Short answers, pulled from the story.
Agrostemma is a genus of annual flowering plants in the family Caryophyllaceae, the carnation family. Its members are commonly called corncockles. The best-known species is A. githago, the common corncockle.
A. githago, the common corncockle, is native to Europe. Its center of origin is thought to be the eastern Mediterranean. It has since spread as a weed worldwide through contaminated cereal crop seeds.
Corncockle is declining in Europe because improved seed cleaning has made it easier to remove its seeds from cereal crop stocks before sowing. With that pathway closed, the plant can no longer spread as readily through agricultural fields.
The genus Agrostemma contains two recognized species: Agrostemma githago, the common corncockle, and Agrostemma brachyloba, the narrow corncockle. Agrostemma gracile is treated as a synonym of Agrostemma brachyloba.
Yes. Corncockle is considered an attractive plant and its seeds are commercially available to gardeners. Despite its history as a crop weed, it is cultivated deliberately in garden settings.
A. githago grows as a weed among cereals and other crops. It spread worldwide by having its seeds mixed in with crop seeds, traveling through agricultural trade and farming practices.