Just save the end from your lettuce and put into a little water (about a half inch). Change out the water every day. The lettuce will grow back incredibly quick too.
The best and most amazing thing about lettuce, besides eating it, is that if the stump is intact it will regenerate and regrow new shoots for a second harvest. I took three photos romaine “stumps” in various levels of regrowth in my herb garden. All I did was put each stump in a half inch of water, check the level daily, and wait for tender new growth. I've also planted the stump right in the ground and had success as well when the weather is cool.
The perfect environment for re-growing leafy greens is a cool and bright window with all day light. Give it a try the next time you have a head of lettuce with the stem still intact. You have nothing to lose and a free harvest of lettuce to gain.
Important Tip: Romaine Lettuce is one type of produce that is easy to find in the organic form. It is often packaged as three heart of romaine and it pays to spend just a dollar or two more and have your salad free from any unnecessary pesticides.
I understand it works with celery as well.
