Regrowing bok choy, cabbage, lettuce and celery all require the same method. They are very fast growers and typically show signs of new growth from as little time as 1-5 days. With these plants though, unlike herbs, to get a proper harvest, transplanting into soil is a must.
The first step is to get your plant and chop off the bottom two and a half inches. You need to make sure your vegetable is fresh and healthy to attempt propagation. I normally evenly stab three toothpicks in the sides of the vegetable base, around 1 and a half inches from the bottom.
Then, set the bottom in a shallow tray, glass bowl or jar. I typically use plastic trays from the recycling bin.
Fill with the vessel with enough tap water to submerge an inch of the root end. Place the bowl or jar where it can get good natural light for several hours a day. Change the water every couple of days, making sure the plant root end is always submerged and to prevent bacteria growth. (The optional toothpicks around the sides keep the plant from touching the bottom of the bowl. I've tried regrowing these plants in water without suspending the root end off the bottom, but found that the outer stalks rot more quickly).
After a few days, you should start seeing small leaves emerging from the center of the top. In about a week, you may see tiny roots emerging around the base. The cut stalks/leaves around the outer base may start deteriorating and turning brown. Don't panic—this is normal. But if you leave the plant in water for too long, the outer stalks will get serious rot, so it's best to either plant before that happens or pull off the brown pieces before they turn to mush.
When the new roots are about an inch long, you can transfer the plant in potting soil or directly into your garden. If you use potting soil, choose a mix without pesticides, and suitable for vegetables and herbs. Make a hole deep and wide enough to hold the plant from the root end up to the cut end. Set the plant into the soil, making sure there's no air pocket below the root end. Gently fill in and tamp the surrounding soil so a bit of the cut end and all of the emerging leaves and stalks are above the soil. Keep the soil moist but not wet.
Regrowing bok choy, cabbage, lettuce and celery all require the same method. They are very fast growers and typically show signs of new growth from as little time as 1-5 days. With these plants though, unlike herbs, to get a proper harvest, transplanting into soil is a must.