Star magnolia (Magnolia stellata) is one of the smallest magnolias, producing a showy cloud of white flowers in early spring. Originally from the highlands of the Japanese island of Honshu, it is sometimes considered a variety of M. kobus but of garden origin.
M. stellata was introduced to the United States in the 1860’s. It is variously listed as hardy from either zone 4 or 5 to 9.
This slow growing deciduous plant can be grown as a large shrub or pruned up as a small multistemmed tree. It eventually reaches a height of 15-20 feet with a rounded crown spreading to 10-15 feet when mature. Young plants have a more upright, conical habit. It is denser and more compact than saucer magnolias, with a heavily branched, twiggy structure for winter interest, along with attractive smooth, silvery gray bark on the main trunk and shiny chestnut brown bark on the young twigs.
The fat, fuzzy buds open in late winter before the foliage appears to produce clusters of lightly fragrant, 3-4 inch white flowers. The blooms have 12-18 thin, delicate, strap-like petals; some cultivars have more than 30 petals per flower. The flowers need shelter from frost and wind which discolor the blooms. The plant creates a spectacular show when covered in blossoms, with flowers from the top of the plant to the lowest branches. Even very small plants are likely to bloom. They flower just before saucer magnolia blooms.
Later in the season the plant produces attractive fruit and seeds. The large seeds grow within a bright pink, knobby capsule 2-3 inches long. Many of the fruits drop before they are fully developed but those that remain on the tree burst open in early autumn to reveal brilliant orange seeds.
The oblong leaves are about 4 inches long. They emerge with a bronze cast, are a clean medium to dark green in the summer, and turn yellow to bronze in the autumn but do not make a particularly nice fall color display.
Star magnolia does best when planted in moist, organic, acid soil in full sun but it is relatively adaptable. If possible, plant in a protected area to prevent buds from opening early and then being killed by a late frost or freeze. This shrub generally does not need any pruning – unless you wish to train it to a tree shape. Pruning should be done after flowering to avoid cutting off buds set for the next season.
Star magnolia is a great flowering tree in a small yard since it remains small and compact for many years. It makes a good specimen plant but can also be incorporated with other trees and shrubs into a planted bed. It is particularly nice when sited against a dark background, such as a brick wall or grouping of evergreens, which shows off the flowers, or on or near a patio where the scent of the flowers can be enjoyed. It also makes a good addition to the edge of woodland gardens or other partly shady areas.
Star magnolia is a relatively trouble-free plant. It is soft-wooded, however, so may be damaged by heavy snow and ice. Also, the blossoms are susceptible to injury by frost or wind. Magnolia scale is about the only reported insect pest. Infested plants have shiny sticky leaves, often with black sooty mold growing on the honeydew produced by the adult females that look like brown buds on the twigs. Heavily infested plants can be more prone to winter kill problems. Apply insecticide treatments in early September to kill the susceptible crawler stage when they are out on the new growth (look for small flat insects on leaves and stems).
Star magnolia may be propagated by taking cuttings in early to mid-summer or from seed. Some sources indicate germination is slow and seeds require special handling.
There are many horticultural cultivars that are preferable to the species. Some of the most commonly available include:
- ‘Centennial’ was released by Harvard’s Arnold Arboretum to commemorate the institution’s 100th anniversary in 1972. The large (5½ inch), many-petaled flowers are white with a slight tinge of pink on the outside, while the upright plant with a central leader reaches 25 feet tall.
- ‘Jane Platt’ has flowers with many light pink tepals that open to white. The tree grows 10-20 feet tall.
- ‘Pink Stardust’ produces numerous large, fragrant flowers (4-5″) on a pyramidal tree to 12 feet tall.
- ‘Rosea’ includes several light pink-flowered clones. The flowers fade to white at maturity and are not fragrant.
- ‘Royal Star’ is possibly the best and is quite common in commerce. It produces abundant, double, snow-white flowers with 25-30 petals each from pink buds. It grows 10-12 feet tall.
- ‘Rubra’ has purplish rose flowers that fade to pink. The compact shrub has yellow-green leaves.
- ‘Waterlily’ includes several clones under this name. Their flowers have larger, narrower, more abundant petals. The pink buds open to highly fragrant flowers. It is a bushy, upright grower that may flower slightly later than other cultivars.
– Susan Mahr, University of Wisconsin – Madison