After 35 years I have two large Magnolias about 30 feet high. I
intend to take one down for more light as it blooms too early an burns
due to frost. The other is close and one branch could rub against our
roof. The latter concerns my prune question.

Can I trim it back ?

Been here.

http://www.rhs.org.uk/advice/profiles0603/magnolia_prune.asp

Bill

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Garden in shade zone 5 S Jersey USA