A RENOWNED horticulturist who developed the world famous Sissinghurst Castle gardens in Kent died from a lung cancer caused by asbestos exposure, an inquest heard.

But how award-winning gardener Pamela Schwerdt came to be exposed to the deadly mineral remained a mystery after her inquest in Gloucester.

Gloucestershire coroner Alan Crickmore heard that in the late 1940s and through most of the 1950s she stoked boilers at a horticultural school near Oxford.

But there was no other evidence of asbestos exposure either in her working or social life, said the coroner. He recorded an open verdict.

Miss Schwerdt was 78 when she died of malignant mesothelioma on September 11 last year.

The Royal Horticultural Society Associate of Honour, of Manor Farm, Condicote, near Stow-on-the-Wold, Gloucestershire, had been diagnosed with the disease - normally associated with exposure to asbestos in a working environment - in August 2008.

In a statement, Miss Schwerdt's sister, Rosemary Booth, said she could recall her sister stoking boilers at the Waterperry Horticultural School near Oxford when she worked there from 1948 to 1957.

She had become very poorly on September 9 last year and she was admitted to Cheltenham General Hospital.

Miss Schwerdt's condition deteriorated and she died two days later.

The inquest was told the Waterperry school had closed in 1971 and its heated greenhouses had been removed. The company that took over the site did not have records that stretched back that far.

A post mortem examination revealed the cause of death as haemorrhagic pericardial effusion - an abnormal build-up of fluid around the heart - caused by malignant mesothelioma.

Analysis of her lung tissues revealed 57,047 mineral fibres per gram of dry lung tissue - a 'moderate' accumulation of fibres consistent with causing mesothelioma.

Coroner Crickmore said he was satisfied that absestos exposure led to Miss Scwerdt's death, but the question remained on how she came to be exposed.

Mr Crickmore said after hearing the evidence he could not say that she was exposed to asbestos during her employment or could he say the exposure occurred in a social setting.

Pamela Schwerdt was joint head gardener for 31 years, with Sibylle Kreutzberger, at the garden at Sissinghurst Castle.

The garden was created by Sir Harold Nicolson and his wife Vita Sackville-West and has become one of the most famous and celebrated gardens in the world.

During the stewardship of Miss Schwerdt and her partner, the garden visitor numbers soared from 6,000 to more than 170,000 per year.

Miss Schwerdt’s eye for colour, knowledge of plants and her first-hand experience of growing them made her an invaluable member on the two Royal Horticultural Society floral committees on which she served.

She was modest and shunned publicity, but was keen to share her passion for plants through her lectures in Britain and abroad - she was a lecturer on the English Garden School’s Plant and Plantsmanship course for eight years - and through her personal contacts with many younger gardeners and growers whom she helped and encouraged.

She retired from Sissinghurst in 1990. She and Kreutzberger then started a new and smaller garden in Gloucestershire, on alkaline soil.

Here she had to curb a lifetime habit of taking cuttings, but revelled in the pleasure of making new colour-themed borders.

Miss Schwerdt was awarded the RHS Associate of Honour in 1980 and appointed MBE in 1990.