Grzegorz Krystian Pieton, of Terrington St Clement, was killed in July 2010, when the trailer he was towing touched an overhead power line.
It happened while he was working at Belmont Nursery in the village.
The jury at Norwich Crown Court cleared the nursery's operator PS & JE Ward of manslaughter, but found it guilty of a breach of health and safety law.
The firm, which is based in King's Lynn, is due to be sentenced on 6 June for "breaching general duty to an employee".
Judge Stephen Holt adjourned sentencing until June 6 and indicated it would a be financial penalty.
Peter Ward began Belmont in the 1960s. Now the company produces up to 24 million tulips per year, plus 33 million other cut flowers including daffodils, gladioli, lilies, stocks and Sweet William.