Magnolia sieboldii is a large deciduous shrub or small tree 5–10 m (16–33 ft) tall. The stalks, young leaves, young twigs and young buds are downy. The leaves are elliptical to ovate-oblong, 9–16 cm (rarely 25 cm) long and 4–10 cm (rarely 12 cm) broad, with a 1.5-4.5 cm petiole.
The flowers, unlike the spring flowering magnolias, open primarily in the early summer, but continue intermittently until late summer. They are pendulous, cup-shaped, 7–10 cm diameter, and have 6-12 tepals, the outer three smaller, the rest larger, and pure white; the carpels are greenish and the stamens reddish-purple or greenish-white.
Magnolia sieboldii subsp. japonica. Japan. Low shrub; flowers with 6 tepals and greenish-white stamens.
Magnolia sieboldii subsp. sieboldii. Japan, Korea, eastern China. Tree or large shrub; flowers with 9-12 tepals and reddish-purple stamens; leaves smaller, rarely over 16 cm.
Magnolia sieboldii subsp. sinensis. Southwestern China (Sichuan); flowers as subsp. sieboldii; leaves larger, commonly to 22 cm.
Friedman, William (Ned). "Oyama magnolia in full bloom."Posts from the Collections, Arnold Arboretum of Harvard University website, June 12, 2016. Accessed 1 October 2019.