Objectors said the development could damage local town businesses.
Northumberland County Council officers said there would be no "significant adverse impact" and a committee has voted in favour.
The proposal followed the serving of a planning contravention notice on the centre in 2011.
Wyevale Garden Centres said: "We are pleased that the application was approved and look forward to continuing to share our passion for gardens and gardening with the local community".
A previous statement had said: "Heighley Gate, a Wyevale Garden Centre, plays a vibrant part in the Northumberland community, providing employment opportunities, a restaurant, children’s play areas and community events for the whole family throughout the year.
"We attract visitors from as far as Edinburgh and Yorkshire and offer our customers an experience based on the joys of the garden and gardening, which complements the diverse retail options in Morpeth and other nearby communities."