New onions can be grown from old onions, as long as the old onion contains part of the root mass. This simple process involves hydrating the onion, planting the onion, fertilizing it and waiting for the new onions to grow.
Hydrate the onion
Before planting the old onion, you need to hydrate it. Put it in a bowl of water at room temperature. Let it soak for several hours.
Plant the onion
If you are planting the old onion in soil, dig a hole big enough to house it and fill the hole with soil. If you are growing it hydroponically, put the onion in a jar of water; make sure the roots are facing downward and are completely covered in water.
Fertilize the onion
If the onion is planted in soil, use a fertilizer that is high in phosphorous. The fertilizer ratio needs to be 10-20-10. Keep the soil moist after fertilization and after transplantation.
Allow it to grow
The onion grows larger when it is given more water. When growing onions hydroponically, put the jar in a location with plenty of direct sunlight, such as a window or outside. Make sure to keep the jar filled with water at all times.