Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
herbesco, ĕre, v. inch. n. [id.], to grow into green stalks or blades: elicere herbescentem viriditatem, Cic. de Sen. 15, 51: campi semper herbescunt, Amm. 31, 2, 19.