Tropical spiderwort has recently become the most troublesome weed in Georgia cotton. Most of Georgia's cotton is glyphosate resistant (GR), and glyphosate is only marginally effective on tropical spiderwort. An experiment was conducted at four locations to determine tropical spiderwort control in GR cotton by 27 herbicide systems. Treatments consisted of three early-postemergence over-the-top (POT) herbicide options and nine late–postemergence-directed (LPD) options arranged factorially. Glyphosate POT controlled tropical spiderwort only 53% 21 d after treatment (DAT). Glyphosate plus pyrithiobac or S-metolachlor controlled tropical spiderwort 60 and 80%, respectively. Pyrithiobac improved control of emerged spiderwort, whereas S-metolachlor provided residual control. Pooled over POT treatments, glyphosate LPD controlled tropical spiderwort 70% 21 DAT. Dimethipin mixed with glyphosate did not improve control. Carfentrazone, diuron, or flumioxazin mixed with glyphosate LPD improved control 9 to 15%. MSMA and MSMA plus flumioxazin were 8 and 19% more effective than glyphosate LPD. At time of cotton harvest, systems without residual herbicides at LPD controlled tropical spiderwort 42 to 45% compared with 64 to 76% with LPD treatments that included diuron or flumioxazin.

Nomenclature: Carfentrazone; dimethipin; diuron; flumioxazin; glyphosate; MSMA; pyrithiobac; S-metolachlor; tropical spiderwort, Commelina benghalensis L.; cotton, Gossypium hirsutum L. ‘DP 458 B/RR’, ‘FM 989 B/RR’, ‘ST 4793 BR’.

Additional index words: Invasive weed, noxious weed, weed shift.

Abbreviations: DAP, days after planting; DAT, days after treatment; GR, glyphosate resistant; LPD, late postemergence directed; POT, postemergence over-the-top.