The Leyland cypress, Cupressus × leylandii, × Cuprocyparis leylandii or × Cupressocyparis leylandii, often referred to simply as leylandii, is a fast-growing coniferous evergreen tree much used in horticulture, primarily for hedges and screens. Even on sites of relatively poor culture, plants have been known to grow to heights of 15 metres (49 ft) in 16 years. Their rapid, thick growth means they are sometimes used to achieve privacy, but such use can result in disputes with neighbours whose own property becomes overshadowed. The tree is a hybrid of Monterey cypress (Cupressus macrocarpa) and Nootka cypress (Cupressus nootkatensis). It is almost always sterile, and is propagated mainly by cuttings.
History
In 1845, the Leighton Hall, Powys estate was purchased by the Liverpool banker Christopher Leyland. In 1847, he gave it to his nephew John Naylor (1813–1889). Naylor commissioned Edward Kemp to lay out the gardens, which included redwoods, monkey puzzle trees and two North American species of conifers in close proximity to each other – Monterey cypress and Nootka cypress. The two parent species would not likely cross in the wild, as their natural ranges are more than 400 miles (640 km) apart, but in 1888, the hybrid cross occurred when the female flowers or cones of Nootka cypress were fertilised by pollen from Monterey cypress. John Naylor's eldest son Christopher John (1849–1926) inherited Leighton Hall from his father in 1889. Christopher was a sea captain by trade. In 1891, he inherited the Leyland Entailed Estates established under the will of his great-great-uncle, which passed to him following the death of his uncle Thomas Leyland. On receiving the inheritance, Christopher changed his surname to Leyland, and moved to Haggerston Castle, Northumberland. He further developed the hybrid at his new home, and hence named the first clone variant 'Haggerston Grey'. His younger brother John (1856–1906) resultantly inherited Leighton Hall, and when in 1911 the reverse hybrid of the cones of the Monterey cypress were fertilised with pollen from the Nootka, that hybrid was baptised 'Leighton Green.' The hybrid has since arisen on nearly 20 separate occasions, always by open pollination, showing the two species are readily compatible and closely related. As a hybrid, although fertility of certain Leyland cypress forms were recently reported, most Leyland cypress were thought to be sterile, and nearly all the trees now seen have resulted from cuttings originating from those few plants. Over 40 forms of Leyland cypress are known, and as well as 'Haggerston Grey' and 'Leighton Green', other well-known forms include 'Stapehill', which was discovered in 1940 in a garden in Ferndown, Dorset by M. Barthelemy and 'Castlewellan', which originated from a single mutant tree in the Castlewellan estate arboretum in Northern Ireland. This form, widely propagated from the 1970s, was selected by the park director, John Keown, and was named Cupressus macrocarpa 'Keownii', 1963.
Description A large, evergreen tree, Cupressus × leylandii reaches a size between 20 and 25 m high, with its leaves giving it a compact, thick and regular habit. It grows very fast with yearly increases of 1 m. The leaves, about 1 mm long and close to the twig, are presented in flaky, slightly aromatic branches. They are dark green, somewhat paler on the underside, but can have different colors, depending on the cultivar. The crown of many forms is broadly columnar with slightly overhanging branch tips. The branches are slightly flattened and densely populated with scaly needles. The tree bark is dark red or brown and has deep grooves. The seeds are found in cones about 2 cm in length, with eight scales and five seeds with tiny resinous vesicles. With the tree being a hybrid, its seeds are sterile. Over time, the cones shrink dry and turn gray or chocolate brown and then have a diameter of 1 cm.
Taxonomic status
Cupressus × leylandii is a hybrid of two other cypress species: Monterey cypress (Cupressus macrocarpa) and Nootka cypress (Cupressus nootkatensis). The taxonomic status of Nootka cypress has changed over time, and this has affected the taxonomic status of the hybrid. Nootka cypress was first regarded as belonging in the genus Cupressus, but was later placed in Chamaecyparis. It has become clear, however, that when the genus Cupressus is defined to include Chamaecyparis, it is paraphyletic unless it also includes Juniperus. In 2004, Little et al. transferred the Nootka cypress to Callitropsis. Little (2006) proposed another alternative by transferring all the North American species of Cupressus, including the Monterey cypress (C. macrocarpa), to Callitropsis. In some of these classifications, this and other hybrids of Nootka cypress become very unusual in being intergeneric hybrids, the only ones ever reported among the gymnosperms. In 2010, Mao et al. performed a more detailed molecular analysis and redefined Cupressus to exclude Chamaecyparis, but to include the Nootka cypress. It may be added that attempts to cross Nootka cypress with other Chamaecyparis species have been universally unsuccessful. The scientific name of Leyland cypress depends on the taxonomic treatment of Nootka cypress. Where Nootka cypress is considered as Cupressus nootkatensis, the hybrid is within the Cupressus genus and is therefore Cupressus × leylandii. If both Monterey and Nootka cypress are considered as species of Callitropsis, the hybrid is Callitropsis × leylandii. However where the parents are treated as being in different genera, Leyland cypress becomes an intergeneric hybrid: if Nootka cypress is within Chamaecyparis, the name of the hybrid becomes ×Cupressocyparis leylandii, and where it is treated as Xanthocyparis, the hybrid becomes ×Cuprocyparis leylandii. Two other similar hybrids have also been raised, both involving Nootka cypress with other Cupressus species:
Cupressus arizonica var. glabra × Cupressus nootkatensis (Cupressus × notabilis) Cupressus lusitanica × Cupressus nootkatensis (Cupressus × ovensii)
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