorite green.