HILL farmers have been warned their livestock could be at risk from a poisonous menace following the discovery of hemlock plants growing at unprecedented altitudes on a Borders summit.

A leading botanist who has studied the spread of the potentially deadly weed through Midlothian and the Borders over the past 12 years believes global warming has assisted the new hemlock colony to become established on Soutra Hill, 371 metres [1,200 feet] above sea level.

Dr Brian Moffat, the director of a long-term archaeological project at Soutra’s medieval hospital, near Lauder, told The Scotsman hemlock had never been recorded anywhere in the UK above 305 metres [approximately 1,000 feet].

The official highest listing was logged at Llynheillyn, Radnorshire, according to the scientific publication Altitudinal Limits of British and Irish Vascular Plants 2003. But now Soutra can claim a record the area would probably rather be without.

“This is a considerable jump above the previous altitude limit for hemlock, and will be a cause for some concern to farming communities in the uplands of southern Scotland,” Dr Moffat said.

He explained how a combination of global warming, the existence of a corridor of suitable habitats, including the A68 trunk road, and the well known property which allows hemlock seeds to lie dormant for many years in the soil before springing to life had been responsible for the surprise appearance of the plants at Soutra Aisle.

Dr Moffat has passed details of his find to staff at the Scottish Agricultural College. They have told him hemlock is becoming a troublesome weed in arable areas, particularly in fields of winter cereals, with no existing herbicide capable of exterminating it.

“Livestock in upland areas will not be familiar with hemlock so may consume it ,should it be allowed to spread in large, unfenced pastures on upland farms”, Dr Moffat said. “The migration of hemlock south from Dalkeith has covered around 10 miles in 12 years.”

The plant takes over distinct sunny, sheltered spots, and even on the exposed hill-top at Soutra, the plants are in the lee of the ancient aisle set in a dip among boulders.

The abundance of bare ground and repeated roadside excavations for pipe-laying and other construction projects have helped the hemlock thrive and spread.

Although the large majority of plants have spread along the A68 roadside verges, Dr Moffat has noted hemlock invading corn fields on at least one Midlothian estate as well as cattle pastures close to the trunk route.

He has said there is a strong case for the introduction of formal measures to keep hemlock out of cultivated fields.

A range of animals have been poisoned after ingesting hemlock, although livestock familiar with the plant tend to avoid it. Cases of poisoning, however, have been recorded involving cattle, horses, pigs, sheep, rabbits, poultry and deer. It can also cause birth defects in pigs and cattle.

Farming experts also warn that hemlock becomes palatable to grazing livestock after plants are cut or sprayed as toxins are retained in the dead tissue.

There can even be a danger to the general public if an unsuspecting consumer mistakes the celery-like stems for sweet cicely, a plant often gathered from the roadside.