What chapters/episode are the Hore Herb & Horedan Flower in To LOVE-Ru?
The Hore Herb (ホレ草) is inferred in To LOVE-Ru volume 5 chapter 40 and the Horedan Flower (ホレダンの花) is from an original scenario on a booklet packaged with The First Edition of To LOVE-Ru Darkness 2nd BD/DVD.

In volume 5 chapter 40 of To LOVE-Ru (凶悪バレンタイン, A Brutal Valentine' Day), 


   when Rito asked Mikado-sensei what happened to the students, she replied that she had told Lala to add an aphrodisiac from a plant from Planet Hore in her chocolate.

 御門: 催淫効果のあるホレ星の薬草を入れるように教えちゃったのよね

 (The original panel in Japanese)


Regarding The First Edition of To LOVE-Ru Darkness 2nd BD/DVD, this is the booklet.



However, there's no review/further information about the content of the booklet, aside from the Japanese Wikipedia putting a footnote for the source of Horedan Flower.