Coupe, currently Waitrose flowers buyer, will take up the role when Sue Steptoe on retires on 2 December. Steptoe oversaw the relaunch of the Waitrose horticulture offer in April. Sales have increased by 16 per cent in the year to date.
Victoria Mason will become garden plant buyer and Allison Burdett flower buyer from 2 December.
In April, Waitrose launched 6,000 gardening products and 49 outdoor garden retail spaces, which will more than double in 2014.
Waitrose chairman Charlie Mayfield said: "A particularly exciting development has been the introduction of our gardening range with new brand ambassador Alan Titchmarsh. Horticulture is proving particularly popular with Waitrose customers."