The ban costs garden centres an estimated £10m a year in lost sales on what could be one of the busiest days of the season.
GIMA director Neil Gow said: "Trade in your outdoor area and café where your layout allows. You can do it legally and you will get as many people using those areas as a normal day plus some extra spend on plants because people can't buy houseplants or cards or fertiliser.
"The law is ridiculous and an infringement of people's liberties. A few misguided individuals spoil it for everyone else and successive governments have not sorted it out."
Retailers are not allowed to sell from a shop building more than 280sqm on Easter Sunday (or Christmas Day). Scotland has no such laws.
HTA marketing director Andrew Maxted said: "The legislation only effects covered areas so we encourage retailers to consider opening cafes and outdoor areas."
He added: "The legislation is an issue for many gardeners. It is 15 years since the Sunday Trading Act and consumers still don't understand why garden centres should be closed on Easter Sunday."
HTA is hoping to tack on legislation to deregulation or devolution bills to make Sunday trading the responsibility of council planning and trading standards officers rather than that of the Government.