Boverton Nurseries, Lubbe and Sons, The Willows Garden Centre, J Coles and Sons, Hillier Nurseries and Mahood Brothers were all chosen by the Vale of Glamorgan Council, which tendered a supply contract on behalf of The Welsh Purchasing Consortium.
The consortium set up a framework for the geographical area covered by its members, which stretches across the whole of south Wales, and includes Wales' biggest county, Powys.
Mahood Brothers managing director Peter Herbert said he hoped to see the first orders in the autumn, but had no guarantee the councils would buy anything.
"This kind of arrangement is unusual but it makes it easier, especially for the smaller local authorities," he said.
In making its decision, the awarding authority put 60 per cent of the weighting on quality, with price the remaining 40 per cent.