Droughtwort was an herb used in the Emelanese Universe. It causes any man who eats it is rendered sterile for several days. 
Briar uses it to avoid fathering children who might end up with the same childhood he had.
Reference and Notes
- ↑ 1.0 1.1 The Will of the Empress, Cp. 4 (p. 61; Scholastic ebook)