I grow onions from seed , start in early September, transplant in December. Over wintering works great for smooth seeded peas like Willet Wonder or Early Alaska. Too many losses with wrinkled seeded varieties. Just for information, sets are dormant long day varieties, that are not good for any more than green onions here. Most big box stores will have little bags of them. Most of us whether we grow them ourselves or order from some place like Dixondale, Brown's Omaha etc use live plants.
