Experts and BHG readers answer.
You do not say which species of Magnolia you are growing or what size container it is in. Magnolias are not ideal plants for container work unless the container is very large. Their roots require enough space to spread out and they are sensitive to very cold weather unless given extra protection. The preferred species for container growing include sweetbay magnolia (M. virginiana) and star magnolia (M. stellata). If the plant is growing well, fertilize it lightly with a high phosphate fertilizer after the normal bloom time. This will encourage the development of new flower buds for the following season. Be sure that the container does not dry out during the warm summer months as this will cause buds to abort. Magnolias are usually best in lightly shaded conditions, rather than in the blazing sun.