Henkyou Ryoushu Reijou no Shiroi Kekkon

Hortensia, the daughter of a frontier lord, suddenly receives a marriage proposal. Her troubled marriage partner is Turaibis, the second prince, whose life is in danger from the queen. It was supposed to be a contract marriage to protect Turaibis, but Hortensia gradually becomes attracted to him, as he shows his kindness and interest in the flora and fauna of the frontier. However, Hortensia harbors a secret...

(Source: Kadokawa, translated)