"The buying patterns of garden centres are getting stupid to some extent. You have the likes of Suttons and Thompson & Morgan (T&M), which contract big buying groups," said Ward.
"Their contracts with retailers state they can't sell seed products produced by others. So what is happening is Joe Public isn't being offered the wide choice that should be available to them.
"Because T&M and the Suttons range is very similar and it's often the only option for the consumer, the market as a whole has seen mail-order and online sales increase, so in fact these contracted buying groups like Tillington are actually slowing up sales for garden centres and halting diversity."
Tillington did not comment but other garden centres contacted suggested that garden centres generally limited numbers of products stocked for space reasons. They said that they did not believe they were losing sales.