Southern magnolias grow from 1 to 2 feet a year, reaching 60 to 80 feet tall with canopies spreading 30 to 50 feet wide. This is rated as a slow-to-medium rate of growth, but how fast a tree will grow varies with the cultivar and growing conditions.Southern magnolias are found on every major plantation of the old South and flank the club house at Augusta National golf course in Augusta, Georgia.his is rated as a slow-to-medium rate of growth, but how fast a tree will grow varies with the cultivar and growing conditions. Southern magnolias are found on every major plantation of the old South and flank the club house at Augusta National golf course in Augusta, Georgia.