I was reading an article in the local paper that mentioned Magnolia grandiflora ‘Alta’ and was hoping that you could tell me more about and its hardiness in the Pacific Northwest and what its mature dimensions would be.
Magnolia grandiflora ‘Alta’ is a trademarked Monrovia introduction. According to their website, it is “very slow growing to 20 ft. tall, 9 ft. wide in 10 years.” Since this is a relatively recent introduction, there is not going to be much information about its hardiness in our area until more gardeners have grown it and shared their experiences. The longevity of the species Magnolia grandiflora and its cultivars can only be estimated (between 50-150 years, according to SelecTree.) Trees grown in urban settings are often affected by root disturbance, pollution, and the like, so their lives may be somewhat toward the short end of the expectancy range.
The local website of Great Plant Picks lists two different cultivars of Magnolia grandiflora, which may give you some idea of how well they do in our area. Here is an excerpt:
“Provide southern magnolias with good drainage and full to partial sun. They thrive in hot spots, where the extra heat encourages better flowering. These flowering evergreens prefer well-drained, sandy soil, but they tolerate average garden soil. Best growth and flowering requires occasional summer watering, but once established, southern magnolias withstand considerable drought. Garden gently under magnolias, for they have fleshy roots that can easily be damaged. The best approach for companions plants is to tuck in natural spreaders and let them flourish untouched.”
From my observations, they do not do well in the occasional winters when we have heavy snowfall, as their evergreen leaf-laden branches are prone to breaking under the weight of snow. Otherwise, they seem to survive here.