Steer, who works at Madingley Hall University Institute of Continuing Education, beat RHS Wisley student Mia Laucht after the competition went down to the last three questions.
Third was Warwickshire College Pershore's Douglas Mackey. RBG Edinburgh's Frances Keeton, Reaseheath College's Kathryn Owen, Askham Bryan worker Aaron Hickman, Glasnevin's Michael Beckett and Bicton College's Rowanna Shorney also reached the final, which Kew hosted.
Among guests were Lord and Lady Taylor of Holbeach and Kew director Professor Stephen Hopper. Sponsors included Shropshire Horticultural Society/Percy Thrower Trust, MorePeople, HDC and Bulldog Tools.