The horticulturalist, who shares three grown up children, Tom, Adam, and Freya, with his wife Sarah Don, announced the happy news.
Monty and Sarah were already proud grandparents to their grandson George, and they have now welcomed a granddaughter into the family.
Fans rushed to the comments section of Monty’s tweet to congratulate him on the announcement.
Heather Sharpe penned: “Aww congratulations grandad, love the name Daisy. Just so happens, daisies are one of my favourite flowers, such a pretty, simple little flower xx.”