Having been frequently disappointed by the short life span of green onions, I decided to try a technique of which I had only heard rumor.
When you purchase green onions at a grocery store, you get the whole plant. Since you’ve got the roots, you should be able to put it back in the ground to stay alive and fresh until you need it. Actually, you don’t even need soil. The green onions will stay alive with just an ordinary coffee mug and tap water.
The key is to trim the green tops before they start to go limp. Even if you don’t need them for your meals that day, snip them and keep them in a Ziplock bag in the freezer. Don’t worry, another shoot or two will shoot up with in the next day.
Of course, the water won’t sustain the green onions indefinitely. I’m not an expert on plants or gardening, but I think it would be safe to say that they would need nutrients from soil. I would recommend changing the water daily, so that you don’t end up “growing” bugs as well.