I recently purchased a magnolia that had no tags on it. I have an extremely large pot that I would like to plant the magnolia in. My books at home lead me to believe that I should plant it in Azalea and Camellia potting mix. A local nursery has advised me that this would be fine, although another has said no. They also disagreed with my plan of putting rocks, bitumen, and old leaves in the bottom of the pot to help with drainage. They believe a quality potting mix and nothing else is the way to go. What are your suggestions?
Here is what the book, Magnolias: A Gardener’s Guide, by Jim Gardiner
(Timber Press, 2000) says about growing Magnolias in containers:
…considerable experience is needed to retain magnolias in a container
for any length of time. The roots are particularly sensitive to being hot
and dry during the summer months and frosted during the winter months…
Evergreen magnolias and clones of Magnolia grandiflora, in particular M.
grandiflora ‘Gallissonniere,’ can be grown in very large containers for
indoor use in atria.
I think if you take the matter of extreme heat and cold into
consideration, you should be able to grow your magnolia in a container. I
would be curious to know which species you have, because some get very
large, and for these a container might not be a good choice.
Magnolias prefer good, free-draining acidic soil that does not dry out,
according to Rosemary Bennett’s book, Magnolias (Firefly Books, 2002).
Since Azaleas also prefer acidic soil, the idea of using Azalea and
Camellia potting mix makes sense.
You may find the following information on growing trees in containers
helpful:
Virginia Cooperative Extension: Trees for Landscape Containers and Planters
University of Tennessee Extension: Trees to Plant in Containers or Wells
UBC Botanical Garden Forum: A discussion on requirements for magnolias in containers
UBC Botanical Garden Forum: A discussion on potting guidelines for a particular magnolia This discussion suggests that the container should be filled with soil-based compost which provides some nutrients to the plant.
As for container drainage, here is what Prof. Linda Chalker-Scott of Washington State University says. In short, she says that putting coarse material in the base of a pot for better drainage is a myth.