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September 2003
Plant of the Month
Japanese-like Maple
(Acer pseudosieboldianum)
tree
zone: 4
by Stan Hokanson
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Acer pseudosieboldianum-A Japanese-like maple for the North?
Japanese maples, Acer palmatum, with their various forms, leaf colors, degrees of leaf dissection, and brilliant fall color are much admired and highly valued garden trees. However, in Minnesota, these beautiful trees largely assume a cruel temptress role in the garden. Their USDA Zone 5 hardiness designation puts them in the marginal hardiness category for the southern third of the state. As such, the species will prosper nicely for several years only to be reduced to a near death appearance after a hard winter. Thankfully, the genus Acer is large, containing a number of species with similarities to the Japanese maple, some with greater cold hardiness. Among these is Acer pseudosieboldianum, commonly referred to as Korean maple or purplebloom maple. Korean maple is a small tree or shrub that is actually classified in the same maple section as Japanese maple. Generally listed as being USDA Zone 4 hardy, the species has been grown successfully in Bismark, ND (USDA Zone 3), suffering no damage at temperatures to - 43º F.
In its native habitat in Manchuria, Korea, and China, Korean maple grows in mixed forests on well-drained, stony soils where it can attain a mature height of 15-25 feet with greater width. The species is an openly branched tree with gray, black striped branches. The oppositely arrange, doubly serrated leaves emerge with a reddish tinge that turns to a dark green above with a slight white pubescence on the undersides. The leaves, 4-6 in width, are borne on 1-2 inch petioles and generally possess 9-11 lobes. Fall leaf colors have been described as brilliant yellow, orange and/or red color combinations. The withered leaves are retained through the winter, falling off as growth commences in the spring.
Like any garden plant, the species is not problem-free. Although individual trees are reported to be winter hardy in Zones 3 and 4, some seed lots have been found to be considerably less winter hardy. Similarly, while the species has been reported to have vivid fall colors, the literature contains reports of the species displaying muddy greenish-brown fall colors. Like many maples, Korean maple is thin barked, rendering it susceptible to mechanical damage and winter sunscald damage, the latter most often occurring at the site of mechanical damage. Mechanical damage to the trunk and stems can also provide sites for disease infections. Currently, the main liability of the species appears to be its susceptibility to a canker disease (Nectria, Eutypella, Valsa and/or Cryptosporiopsis) which enters the tree at wound sites, causing dieback of the infected branches or stem. Fungicides can be used in conjunction with pruning or at wound sites to protect against infections, although there are no fungicides labeled for such use. The principle management strategies include maintaining plant vigor with adequate water and fertilization and the removal and destruction of infected twigs and limbs to prevent the spread of additional inoculum.
While the species tolerates full sun and wind with only minor leaf burning and tatter, it performs much better with protection from afternoon sun and prevailing winds. The species will grow perfectly well in heavy shade, but will develop its best fall color when sited in sunnier locations. The Korean maple would be best used as a focal point in a naturalized garden in company with low growing ferns and shade tolerant perennials. I can also envision the tree planted with smaller statured rhododendrons such as the R. dauricum hybrids, the PJM hybrids and/or Korean (R. mucronulatum) hybrids.
Acer pseudosieboldianum can be reliably grafted on A. palmatum. We have had some success rooting terminal softwood cuttings collected in June and treated with 8000 ppm IBA. The species can also be grown from seed, although locating viable and true-to-type seed can be difficult. Several subspecies, and varieties of A. pseudosieboldianum have been named in the literature, so some effort may be required to verify the identity of plant material acquired from commercial sources. A search of the Minnesota Landscape Arboretums online plant and seed source locator revealed five sources for seed and numerous sources for plants, including Bachmans Nursery Wholesale as a local source.
In the Woody Landscape Plant breeding program at the University of Minnesota, we have been evaluating a small collection of Korean maple seedlings as part of a larger project to assess a number of Asian maple species with potential for use in Minnesota. Programmatic goals include selecting reliably cold hardy seed sources and genotypes that produce consistently good fall color. In addition, we intend to determine the causal agent(s) of the aforementioned canker disease(s) and identify sources of resistance to the pathogen(s).
By Stan Hokanson
Assistant professor, Woody Landscape Plant Breeding/Genetics
University of Minnesota
Dept. Of Horticultural Science
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