I am looking for a novel by Agatha Christie in which she describes a murder with the water of Convallaria majalis (lily of the valley). Does anyone know the title?
Are you sure that it was an Agatha Christie story?
Anne Perry's murder mystery Weighed in the Balance (2010) features a murder using the water of lily-of-the-valley. From this summary:
Princess Gisela (39) is the woman Zorah accuses of killing her husband. Gisela and Frederich were exiled after he renounced the throne to marry Gisela. His family wouldn’t approve of Gisela, and during the trial it is discovered that the reason she wasn’t liked was because she had had a child out of wedlock and tried to have a termination. Then she aborted the baby she expected with Friedrich. In the end Rathbone proves that she did poison her husband with the water of the lilies of the valley she had in the bedroom because she felt trapped in her marriage.
I found this from the comments on this blog post "Q and A: Lily of the Valley Poisoning", but was unable to find any mention of lily-of-the-valley being used as a murder weapon in an Agatha Christie story. Some Christie stories used other common garden flowers as murderous poisons, such as foxglove, which has similarly shaped flowers to lily-of-the-valley but is apparently not closely related.
Lily of the valley was, however, Agatha Christie's favourite flower in real life! Maybe your memory conflated this fact about Christie with the novel by Perry, or with another murder mystery featuring lily of the valley?