Can grocery-store green onion roots regrow in water?

Yes, grocery-store green onion roots can often produce new leaves in water. Treat water as a way to restart growth, not a guarantee of a lasting, productive crop. Results depend on the condition of the roots, light, temperature, and the individual plant.

Keep the roots and a short section of the base when you trim the onion. Place the roots in a clean container with enough water to reach them, without submerging the whole cutting. Change the water when it becomes cloudy or smells unpleasant. If the base softens or begins to rot, discard it.

Once the roots are growing, you can move a healthy cutting into soil if you want to keep growing it. New leaves show that the plant is growing, but they do not guarantee a particular harvest or that every store-bought onion will regrow.