Horticulture head Raoul Curtis-Machin says its research shows £73 million is the potential gain throughout the year – including Easter Sunday - if Parliament voted to extend trading to more than six hours on Sundays and to include Easter Sunday.
He said the HTA was having talks with the Department of Business Innovation and Skills for an exemption for garden centres.
He said for members it's an "archaic law", adding :"It’s a daft law given technology today, where you can sit outside a garden centre and buy whatever you want but you can’t go inside."
However, the HTA is in sympathy with some of the arguments of the family Sunday but say Easter is one of the busiest times of the year and "you can’t really get that trade back".
He said in meetings with MPs many are "gobsmacked and surprised the law still exists".
HTA retail committee chairman Adam Wigglesworth said: "The reality is that we have been frustrated for a long time and it’s not right in the current multicultural world we live in that there is a restriction on Easter Sunday trading.
"We have traded for many years on Easter Sunday so when it came into play in 1994 - staffing levels didn’t change and staff wanted to continue working on Easter Sunday.
"We will be open but we can’t sell to customers, the law doesn’t make sense. We are now 20 years on from when the law was passed and people still don’t understand. We get lots of customers pulling in on Sunday and asking why we are closed.
"The law doesn’t reflect modern day thinking and the public don’t understand it. Lots of people are living different lives to how we used to live 50 years ago and Sunday is regarded as another day of the week for many.
"The law is historical and needs to be reformed."
He said an in-store petition had attracted thousands of signatures and that farm shops and motor shops had exclusions "so the precedent is already set".