The magazine has been published for 139 years.
Future PLC is the publisher. Contributors were told on 11 September.
Editor Garry Coward-Williams said: "It was simply a question of ever-rising costs. Paper went up 20% last year, print costs have risen sharply, the cost of the cover mounted seeds has gone up this year and advertising revenue has dropped significantly in the last six months. It’s a cocktail we could not survive despite good news stand sales performance.
"No one wanted to see the oldest gardening magazine in the world close, least of all the staff and its publishers, who thought very highly of Amateur Gardening. I am sorry for the thousands of loyal readers and our long-standing and much respected contributors, but it’s time to say au revoir.”
Alan Titchmarsh is a former deputy editor, Peter Seabrook was a contributor for 35 years, and Mr Fothergill's is seed supplier for cover mounts.
Titchmarsh said: "I'm sad that AG will soon be no more. I remember reading it in my youth when I built m first greenhouse in our Yorkshire back garden. After training at Kew I entered the world of horticulture journalism, working with Percy Thrower who had been my hero as a boy. I suppose 139 years isn;t a bad run and it will always hold a special place in my affections."
Contributor Toby Buckland said: "Amateur Gardening wasn't simply the oldest garden magazine title in the world it was unique because it spread the gardening word in a friendly conversational way. Writing the back page has been a pleasure and an honour and like all of its many thousands of loyal readers, I'm so sad to see it go."
Writer Val Bradley said: "Sad to see Future are finishing off Amateur Gardening magazine next month after over 100 years. Generations of gardeners learned so much from publications such as this, but the money comes first. They don't care about the customers."
Former news editor Marc Rosenberg said: "Truly sorry to hear the news that Amateur Gardening magazine is to close after 139 years of publication. The end of an era for a piece of British gardening history."
AG editors: Shirley Hibberd (1884–1886), T. W. Sanders (1886–1926), A. J. Macself (1926–1946), Arthur Hellyer (1946–1967), Anthony Huxley (1967–1971), Peter Wood (1971–1985), Jack Kendall (1985–1986), Graham Clarke (1986–1997), Adrian Bishop (1997–2001), Tim Rumball (2001–2017) and Garry Coward-Williams (2017-date).