The British Protected Ornamentals Association has criticised Homebase for asking for changes to trading terms with grower suppliers after signing contracts.
BPOA chairman Ian Riggs said Homebase and Argos asking for two per cent extra rebates and to extend payment dates went against the NFU Code of Good Trading Practice, which was published in October 2013 and designed to help growers negotiate with retailers:
He said: "Regrettably the reality is this arbitrary action can be imposed without proper discussion or negotiation with grower suppliers, as contract prices have already been agreed. For most growers this will effectively hit as an additional retrospective rebate.
"Adopting the principles of the NFU/BPOA code of good trading practice for flowers and plants would ensure that such measures would need prior-agreement between all parties and would ensure that declining to implement such measures would not result in impacts on existing commitments.
"This case is a compelling reason for growers to work with the BPOA and NFU to ensure the principles of the code are adopted."
Homebase and Argos, which are owned by Home Retail Group (HRG), have sent letters asking for a rebate for orders placed on and from March 1.
HRG also want to "take this opportunity to discuss and agree with you proposed changes to the existing payment terms".
The letters say from 1 March 2014, "we will be amending our standard trading terms to include an additional 2pc turnover rebate for orders placed on or after that date".
A Home Retail Group representative said: "Our suppliers are very important to us. Payment is made in accordance with pre-agreed terms and reflects the markets in which we and our suppliers operate.
"Details of these agreements are commercially sensitive and therefore will not be disclosed."