I cut the bottom of a stalk of celery and set it in as much water as possible before it began floating. I didn't count how many days it took for this to grow, but it was pretty fun to watch happen.
This is about the maximum height the celery got after a few weeks. Unfortunately, I didn't harvest it before I went on a trip to Yosemite for a few days. When I returned, it had shriveled because the root at the bottom had gotten moldy. Throughout the growing process, I had to pour out and add fresh water and peel away molding stalk. I plan on trying again and keeping more on top of it, as well as harvesting some celery before it dies.
I also tried growing green onions. At first I had them resting in this jar with water as you see here, but they kept tipping over. My solution was to wrap rubber bands around them and then secure them to the side of the jar with a paperclip. It worked!
These grew great, and super fast, too! It seemed as though they grew right before your eyes some days...
This was the longest set that I ended up with, and we cut them up and ate them. I put the bottom part back into the water and it's still growing. As with the celery, I had to peel away some molding exterior layers.
This project was fun and delicious, and I can't wait to try more of the water-only regrowing methods! It sounds like the onions will also grow in dirt if you transplant them after a bit of water-growing (source). I'll have to try that, too.
Have you ever regrown anything from just water before? I'm curious what you think is worth growing in this method. So far, I vote green onions are more worth it than celery...