Onions can be used to grow new onions. Unlike many other vegetables, onions can be used to regrow new onions by planting either a whole onion bulb or part of the onion that contains the root mass on the bottom.Once planted and hydrated, the roots on the bottom of the onion will begin to grow, and green onions will form out of the top of the vegetable.Onions can be used to grow new onions either using water — which is called hydroponic growing — or traditionally, using soil.nce planted and hydrated, the roots on the bottom of the onion will begin to grow, and green onions will form out of the top of the vegetable. Onions can be used to grow new onions either using water — which is called hydroponic growing — or traditionally, using soil.