Want Big Onions? -- Get your onion plant in the ground as far in advance of June 21st as possible. This gives the plant a long time to develop a large green top, which is the key to developing a large bulb in the summer. Grow your onions from seed under lights and then set them out in the garden no later than April 15th. You can buy sets or bulbs--I have had better luck with sets. This video shows you how to plant them.