Last month Horticulture Week revealed that business minister Sajid Javid would make an amendment to the Enterprise Bill to force through extra Sunday hours.
He expects the legislation to pass through Parliament to devolve to the decision on extending Sunday trading in large shops to local authorities.
Bellenden chief executive Mark Glover told Horticulture Week's Garden Retail Summit 2016, held at the Connaught Rooms in London, the Bill must be heard by 25 February.
He said: "If you want extended Sunday Tradng there has never been a more important time to write to your MP about these issues."
Glover said George Osborne and David Cameron support Javid's aim to drive the competition agenda and they are all feeling confident they have the vote in the House of Commons. He said English Votes for English Laws rules are likely to be implemented by the Speaker to see off opposition from SNP MPs.
Glover suggested with LibDem support, a Conservative majority would be 100, minus 20 Keep Sunday special Tories, making a 60-vote majority.
However, Glover said the vote could be a "close thing" as Usdaw and some Tory and Labour MPs were against.
He said garden centres could have staff write to MPs and they could also lobby councils to make a decision on the issue, which would probably be decided in Parliament by the summer recess after a battle with bishops in the Lords and would then be for councils to decide on individually this autumn.
He said Lord Taylor of Holbeach's chief whip role in the Lords would also be important with the Bill.