Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sēmentīvus, a, um, adj. [sementis], of or belonging to seed or sowing: feriae, that occur at seed-time, Varr. R. R. 1, 2, 1; cf. id. L. L. 6, § 26 Müll.; Macr. S. 1, 16; Fest. p. 337 Müll.; called also sementiva dies, Ov. F. 1, 658: pira, perh., that are not ripe till late in autumn, late pears, frost-pears, Cato, R. R. 7, 3; Varr. R. R. 1, 59, 3; Plin. 15, 15, 16, § 56: pirum sementivum serum, Cloat. ap. Macr. S. 2, 15 fin.: genera frumenti, that are sown in autumn (opp. verna): sementiva autem (appellant) triticum, hordeum, fabam, Plin. 18, 7, 10, § 50: faenum, proper for sowing, Tert. Jejun. 4.