Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† pĕristĕrĕon, ōnis, and pĕristĕre-os, i, m., = περιστερεών, vervain, peristereon, Plin. 25, 10, 78, § 126; 25, 9, 59, § 105; App. Herb. 3; Cael. Aur. Acut. 3, 17, 160.