They look easier than seeds, but they don t offer much variety compared with what we see in catalogs. The big thing to know about onions is that the switch from green growth to bulb building is brought about by day length, not by temperature or by the age of the plant. You need lots of big leaves at the magic moment to get large, sound onions. Northern varieties head bulbward in the long days of late spring. If you sow seeds outdoors right this minute, they might grow to respectable size in time, but at this point it would also be smart to plant the tiny onions called sets or else prestarted plants.
Sets are convenient, but that s about it.
Because they are grown the year before planting, they must be long-storage types, more durable than delicious. If you do want to plant them, be sure to use small ones. Sets larger than a half inch in diameter often split or send up flowers instead of making sound bulbs. For sweeter, larger, more interesting onions, prestarted plants are the way to go. The choice among commercial ones is paltry compared with that for seeds, but it s a lot wider than for sets.