| Ulmus hybrid | |
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'New Horizon', Groningen. Photo: Ronnie Nijboer, Bonte Hoek kwekerijen |
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| Details | |
| Hybrid parentage | Ulmus pumila × U. davidiana var. japonica |
| Cultivar | 'New Horizon' |
| Origin | WARF, Wisconsin, USA |
An American cultivar raised by the Wisconsin Alumni Research Foundation (WARF), 'New Horizon' was derived from a crossing of Siberian Elm Ulmus pumila with Japanese Elm Ulmus davidiana var. japonica [1] and patented in 1994.
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Unlike an earlier crossing of the same species 'Sapporo Autumn Gold', the tree has a compact pyramidal form, with comparatively dense foliage comprising glabrous, dark-green, elliptical leaves < 12 cm long by 7 cm broad, occasionally without the asymmetric bases typical of the genus. The perfect, apetalous wind-pollinated flowers appear in March, followed by the seeds in April; flowering usually begins when the tree is aged 8 years.[2]
The tree's growth is unusual; in an assessment at U C Davis as part of the National Elm Trial, its d.b.h. increased faster than any other of the 15 cultivars, but increase in height was one of the slowest, averaging a modest 0.9 m per annum.
In commerce in the USA, the tree is occasionally propagated by grafting onto an Ulmus pumila rootstock, rather than simply rooting cuttings as normally practiced in North America and Europe. More recently in the Netherlands, cuttings have been successfully grafted onto Belgian Elm Ulmus × hollandica 'Belgica' rootstocks in the hope of making the tree more adaptable to heavy, clayey soils.
'New Horizon' has a high resistance, rated 5 out of 5 [3] to Dutch elm disease, and is also resistant to elm leaf miner, and verticillium wilt [4]. However, it has proven susceptible to attack by elm leaf beetle Xanthogaleruca luteola [5] [6] and Japanese Beetle in the USA [6].
The tree has not been an unqualified success. In the elm trials conducted by the University of Minnesota, 'New Horizon' was found to need relatively high levels of maintenance, largely owing to its predilection for co-dominant leaders and heavy side branches [7]. In the Netherlands, removal of sideshoots from the lower trunk was found to be necessary twice a year. In trials in eastern Arizona [7] it often exhibited > 25 % crown dieback over winter and a very high level of leaf scorch in summer. The tree is currently being evaluated in the National Elm Trial [8] coordinated by Colorado State University.
'New Horizon' was introduced to Europe by the Conrad Appel nursery (ceased trading 2005) in Darmstadt, Germany, which propagated the tree under licence as one of the hybrid elms offered in the Resista series [9]; the tree is currently propagated by Eisele GmbH. Introduced to the UK, 'New Horizon' was named 'Best New Plant Variety' by Horticulture Week in 2005, however, an assessment by Butterfly Conservation found its growth on heavy, poorly-drained ground negligible. Nevertheless, the tree has tolerated flooding, by both freshwater in England, and seawater along the Baltic coast in Germany [10]. Trees at exposed sites in Hampshire exhibited much the same degree of dieback experienced in the Arizona trials despite the extreme differences in climate. However no losses have been sustained, and in sheltered conditions on deep loam over chalk, 'New Horizon' grew healthily if relatively slowly, increasing in height by approximately 50 cm per annum, less than half the speed of the Dutch hybrids such as 'Dodoens' planted with it. The trees in the English trials first flowered aged 10 years, in late March [8].
In 2004, 80 trees were donated to the Greater London Council as part of the grower's European Elm City promotion; similar gifts were also made to Belfast, Cardiff (Pontcanna Park), and Hamburg (the central City Park).[9] [10] In 2010, 100 trees were planted in the London borough of Enfield to aid and abet the conservation of the White-letter Hairstreak Satyrium w-album. [11] Another hundred have been included in the tree planting at the Olympic Park.[11] Specimens have also been planted in various parts of Edinburgh, such as Castle Terrace, the Meadows and Inverleith Park.