The HTA said: "Since we were given notice of this change in legislation we have been working closely with appropriate bodies to try and mitigate the effects of this change on members.
"To ensure that enough time has been allowed to train staff in this matter, we are pleased to confirm that an amnesty on vouchers with an expiry date of 31 December 2016 has been agreed until 31 August 2017."
Until now the HTA has helped all members redeem National Garden Gift Vouchers no matter how old. But the organisation said that due to this change in circumstances "this situation must now change".
There are potentially a high value of unredeemed vouchers that have been sold in the past, which means organisations had a potential expensive backlog if many or all were redeemed.