Q: Why do they call some trees and shrubs that lose their leaves evergreen (like Magnolias)?
A: The term evergreen is a loosely used term by the general public, which generally means a tree retains its leaves year-round. Deciduous refers to those trees that drop all or most of their leaves when the temperatures drop and the amount of sunlight exposure diminishes. It is interesting that you chose magnolias because this family has some trees that are deciduous; some are evergreen and even a semi-evergreen. However, it is important to note that even if a tree is classified as “evergreen’, it does not mean it never loses its leaves. It simply means they do not lose all of their leaves at the same time or season.
Pines are considered evergreen but obviously they loose their leaves occasionally because we collect them and use them as mulch. Even the Southern Magnolia, which is considered evergreen, loses some of its leaves throughout the year. ‘Little Gem’ and ‘St. Mary’ are two of the more familiar cultivars of Southern magnolia. Sweetbay Magnolia is the member of the magnolia family that is classified as semi-evergreen.Many of the magnolia trees are classified as deciduous. One such tree is the Cucumber Magnolia which produces pale yellow flowers. Another large group of deciduous magnolias growing in this area include the cultivars ‘Alexandrina’, ‘Brozzonii’, ‘Deep Purple Dreams’, and ‘Grace McDade’. Star Magnolia is another group of deciduous magnolias which produce small white flowers and can reach a mature growth height of about 20 feet. The Gresham hybrids are the tallest and produce the largest flowers of all the other groups. Many of the Gresham hybrids are well suited for Florida such as ‘Jon Jon’, ‘Pink Goblet’, ‘Royal Crown’, and ‘Winelight’.
I suppose this is more information than you wanted but I think is it important to note that although a family of plants may contain similar traits they may differ on one or more specific characteristics. I was somewhat surprised myself at the large number of magnolia cultivars that are deciduous. Check out Dr. Gary Knox’s publication titled, “Magnolias” for a more complete list of deciduous and evergreen magnolias. This publication can be found at: http://edis.ifas.ufl.edu/mg270