The time for planting onion starts in our area (Southern California) is in December/January and we will be selling the correct types of onion plants which are SHORT DAY TYPES. These will make larger onions in the southern United States. Or you can plant your own seeds around October.
You know onion plants are ready to be pulled when they start to fall down.
One of the keys to large onions is to fertilize with lots of nitrogen which makes lots of leaves. Each set of leaves equals one ring on the onion, hence you want lots of leaves!
Additionally, be sure to plant shallow - not deep - making it easier for the bulb to expand.