MacKenzie first noticed this clematis, a much stronger, larger and more vigorous type than the ordinary species, in 1968 while he was visiting the Waterperry School of Horticulture, near Oxford. Another eminent gardener, Valerie Finnis, named it after him since it was his observant eye that had first spotted it. On 7 September 1976, she showed the clematis at a Royal Horticultural Society Show where it immediately won an Award of Merit.
MacKenzie, who lived the last 22 years of his life in Frimley Green, Surrey, was born in Scotland, where his father was head gardener at Ballimore, near Loch Fyne in Argyllshire, an estate belonging to Col John McRae Gilstrap. In his day, he used to explain, "boys either went into farming or gardening", and before he had finished school, he went to live with his grandfather who was a dairy farmer. "There was little other choice," he told Valerie Finnis, who recorded a conversation with him earlier this year. "But I had three cows to milk at five in the morning before school and another three to do each evening when I came home."
That was enough to put him off farming for life. Instead, he moved back to Ballimore where he was taken on as vegetable boy, responsible for delivering supplies from the vast kitchen garden to the Ballimore cook. He evidently found favour there, for the family suggested he abandon gardening to train instead as their butler.
But gardening had him firmly in its grasp, and when he was 24, he became a student at the Edinburgh Botanic Garden. The Twenties and Thirties were heady times to be gardening there, for George Forrest was bringing back seed of new rhododendrons and primulas from his journeys in Yunnan, in China; Frank Kingdon Ward was sending seed of meconopsis, gentians and lilies from his plant-hunting trips in China, Burma and Tibet. It was MacKenzie's job, as deputy foreman in the propagation department, to raise all these new plants from the plant hunters' seed. Nomocharis from China, omphalogramma from Sikkim and Upper Burma, and many other plants (including the first plants ever seen at the botanic garden of the famous blue Himalayan poppy, Meconopsis betonicifolia) were coaxed into flower by MacKenzie's patient care. Like all the best gardeners, he was an acutely observant man.
While at the botanic garden MacKenzie developed a great love of alpine plants, particularly gentians, and bred the beautiful autumn gentian "Inverleith", which is an intense Cambridge blue, striped on the outside with darker blue. In 1933, he co-founded the Scottish Rock Garden Club which flourishes still. Last year the club presented with him with a silver salver to mark the occasion of his becoming their honorary life president.
After nearly 20 years at Edinburgh, MacKenzie accepted the prestigious post of curator at the Chelsea Physic Garden, at that stage rather a mysterious place, not open to the general public, but shut away behind high walls in the Royal Hospital Road, by the River Thames. He stayed there until he was nearly 70, his retirement coinciding with the tercentenary of the garden in 1973.
Just after the Second World War, when MacKenzie first came to London, the chairman of the committee who ran the garden for the Society of Apothecaries was the great plantsman E.A. Bowles, who gardened at Middleton House, in Enfield. "Bowles was a very wise judge of character," says the distinguished botanist and taxonomist William Stearn, who remembers MacKenzie's arrival at the Physic Garden. "MacKenzie was a first-class curator, very well trained. His sheer competence in all departments of gardening is what I remember best about him. And he was such an agreeable, genial person."
When MacKenzie came to the Physic Garden, it was suffering very much from the neglect of the war years. MacKenzie described how he used to "roll up the chickweed, like rolling up a carpet". But despite this, he found time to serve on various committees for the Royal Horticultural Society and it was they who in 1961 awarded him the Victoria Medal of Honour, the highest accolade available to gardeners. You would have to dig hard to get him to tell you that, for as well as being a great plantsman, Bill MacKenzie was imbued with the rarest of all qualities, modesty.
William Gregor MacKenzie, plantsman, gardener: born Ballimore, Loch Fyne, Argyllshire 14 June 1904; married; died Frimley Green, Surrey 16 October 1995.