If you want to regrow celery, place the base of the remaining stalk in water for three days. When roots appear, plant it again in the same container allowing leaves to stick out like any other plant.
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Bok Choy is similar to celery and can be regrown in the same manner b submerging it up face upwards in water. Bok Choy makes a fantastic stir fry with garlic. You can also add it as part of a mixed vegetable stir fry too.
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Scallions are like any usual bulb. After using the stems, place the remaining bulbs in a glass of water. The green shoots will grow again in about 10 days. Scallions can be used in making fried rice dishes and soups.
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