Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
herbŭla, ae, f. dim. [herba-potens], a little herb: cervae perpurgant se quadam herbula, quae seselis dicitur, Cic. N. D. 2, 50, 127; Sen. Vit. Beat. 9; Quint. 1, 3, 5.