Don was the programme's lead presenter from 2003 to May 2008, when he stepped down after a minor stroke. He will present the programme from his own garden in Herefordshire.
Broadcaster and Buckingham Garden Centre representative Chris Day said: "That's a bit of a bolt from the blue. I thought Gardeners' World was getting back on track this year and Toby Buckland was doing a much better job of presenting. I'm uneasy at Monty's return - many people simply don't get him. The jury is out on this one for me."
Don will be joined by Carol Klein, Joe Swift and Rachel De Thame. Viewing figures fell from five-million to two-million under Don but were under 1.4 million at times this year. The HTA said it received an "underwhelmed" response to a survey asking members about the return.