The Collaborative International Dictionary of English v.0.48:

Knawel \Knaw"el\ (n[add]"[e^]l), n. [Akin to G. knauelk,
   kn[aum]uel, prop., a ball of thread, coil. Cf. Clew.]
   (Bot.)
   A low, spreading weed (Scleranthus annuus), common in sandy
   soil.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Scleranthus annuus
    n 1: widely distributed low-growing Eurasian herb having narrow
         leaves and inconspicuous green flowers [syn: knawel,
         knawe, Scleranthus annuus]