Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
fermentesco, ĕre, v. inch. n. [fermentum] (lit., to ferment, to rise in fermenting; transf., of the earth), to swell, rise, become loose (Plinian): tellus quoque illo modo (i. e. ab nive diutius sedente) fermentescit, Plin. 17, 2, 2, § 15; 28, 8, 28, § 109.