How to Re-Grow Celery, Bok Choi, Romaine Lettuce & Cabbage from Kitchen Scraps
After a few days, you should start to see roots and new leaves appear. After a week or so, transplant it into soil with just the leaves showing above the level of the soil. The plant will continue to grow, and within a few weeks it will sprout a whole new head.
Alternatively you can plant your cutting directly into soil (without starting the process in water) but you will need to keep the soil very moist for the first week until the new shoots start to appear.