M&S displayed a home-grown cut flower exhibit at Chelsea this year and continued to support in-store.
The BPOA and plant hunter Tom Hart Dyke presented the award at the National Plant Show, Stoneleigh, last month.
The BPOA launched the awards in 2014 to recognise excellence and support in growing, retailing and promoting British ornamental plants.
Initially, the idea was introduced to show that a multiple retailer was principally responsible for the large scale promotion of British-grown poinsettia.
This was presented to Sainsburys at the National Plant Show in 2014 for its commitment to using 100 per cent British-grown poinsettias and communicating this to the buying public.