I have used that method to grow both bok choy and green onions (scallions). Once you use the top green parts of the scallions, plant the the bottom part with the roots. They will grow new green tops that can be continually cut multiple times. Rosemary is another prolific herb. Just put a stem or branch in water and after a couple of weeks it will produce roots. Then plant it and you will have fresh rosemary forever.