Allium giant onions in the fall, in a sunny to partially shaded location, 4 to 8 weeks before the ground freezes. Most bulbs, including giant onions, do poorly in wet, clayey soils. If necessary, improve drainage by building beds higher than the surrounding soil surface and add lots of organic matter (such as rotted manure, leaf mold or peat moss) to the soil. Loosen the soil at least 1 foot deep. It should be slightly acidic to neutral (pH 6.5 to 7.0).
Allium bulbs 12 to 18 inches apart 6 to 8 inches deep, measuring from the bottom of the hole. Point the growing tips of the bulb up and press their rounded bottoms into the soil, using a slight twisting motion to assure good soil contact for the root plate. Cover the bed with mulch to protect it from heaving during winter frosts and thaws.