Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
helvĕnācus, a, um, adj. [helvus], pale yellow, yellowish: vitis, a particular kind of wine, Plin. 14, 2, 4, § 32: genus palmitum, Col. 5, 5, 16: vinum, Plin. 23, 1, 24, § 47.
Also: helvĕnācĭus, a, um, adj.: vites, Col. 3, 2, 55.