Mary Delafontaine (How Does Your Garden Grow?)

In the short story How Does Your Garden Grow? Mary Delafontaine was the niece of Amelia Barrowby. She and her husband Henry had been embezzling her aunt's money, investing her securities and making profits for themselves. They later learned that Amelia had found out about it and had engaged Poirot to investigate. They also learned that Amelia had made a will leaving most of her wealth and especially the house and garden to her companion Katrina Rieger. She therefore murdered her aunt before Poirot could come to see her.

At the end of the story, Mary confessed that she and her husband had murdered her aunt with Strychnine hidden in oysters.

Portrayals
Mary Delafontaine was played by Anne Stallybrass in the film adaptation of How Does Your Garden Grow? in Series 3 of ITV's Agatha Christie's Poirot. She was portrayed as the one who took the more active role in plotting the poisoning. Her husband was essentially an alcoholic and quite passive in the process.