Cosgrove, who beat off stiff competition from other reporters for the award, was also shortlisted for the Alan Titchmarsh New Talent Award.
HW's garden industry correspondent and deputy editor Matthew Appleby was shortlisted for the News Story of the Year award and Trade Journalist of the Year.
The Garden Media Guild awards were held at the InterContinental London Park Lane hotel in central London this week (27 November) and recognise the best communicators and media professionals across the gardening and horticultural industries.
The judges thought that Cosgrove’s stories stood out because they were engaging, well-written and demonstrated a good grasp of relevant facts and figures.
They said: "Sarah had clearly put in the hours interviewing subjects and researching her stories, resulting in balanced, informative copy that kept readers - and the judges - reading to the end."
Cosgrove, who started her career in a series of local papers in east and north London, said she was both really surprised and pleased to win the award.
"Gangs, terrorism and endless council meetings don’t really prepare you for people who switch to talking Latin mid-sentence but thankfully everybody in the industry, and at Horticulture Week, has been incredibly kind and generous and helped me find my way.
"Horticulture is a fascinating and vibrant industry with important issues to address and I hope to write more and better stories in the future for HW's readers and help highlight the good work horticulture does."
The Dr David Hessayon Garden Writer of the Year Award went to Alys Fowler while Christopher Brickell won Reference Book of the Year. Other winners included BBC Gardeners' World Magazine which won Garden Publication of the Year, Guy Barter who won Practical Journalist of the Year and Monty Don for his French Garden series which won TV Broadcast of the Year. News Story of the Year went to Ian Hodgson and the New Talent Award went to Nick Turrell.
Stefan Bucazcki won the Lifetime Achievement Award.
The Garden Media Guild works to raise the quality of writing, photography and broadcasting in the gardening and horticultural world and works with professionals across all communication disciplines.