His warning came after consultation on the EU Sustainable Use Directive legislation, which cuts the pesticides available to the industry, ended on 5 May.
Adlam said the legislation, which must be implemented by 25 November 2011, would have a big impact. "A reduction in active ingredients is going to be serious."
He added: "There are 12 active ingredients for the whole of the garden centre industry served by retail products. Withdrawing any one of these quickly will seriously effect the variety of control products gardens can use. We have a reliance upon very few insecticides and fungicides in the industry."
He warned that legislation on "micro-distributors"
- small garden centres or plant centres with 10 or fewer staff - could be damaging and needs an exemption because it requires qualified staff to be on hand to advise customers at all times.
HTA president Caroline Owen said: "We're losing products all the time from not re-registering. But we will just have to wait and see what happens."
GAN operations director Simon Edwards added: "There is a problem. I spent last Saturday in one of our garden centres trying to advise customers on what to choose but there is not much to advise them on any more. As a retailer, the ability to offer choice is severely limited."