Tree of the month: Much maligned, but with plantation potential

ASK an arborist to name a typical ‘common or garden’ tree and the much-maligned Leyland cypress will almost inevitably pop up. However, this is a tree with hidden forest plantation potential.

Depending on the pack of taxonomy terriers you run with, the scientific name is x Cuprocyparis leylandii or x Cupressocyparis leylandii. And in these two names lie the first clues as to the complexity of the tree’s origins. Today’s 20 or more varieties are universally derived from pockets of vegetative propagation material with roots stretching back almost 150 years.

The symbol preceding the names correctly suggests a hybrid, but achieved with minimal contribution from man. The parents are Cupressus nootkatensis (Nootka cypress) and Cupressus macrocarpa (Monterey cypress), two North American native conifers with natural distributions on the north-west Pacific coast.

However, the conundrum contained in Leyland cypress does not stop there, because the cross, though taking place spontaneously in the wider environment, was by no means totally natural. Indeed, it did not even occur in North America, but thousands of miles away in the British Isles.

A line of Leyland cypress planted along the perimeter of a golf course in South Hertfordshire to screen out the London-to-York railway.

CHANCE CROSSES

The reason it was not found first in North America appears to be down to disparate distribution of the two parent species – Monterey cypress is found within a coastal enclave in Central California and Nootka cypress along a coastal strip mainly in British Colombia, Canada – essentially 1,000 miles apart.

It is 18th-century British botanists and plant hunters we have to thank for the Leyland cypress tree after they brought back a wide range of native North American conifer species and unwittingly planted the trees alongside each other at various estate locations in the UK.

One such site was the Leighton Estate near Welshpool in North Wales, to which the first identification of Leyland cypress is generally assigned. Actual identification is attributed to a Mr C. J. Leyland who, after taking some seeds from a Nootka cypress tree, noticed six of the resulting plants were different from the parent species. This took palace in 1888. He subsequently took these trees to Haggerston Castle in Northumberland where they were grown on to form a pocket of propagation material.

However, renowned and irrepressible arborist Alan Mitchell subsequently discovered the first occurrence of Leyland cypress was 18 years earlier at Rostrevor in County Down, Northern Ireland, with cuttings planted at Castlewellan (also in County Down), hence another pocket of propagation material. Who to believe is up to you. The Leighton event was clearly documented in real time, but it would be dangerous to doubt someone as eminent as the late Alan Mitchell.

As far as I’m concerned, it doesn’t really matter, as plant material from both has been used as propagation material to multiply this clonal tree which as a hybrid bears sterile seeds. At least another 20 natural hybrids have been identified since then.

However, just to muddy the waters, the hybridisation found at Leighton in 1888 occurred with Nootka cypress as the female parent, while the one identified in 1911 occurred with Monterey cypress as the female parent. Hybridisation has never been achieved by controlled pollination.

Recently planted Leyland cypress duly staked and tied. However, given the potential growth rate it will have to be checked and adjusted on a very regular basis.

ACHILLES HEEL

Amazingly, today’s millions of Leyland cypresses are derived from propagation pockets planted more than 100 years ago.

Its breakthrough came in 1925, when commercial nurserymen specialising in conifers were searching for trees that were both fast-growing and hardy in windswept, salty environments like Cornwall. Eventually, they found the original six trees identified at Leighton and planted at Haggerston Castle and began to propagate the hybrid tree. Rapid growth was conferred by the Monterey cypress parent and hardiness by the Nootka cypress parent.

Commercial plant nurseries spotted the tree’s potential and, in no time at all, it became the biggest-selling tree in garden centres throughout the UK, at one time accounting for 10 per cent of total sales. However, as we all now know, its ultra-rapid growth rate can be an ‘Achilles-Heel’, causing legal disputes between neighbours up and down the country, leading to violence and, in at least one instance, murder.

Just 30 years of growth was required to produce these trees, with clear commercial potential if grown on a plantation scale.

BASIC BOTANY

Leyland cypress grows rapidly (up to 1 m extension/year) reaching 20 to 25 m and, in exceptional cases, 40 m. The crown is broadly columnar, branches slightly flattened with slightly overhanging tips to give a compact canopy with a thick, regular form. Bark is very thin, dark red or brown, with longitudinal grooves. Leaves are borne close to the twig and are presented on flaky, slightly aromatic branches. Leaf colour is generally dark green but somewhat paler on the underside. Cones are about 2 cm in length, covered with eight scales and contain five seeds with tiny resinous vesicles, but are invariably sterile in line with the tree’s hybrid status.

Taking down big Leyland cypress trees in suburban situations is a painstaking task requiring the removal of all side branches on the way to the top.

TIMBER TRAITS

Trees for timber are typically 20–30 m tall with wood having an average dry weight of 500 kg/m3. Heartwood is pale yellowish or reddish brown with a narrow band of paler sapwood clearly demarcated from the heartwood. The grain is usually straight, although small knots may be present to create a more irregular pattern. It has a fine, uniform texture with a good, natural lustre.

Leyland cypress is rated as moderately durable with mixed resistance to insect attack. It is generally easy to work with using hand and machine tools, though wood around the knots may prove problematic. It is difficult to steam bend, but glues, stains and finishes well.

SOFTWOOD TIMBER POTENTIAL

For me, the first chinks in Leyland cypress’s armour as a pure amenity tree came after comments by a long-serving Forestry Commission officer. Deep in the catacombs of the commission, he said, was data documenting Leyland cypress as an alternative softwood species, should something nasty happen to Sitka spruce.

Some years later, I had cause to take down half a dozen Leyland cypress trees that were beginning to unsettle my neighbours. They had been planted in the early 1980s as seedling conifers given to me by my late father. I had come to take the trees, which my arborist described as bog-standard Leylandii, for granted, but closer scrutiny showed they did not deserve this tag. In just three decades they had grown tall, straight as a ship’s mast and with a dbh of up to 0.50 m. The wood was clean and colourful and gave the immediate impression of excellent sawn timber.

MERRY CHRISTMAS FROM LEYLAND CYPRESS

Readers may be surprised to learn that Leyland cypress is one of the most popular Christmas trees in the Southeastern United States. The National Christmas Tree Association attributes this to an attractive natural shape enhanced by shearing the dark green foliage, and a weak aroma compared with other cypresses.

Are there any diseases out there likely to spoil the party? Not at the moment, though it is worth remembering the exotic fungus-like pathogen Phytophthora austrocedri, currently wiping out native common juniper (Juniperus communis) which is our only native tree in the Cupressaceae. Phytophthora austrocedri has been found infecting one of Leylandii’s parent species, Nootka cypress, in Scotland.