Jul 03, · Green onions, or scallions, are the green shoots that come up before an onion bulb begins to form. They are essentially immature onions. Look for a hearty scallion seed, such as the species A. Welsh onions, or simply choose your favorite white, red or yellow onion to grow%(). When onions start to mature, the tops (foliage) We only use onion sets for green onions They never get large bulbs only about 1-inch bulbs. Starting onion plants from seed takes the longest for onions to fully grow, but seeds give you the most choices in variety. It can take up to five months for plants to mature enough to harvest. When starting onion plants from seed, allow the soil temperature to warm to at least 50 degrees Fahrenheit before sowing.