Acer rubescens Hayata

(Sect. Macrantha)

Synonyms

A. morrisonense H.L. Li non Hayata

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New Trees

Tree 10–20 m. Bark smooth, green with irregular white longitudinal stripes. Branchlets somewhat glossy, with whitish stripes. Leaves deciduous, papery, 8–10 × 6–8 cm, palmately five-lobed (rarely seven-lobed), divided to one-fifth of the length, upper surface shiny bronze-green to dark green with caducous rusty hairs on the midrib, lower surface lighter green, dull with persistent rusty hairs, margins coarsely serrate, apex acute to acuminate; petiole 4–8 cm long, red and glabrous; autumn colour bronze to red-orange, finally yellow. Inflorescence terminal and lateral, racemose with 10–30 flowers. Flowers 5-merous, 3–4 cm diameter, dioecious; sepals triangular-ovate, dark green, petals oblong, yellow, stamens eight, inserted outside the nectar disc. Samaras 1.8–2.3 cm long, yellowish brown, wings distinctly veined and spreading almost horizontally. Flowering March to April, fruiting October (Taiwan). Van Gelderen et al. 1994, van Gelderen & van Gelderen 1999, Xu et al. 2008; Gregory, in prep. Distribution TAIWAN. Habitat Montane forest between 1800 and 2200 m asl. USDA Hardiness Zone 7–8. Conservation status Not evaluated. Illustration NT105, NT108.

In horticulture, Acer rubescens has been somewhat confused with A. caudatifolium, principally because the two species share the synonym A. morrisonense, and material of A. rubescens has been distributed as A. morrisonense through the nursery trade in England. There should, however, be little risk of mistake, as (besides the botanical distinctions) A. rubescens often has a strong red flush to all its parts (as its name suggests), with dark olive-green leaves, while A. caudatifolium is rather green in appearance, with mid-green leaves.

Much of the cultivated stock of A. rubescens has come from a tree at Trewithen, Cornwall, 19 m tall (67 cm dbh) in 2004 (TROBI), for long the only specimen known in cultivation. This was planted in 1912, from a seed collection (Yashiroda 109) made in Taiwan (van Gelderen et al. 1994, Johnson 2003). Scions of it are being grown throughout the British Isles, and many have become fine trees. One observed at Hergest Croft is c.12 m tall (dbh 56 cm), with stiffly ascending branches. The bark is extremely attractive, being a mixture of dark green and brown, with paler lines and large lenticels, the pattern retained to the ground. The younger growth is deep red, as are the petioles and the expanding leaves. When mature the leaves are glossy dark green, and vary greatly in the degree of lobing. Seedlings are very variable, however, and recent introductions have revealed further variation. A specimen grown from BSWJ 1744 at Hergest Croft is rather gaunt, with wide-spreading limbs, and has green stems with white stripes.

The species is extremely fast-growing (A. Norfield, pers. comm. 2006), but is said to require a sheltered site (van Gelderen & van Gelderen 1999), and in Poland it is damaged by frost (P. Banaszczak, pers. comm. 2007). At Kew it is susceptible to frost until well established (Flanagan & Kirkham 2005), possibly because the leaves tend to emerge rather early – although at Hergest Croft it has not been affected by frost (L. Banks, pers. comm. 2006). The leaves also remain on the tree rather late into the autumn, turning a dull red. It is well established from ETOT 158 (1993) at the David C. Lam Asian Garden at the University of British Columbia (Justice 2002), but seems not to be widely cultivated in North America. Several variegated clones are known, having white mottling and marbling of varying extent; with the red stems this can give a rather striking effect.

The generally reddish tint seen in Acer rubescens does not appear in the bark, but is apparent here in the petioles of the leaves in the background. Image R. Hitchin.

Genus

Acer

Other species in the genus