First ran in Horticulture September 2003.
The four species that belong to the section Oyama of the genus Magnolia–M. sieboldii, M. globosa, M. wilsonii, and M. sinensis–share a set of traits that makes them easy to recognize (if not to distinguish from one another). All are relatively small-growing deciduous trees native eastern Asia that bear white, rounded, nodding or pendent flowers, opening from egg-shaped buds in late spring or early summer after the foliage has appeared. Each fragrant flower carries a conspicuous circlet of rosy pink to crimson stamens, and flowering can continue for up to three months. In the wild, the Oyama magnolias grow as understory trees or large shrubs, and in cultivation do best in partial shade with humusy, retentive, acidic soil.
Of the four oyamas, M. sieboldii is the most commonly available and widely planted, and at first glance, it would seem hard to improve upon its classic beauty. But the late Dr. August Kehr, of Hendersonville, North Carolina, thought otherwise. Kehr was an expert at using colchicine–a toxic alkaloid derived from Colchicum autumnale–to induce polyploidy (an increase in an organism’s normal chromosome number) in magnolias. His work showed that polyploidy magnolias have larger, longer-lasting flowers with heavier substance; wider, more rounded leaves; and increased vigor–sometimes to a startling degree.
One of Kehr’s last releases, M. sieboldii ‘Colossus’ is hexaploid, meaning it has three times the normal 2n number of chromosomes in each cell. In concrete terms, this translates into flowers that are up to five inches across (as opposed to the one to two inches of the plain species) and usually double, with as many as 18 tepals. The bold, broad, elliptic leaves can be long as eight inches, and as for vigor, I advise you to stand back after putting your specimen in the ground. Mine has easily tripled in height and spread in just over two years. ‘Colossus’ also has the advantage of being more tolerant of heat and sun than the wild form.
What about drawbacks? Well, the stamens of ‘Colossus’ are a medium rosy pink, and some gardeners may prefer the deep crimson stamens of the “unimproved” M. sieboldii, furthermore, the faded brown flowers sometimes hang onto the branches longer than one would like. And if you’re in the market for a demure, slow-growing, slender tree, look elsewhere. Still, ‘Colossus’ does boast those big, double flowers–as serenely beautiful as a lotus–and a willingness to grow that makes it easy to establish. All in all, a worthy experiment.