He studied at Cambridge Botanic Gardens and became NHS head gardener for hospitals in Wakefield, West Yorkshire, before moving to the National Trust's Powis Castle in Wales.
Hancock was head gardener from 1972 until his retirement in 1996, and received the British Empire Medal for his work as well as an associate of honour dubbing from the RHS.
Michael Walker, Trentham Estate garden and estate manager and a past student at Powis Castle, said: "It was at Wakefield he propagated cuttings of artemisia, which were later registered with the RHS. They were given the now familiar name Artemisia 'Powis Castle'. Gardening has lost a great character."