Growing onions in your home garden is a bit different than other vegetables, because onions are sensitive to the day length, that is, the number of hours of daylight during the growing season. Hence, all onion varieties fall into one of three categories: Long Day, Mid-Day, and Short Day onions.
Long day onions are used in the more northerly climates because in the summer, the north has very long daylight hours compared to the south. So we can think of long day onions as growing in areas with long summer day lengths, and short day onions as being for southern areas like southern Texas, southern California, Mexico, etc.
When onions are planted, they first develop their vegetative growth, and there is no sign of bulb formation. Suppose you live in the north and have planted a long day onion, and suppose that onion responds to a 15 hour day length. In the spring, when daylight hours are shorter than 15 hours, the onion develops it's vegetative growth. Then, say on May 15th the day length reaches 15 hours. The magic of nature then takes over. The genetic makeup of the onion variety send a signal to the plant to stop producing vegetative growth and to start forming a bulb. At this point, the tops or leaves stop growing, and a bulb begins to form. As the onion matures, the tops will eventually fall and touch the ground, at which point the onion is ready for harvest.
Now, what happens if you plant a short day onion in a long day climate? Well, suppose the short day onion's internal clock is set for 13 hour days. Instead of fully developing it's vegetative growth, the onion senses that it is time to form a bulb while the plant is still very young. The result? You get a small onion, often referred to as pearl onions or cocktail onions.
At the same time, suppose you plant a long day onion in a short day climate, and the day length never gets long enough to trigger the onion's internal clock. Then you get a bunching or "green" onion, with little or no bulb formation at all.
In view of these facts, it is best for you to plant onion varieties sold by a local nursery that are suited to your area. If you live in Oregon and your cousin sends you seed from Florida, you're going to have pearl onions. It is ESPECIALLY important for you to be aware of this when you order onion seed or sets by mail-order. If you're not sure, be sure to specify on your order that you need a variety suited to your geographical area.