Sinclair is to invoice Solus, Stax and Decco customers directly from 1 October rather than through the wholesalers.
The wholesalers have indicated that they may no longer stock Sinclair products if they do not receive their cut of the invoice.
Sinclair says third party invoicing is no longer realistic in such a competitive market.
Sinclair will send a letter to the trade on 5 August saying garden centres must set up a direct account.
Retail managing director Danny Adamson said margins were so tight in the growing media industry that Sinclair was no longer going to allow wholesalers to take a cut on invoicing: "We could lose some business but this is the right thing to do."