Southern Magnolia More Hardy than You Think

 Southern Magnolia

Magnolia grandiflora

The Southern Magnolia is perhaps the most popular broadleaf evergreen tree in North America. Very large glossy (plastic like) green leaves can be up to 10" long and large very fragrant flowers that are citrusy in smell that can be up to 12" long. Due to mass cultivation of this species it has naturalized much further North than its historical native range (I circled). Cultivars exist that are more cold hardy that grow a zone North than the regular species can tolerate. The Southern Magnolia is hardy in USDA Zones 6-10. A couple cultivars that are zone 5 hardy are 'Bracken's Brown Beauty' and 'Edith Bogue'. The large deep glossy foliage adds a lush tropical look to the frigid and gray Northern landscape. 

Magnolias in general prefers soil more on the moist side rather than drier side. The species depends on exactly what they prefer. Southern Magnolia prefers moist well draining soils. They are drought tolerant once established. They can not tolerate wet soils, but can tolerate occasional flooding. The other species of Magnolia The Sweetbay Magnolia tolerates wet soils al the way to even some drought. The main physical difference is Sweetbay has much smaller foliage and blooms. The other phsyical difference between the two is that Sweetbay Magnolia is semi-evergreen/deciduous in Northern regions.

Many cultivars of Southern Magnolia exist from columnar, dwarf, to improved foliage.. the 'Little Gem' cultivar is dwarf not getting bigger than 20' tall, but is less hardy than the regular species hardy only in zones 7-10. 'Alta' is very columnar cultivar for small or tight spaces. Also the the ones more cold hardy are listed above. Southern Magnolia is most popular within the US, but is cultivated around the world due to its unique qualities you can't find with any other tree. It is native to the Southeastern US. It has naturalized all the way to Arkansas, Tennessee, Washington DC and others. The species only is native in USDA Zones 8-10a (10a being Tampa the Southern most tip of the native range) I have seen the tree landscaped as far South as zone 10a in Naples Florida. The trees seemed stunted and performing poorly, likely too hot with limited dormancy periods. Very much like Boxwood in zone 10 that gets more disease issues with the hotter climate. The Southern Magnolia performs best in zones 6-9, but is capable of growing in zone 10, but extra water or some afternoon shade may be required. I have successfully grown 'Edith Bogue' Southern Magnolia in Carmel, IN (USDA Zone 5b). At the coldest point it survived -13°F with severe winter burn, but every branch foliated the following Spring. There is a mature specimen in Carmel as well that has endured temps slightly colder than -20°F in the winter of 2014 with nearly complete defoliation, but every limb survived and foliated the following Spring.

Native Range of Magnolia grandiflora

Area circled is Magnolia grandiflora rough generalization of species being naturalized beyond native range (naturalization may occur outside circle especially along East Coast)

'Edith Bogue' Southern Magnolia in Carmel, IN (USDA Zone 5b) 2014 winter for example saw air temps pummet to -20°F with wind chills close to -50°F. Tree nearly completely defoliated, but every single branch survived and flushed out the following Spring. Winter burn begin when winter temps dip below -5°F to 0°F, but stems are hardy to at least -20°F on the hardiest cultivars. Even if a night or 2 falls below zero, the foliage out of direct winds wind face a better chance at staying green, especially if the subzero is brief.