Flanagan was the Crown Estate's keeper of the gardens in Windsor Great Park, the Savill Garden, the Valley Gardens and the private garden at Frogmore House. He died aged 56 in October 2015 and last week became the first person to be awarded a posthumous RHS Victoria Medal of Honour (VMH).
Speaking at the RHS awards ceremony at Lindley Hall, Kirkham said: "Everything he did was perfect. He was the best plantsman of this generation - sorry Roy (Lancaster) and Jim (Gardiner). He could identify any plant even in a moving vehicle at 30 or 40mph in temperate Asia."
Kirkham said the VMH, which only 63 people hold at any one time, was "a fitting tribute". Flanagan worked at Manchester parks, gained an honours diploma in horticulture from Royal Botanic Garden Edinburgh and a master of horticulture from the RHS. Prior to joining The Crown Estate he spent 10 years working for RBG, Kew, primarily based at Wakehurst Place.
He travelled extensively in search of hardy plants, with visits to Turkey, Russia, various countries in eastern Asia - Japan, South Korea, Taiwan, and China - as well as the USA and western Canada. He lectured widely, contributed regular articles to horticultural journals and jointly authored two books on plant collecting.
Flanagan was chair of the RHS woody plant committee, a trustee of the Chelsea Physic Garden and a member of the gardens panel of the Sir Harold Hillier Gardens & Arboretum. Orchid expert Johan Hermans was also awarded a VMH.
- For full details of the awards, see www.HorticultureWeek.co.uk/.