Check out this wonderful tip from the Thrive Farm.
Originally posted on The Adventures of Thrive Farm:
The next time you have green onions, don’t throw away the white ends. Simply submerge them in a glass of water and place them in a sunny window. Your onions will begin to grow almost immediately and can be harvested almost indefinitely. We just use kitchen scissors to cut what we need for meals and periodically empty out the water, rinse the roots off and give them fresh water.