Cushnie studied at Greenmount College of Agriculture & Horticulture, Antrim.
He specialised in glasshouse crops and fruit growing and spent six years with the Northern Ireland Horticultural Advisory Service, then a further seven years controlling landscape maintenance with the Northern Ireland Housing Executive. In 1978 he started his own landscape contracting business, Cushnie Landscapes, working on private gardens and for local authorities.
His broadcasting career started at BBC Radio Ulster. He was a regular panellist on Gardeners' Question Time for 15 years.
Cushnie became a regular columnist for The Daily Telegraph's Saturday gardening supplements. He also wrote for Gardens Illustrated and Gardeners' World, Ireland's Homes Interior & Living and the Belfast News Letter.
His books included Shrubs for the Garden and Trees for the Garden (both 2004); How to Propagate (2006); and How to Prune (2007). He was also co-author of Gardeners' Question Time Plant Chooser (2003).
More recently he had taken the role of the Hedge Man on Radio 2's Chris Evans Show, and also presented Greenmount Garden for BBC One in Northern Ireland.
Other television appearances included BBC One's Room With A View and RTE's Greenfingers.
BBC Radio 4 controller Mark Damazer called him "a brilliantly knowledgeable panellist" who "laced every programme with warmth and joy.
He continued: "John Cushnie was a towering figure on Gardeners' Question Time. His trademark acerbic wit was deployed with terrific timing against a wide variety of plants he did not like — and it was always done with an affectionate twinkle in his eye, with an exuberance of voice and with unrelenting sympathy for fellow gardeners."
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