Magnolia seedlings, by mid-April, should be hardened off gradually by exposure to….

The outdoor environment. When the daily temperature is at least 40°, place seedlings outside in a sheltered spot, 15 minutes the first day, 30 minutes the second, and so on. In late spring, when weather is reliably warm, transplant to a protected area. Protect young magnolia plants with a wrapping of burlap their first winter outside.