Neiburg supplied herbs to Golden Acres Nurseries and Podington Nurseries before Wyevale Garden Centres bought them in December 2014.
He said: "Both were swallowed up last year. Staff there told us they wanted them to remain true to themselves, that they were very much plants centres and they could continue using their independent suppliers. But we only had one decent order and one smaller once after takeover and within a couple of months we didn't hear anything more thereafter. From my experience they don't allow new centres to source plants from established suppliers."
Neiburg said he was then told he had to set up as a Wyevale supplier to get paid.
He added: "As a niche herb supplier, if they wanted us to supply them now, we'd say no because they'd want large production for not a lot of money."
Other suppliers say they hope to keep contracts with Wyevale-bought centres and Wyevale has promised them they will buy plants for a year at least.