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.