Although red onions are sometimes used for cooking, their sweet mild flavor actually makes them more popular as an onion to appear raw in salads or on top of hamburgers. Sometimes known as purple onions, these large onions are easily identified by their purplish skin color and slightly red flesh. As with other onions, you can grow a new onion with the root section of one that you buy at the store.
Select a firm red onion from the store that has a good root system. The root system of an onion is located on its bottom.
Use a knife to cut 1 ½ to 2 inches off the bottom of the onion.
Place the onion bottom in a dry shaded area for 1 to 2 days or until the flesh of the onion has dried. This process is known as callusing and helps to prevent the exposed flesh of the onion from rotting.
Fill a gardening pot two-thirds full with potting soil. Place the onion piece into the center of the pot. Push the bottom piece of the onion gently into the soil. Cover the onion piece with soil.
Set the gardening pot in a sunny spot near a window. Water the growing onion plant weekly or as needed to keep the soil moist. Allow the new leaves of the onion to appear and grow.
Remove the onion piece from the soil once the new onion plant is established. It should now have a well-developed root system and may have more than one stalk with leaves. Divide the stalks into separate plants with a knife.
Transfer the plants into the ground in a sunny spot outside. Trim off half to one-third of the leaves to encourage the bulb to grow. Harvest the new onions in 108 to 110 days after planting.