Definition of buttonweed:
(n) :
Any of numerous species of plants with button-like flowers, especially:
(n) :
Any of several plants of the genera Diodia and Hexasepalum (syn. Diodella), of the madder family.
(n) :
False buttonweed (Spermacoce spp.).
(n) :
Certain knapweeds (Centaurea spp.).
(n) :
Velvetleaf, Indian mallow (Abutilon theophrasti).
(n) :
Clustered bluet (Oldenlandia uniflora).