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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Sorbus 'Joseph Rock'' from the website Trees and Shrubs Online (treesandshrubsonline.
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A tree so far about 40 ft high in cultivation; branchlets soon glabrous, grey-brown by autumn, with numerous warty lenticels; winter-buds narrow-ovoid, acute, with scattered brown hairs, especially at the edge of the scales and at the tip. Leaves 41⁄2 to 7 in. long, including petiole; leaflets in seven to ten pairs, closely set; rachis grooved above, not winged, soon glabrous. Lateral leaflets narrowly oblong, 1 to 13⁄4 in. long, 3⁄8 to 5⁄8 in. wide, subacute and narrowly apiculate at the apex, obliquely rounded to truncate at the base, incise-serrate almost to the base, glabrous and fairly glossy above, undersurface glabrous by June but earlier with a few brown hairs on the midrib. Flowers in a rather open and irregular inflorescence about 4 in. wide and almost as long, opening in late May or early June; inflorescence-branches lenticellate, at first sparsely clad with brown hairs, glabrous by autumn, by which time they are bright red. Flowers about 3⁄8 in. wide; receptacle funnel-campanulate, with acute lobes. Stamens with creamy filaments, anthers pink, darkening to purple and finally to black. Fruits globular, about 3⁄8 in. wide, up to fifty or slightly more in each cluster, at first green, becoming white and finally amber yellow by October but remaining white on the shaded side and always with a pink tinge at the calyx. Bot. Mag., n.s., t. 554.
This rowan, of uncertain origin, is one of the most beautiful of its group and indeed one of the first small trees of any genus to be considered when only a few can be grown. Given a good soil it should attain 25 ft in height and 10 ft in spread in fifteen years. Pretty in flower, its primrose-yellow fruits combined with the splendid autumn colour of crimson, purple and scarlet makes a unique combination, and even in midsummer it is a pleasant tree, graceful in habit and with fresh green foliage enhanced by the lighter green of the young fruits. It received a First Class Certificate in 1962.
The history of ‘Joseph Rock’ will be found in the article by C. D. Brickell in Journ. R.H.S., Vol. 89 (1964), pp. 19-22. In brief, the original in the R.H.S. Garden at Wisley is believed to have come from the Royal Botanic Garden, Edinburgh, under Dr Rock’s collector’s number 23657. But the corresponding herbarium specimen is S. hupehensis (oligodonta), and it is doubtful if ‘Joseph Rock’ could even be a hybrid of that species. J. R. Sealy, who went into the problem of its identity, remarked on its resemblance to S. ursina (q.v.), and to the Japanese and Korean S. commixta (q.v.).
‘Joseph Rock’ does not come true from seed, and the name applies only to plants propagated vegetatively and belonging to the original Wisley clone. Caution is needed in deducing its parentage from its seedlings, at least when it grows in large collections, but it may be of significance that of two raised by Messrs Hillier ‘Tundra’ has white fruits and resembles both S. vilmorinii and S. koehneana, while ‘Sunshine’, apart from its bright yellow fruits, bears a strong resemblance to S. rehderiana, a Chinese ally of S. ursina not known to be in cultivation. All the evidence, such as it is, suggests that ‘Joseph Rock’ is a chance natural hybrid, raised from seeds collected by Dr Rock in Yunnan in 1932.
It is lamentable that this beautiful tree is no longer being propagated in nurseries, because it is so susceptible to fireblight. In gardens, however, there is no obligation to remove a healthy tree. The original tree at the R.H.S. Garden, Wisley, measures 50 × 31⁄4 ft (1985).