Caerphilly Garden Centre in South Glamorgan and Sunnydale Garden Centre in Cwmbran have joined the group, which includes garden centres from Gloucester and the South West.
Steve Searle, storemanager at Monkton Elm Garden & Pet Centre in Somerset, which is part of the group, said: "The group has been working together for more than 15 years and has built good relationships with the best suppliers in the garden industry.
"We felt it was the right time to increase our presence in other parts of the country and we're thrilled to have Caerphilly Garden Centre and Sunnydale Garden Centre come on board. The two centres fit well with our geographical area and will strengthen our relationship with suppliers."
The Great Western Buying Group is a group of garden centres who join together to share best practice in buying and marketing.
The group is made up of independent, family run garden centres and is currently looking for new members to join.
Phil Taylor, managing director at Caerphilly Garden Centre and Sunnydale Garden Centre, said: "We are very pleased to have become part of such a prestigious and well organised buying group. A number of options were open to us but we felt that the Great Western Buying Group gave us the best fit for our centres.
"Combining our experience and skills in the market we feel that we can benefit from and positively contribute to its continued success.
"It is an exciting time for us, the group and suppliers alike, providing us all with real potential to do more business and build on and develop exciting and new relationships."
The members of the group include Monkton Elm Garden & Pet Centre in Somerset, Highfield Garden Centre in Gloucester, Exeter's Bernaville Garden Centre, Plymouth Garden Centre, Castle Gardens in Sherborne, Dorset, Brimsmore Garden Centre in Yeovil and Poundbury Garden Centre, also in Dorset.