The Collaborative International Dictionary of English v.0.48:

Welk \Welk\, v. i. [imp. & p. p. Welked; p. pr. & vb. n.
   Welking.] [OE. welken; cf. D. & G. welken to wither, G.
   welk withered, OHG. welc moist. See Welkin, and cf.
   Wilt.]
   To wither; to fade; also, to decay; to decline; to wane.
   [Obs.]
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         When ruddy Ph?bus 'gins to welk in west. --Spenser.
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         The church, that before by insensible degrees welked
         and impaired, now with large steps went down hill
         decaying.                                --Milton.
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