Magnolia stellata (figure E) and its close relative Magnolia clovis are the hardiest of any magnolias.Planted or potted, magnolias like moist, fairly well-drained soil that's slightly acidic. If you're repotting one, try not to rough up the roots as you replant. Minimize root distrubance for all magnolia trees and shrubs. They're finicky about having their fibrous roots damaged and often react to changes by shedding leaves.
Containers allow mobility, so if you expect a hard freeze, move the trees to a safer place--inside the garage or basement, for example.
Magnolias are relatively unaffected by pests and disease. What will send them into a tailspin, though, is not getting enough water.
Tidbit: Beetles--not bees or butterflies--do most of the pollinating because magnolia flowers don't produce nectar.