Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
grandesco, ĕre, v. inch. n. [grandis], to become great, to grow (poet. and in postAug. prose): quicque sua de materia grandescere alique (preceded by crescere), Lucr. 1, 191; cf. id. 2, 1122; 1160: lentiscus triplici solita grandescere fetu, Cic. poët. Div. 1, 9, 15: grandescunt frumenta, Col. 2, 20, 2: hordeum, Pall. Jun. 2: lignum intus, Plin. 15, 3, 3, § 10.