From The Collaborative International Dictionary of English v.0.48:
Whiten \Whit"en\, v. i. [imp. & p. p. Whitened; p. pr. & vb.
n. Whitening.] [OE. whitenen; cf. Icel. hv[imac]tna.]
To grow white; to turn or become white or whiter; as, the
hair whitens with age; the sea whitens with foam; the trees
in spring whiten with blossoms.