Warm season vegetables are replacing cool season vegetables about now. Technically, lettuce, Lactuca sativa, is a cool season vegetable. It grows through spring and autumn though. The last of it can continue almost until May. Within the mildest coastal climates, it can continue later. By the time it finishes there, it is almost time to plant more for autumn.

After thousands of years of cultivation and breeding, lettuce is now remarkably diverse. Most popular varieties are leaf, head or romaine types. Leaf lettuce is mostly green, but can be bronze, reddish or irregularly blotched. Most varieties develop loosely ruffly foliar texture. Romaine and head lettuce is denser but larger. Some grow a foot high and wide.

One commonality among lettuce varieties is that they tolerate neither frost nor arid heat. Some are a bit more tolerant of one or the other, which can prolong their season. Some of the larger romaine and head types need nearly four months to mature, though. Only a single phase matures within each season. Smaller types might mature in about a month.