Powell's role as creative director will be to work alongside the event's founder and event organisers Live Industry, to oversee the show's design, its look, feel and personality, and collaborations with exhibitors, brands and the public.
She commented: "This is a wonderful opportunity to create an engaging, fun, different show - a jamboree of all things gardens and gardening. Set to be London's local garden show where families and friends can meet, eat and grow, whether you're a keen gardener, grow your own, or are an armchair supporter of a gardening devotee there'll be something for you to enjoy.
"I look forward to creating a festival of gardening which truly inspires our capital's residents to revel in all that the great outdoors has to offer."
The four-day show will be set in the Georgian country estate Osterley House and Park, with a blend of show gardens, floral displays and growing competitions along with live entertainment, music and art installations.
Sam Speaight, managing director of Live Industry, said: "We are delighted to have Ann-Marie on-board as part of the growing London Flower Show team.
"With her unrivalled knowledge and experience in designing and creating incredible gardens for private clients, companies and charities, her enthusiasm for inspiring the British public about gardening through her books, TV appearances and articles in national newspapers and magazines, Ann-Marie is the perfect candidate to help us to build an exciting, unique, and inspiring program for this year's London Flower Show."