In 1911 at Leighton Hall, Christopher’s nephew, Captain Naylor, after picking up a cone from a Monterey Cypress Growing fifty yards from a Nootka, noticed that two of the resultant seedlings were markedly different from the rest. The difference from the 1888 seedlings was that the Monterey was the female parent. The seedlings were planted half a mile apart on a hill behind the house. Cuttings were sent in 1916 to Bicton and Inverary Castle and, probably, Headfort, all of them growing extremely quickly and to heights in excess of thirty metres.

News of these monster conifers, which the family called Leyland cypresses, only reached a wider audience when a Cambridge University Professor, William Dawson, stayed as a guest at Leighton Hall in 1925. He was so fascinated by the unusual conifers that he sent samples of their foliage to the eminent botanists, Bruce Jackson and William Dallimore, for classification.

The new hybrid, given the botanical name of Cypressus x leylandii, was announced in the Kew Bulletin in March 1926. On a visit to Haggerston Dallimore and Jackson collected material from the by now enormous six trees for propagation. From these cuttings the first leylandii at the Royal Botanical Gardens at Kew and Edinburgh, at Wisley, and Bedgebury Pinetum were grown.

Another hybrid cross appeared around 1940 from seed collected from a Monterey Cypress Growing in Ferndown in Dorset and was sown at Barthelemy’s nursery in Staplehill. Curiously, there was no evidence of a Nootka cypress in the area.

The Leighton Hall leylandii might not have been the first, conifer expert Alan Mitchell of the Forestry Commission unearthing evidence of an even earlier hybrid, which was planted in the early 1870s, based on estimates from its size, by a gardener in Rostrevor in Northern Ireland. No formal records of its origin exist and the tree itself blew down in a storm before the First World War, but not before cuttings were taken from it in 1908 and grown on. These in turn provided cuttings which were planted at Castlewellan in 1949.

The first to capitalise on the public appetite for conifers that were hardy, fast-growing, and tolerant to a wide range of climatic and soil conditions was Hilliers, who began selling leylandii commercially in 1930. Leylandii now account for around 10% of garden centres’ sales. Pandora’s box had well and truly been opened.

Subsequent attempts to raise the hybrid by controlled pollination of Nootka and Monterey cypresses had all failed until 2011 when James Armitage seemed to have achieved the impossible by growing a seedling from seeds he had collected from a female Leylandii in Wisley Gardens. This seemed to disprove the commonly held view that they were sterile and needed a compatible pollen donor. However, some believe that the presence of a Nootka cypress growing nearby might have had something to do with it.

Armitage’s success seems to have been a one-off. Indeed, every one of the more than forty forms of Leylandii in cultivation today is a genetically identical clone or cultivar produced from one of the original plants. Most are derived from one of the six seedlings that were taken by Christopher Leyland to Haggerston, including the Haggerston Grey, from clone 2, which produces the tallest individual specimens and the Leighton Green, from clone 11, which is used for hedging.

Other popular forms of leylandii are Castellan Gold, raised as a seedling at Castwellan in Northern Ireland in 1962 as a cross from the golden forms of the Monterey and Nootka cypresses, and Leylandii 2001, developed in the 1990s by Van den Dool nurseries in the Netherlands. It is not clear whether the latter is a new hybrid cross or the product of a random genetic change. One failure was the Staplehill 20 cultivar, grown commercially for a few years but withdrawn when it was found to be susceptible to drought in the 1970s.

Love them or loathe them, leylandii are truly freaks of nature.