Spring flowering Magnolia x soulangeana can be planted now. Container grown plants from grafts or cuttings will flower sooner than bare rooted shrubs. Check the container is not pot bound and older, grafted plants will perform quicker. It is a case of paying a bit more to get what you want as it can take up to 5 years for Magnolias to start flowering.

Growing Tips

• Blooms may fall during spells of warm weather so avoid planting in very sunny parts of the garden. Also avoid early morning sun and wind which can damage young growth. Other than that magnolias are hardy shrubs and trees.
• You can under plant Magnolias as their structure is open but avoid damaging the shallow, delicate roots. I suggest you try Muscari (grape hyacinth) or other small bulbs.
• You can layer your existing magnolia in August using current year shoots but rooting may take a couple of years.
• Pruning should not be needed except crossing branches and reshaping. hard pruning can set back flowering for several years.

Varieties to Grow

• Alexandriana has fragrant flowers of white sometimes flushed with purple.
• Brozzonii has extra large white flowers and looks good against a wall.
• Magnolia x lilliflora ‘Nigra’ is scented, purple and long flowering
• Rustica rubra may be the best for a larger garden as it is a vigorous spreading shrub.
• Magnolia Elizabeth is a yellow form that needs a lime free soil.
• Lennei is free flowering and there are pink and white versions.

Other Sources

Magnolias can grow into large trees according to the RHS.  20+ feet tall may be common but they tend to be slow to reach that sort of height (10 feet after 10 years).

The American Magnolia Society may help with more detailed queries.

Info on National Collections and Growing Magnolias