1. Do NOT let your lettuce do this. (See left)
This is called letting your lettuce "go to seed." This happens when you don't harvest your lettuce fast enough and the weather gets warm very quickly. Lettuce is basically a grass that blooms. Unfortunately, all those lovely leaves at the bottom were left on the plant far too long and turned hideously bitter and inedible. You know it's bad when even the birds won't eat it.
2. Lettuce does not like hot weather. A morning watering is essential. Do not sleep in if you want lettuce. One morning I enjoyed a lazy morning/early afternoon, waited till the sun was high and the lettuce got very, very sad. And they do not recover well.
3. Extend a single head of lettuce: Starfish Method
Named because when you cut the arm off a starfish, a new one grows in its place. In this system, keep pulling leaves from the side of the plant and then eventually cut the plant down. A little baby lettuce will grow in its place.
If you cut a tree down and don't ground out the stump and roots, new trees will start sprouting out. Lettuce woks the same way. The first crop is the starter that you let it grow into a full head. Then, instead of pulling it out, cut it down as close as possible to the stump. Roly-poly bugs will inevitably nibble a bit at the stump, but then the sweetest little baby lettuce "trees" will grow in the stump. Repeat, but do not let the lettuce go to seed (See #1). After re-growth #3, you're at diminishing returns. Not worth the 'micro-greens.'
Eat Well. Be Well.