Integrated Systems of Weed Management in Organic ‘Vidalia®' Onion

Trials were conducted in southeastern Georgia to develop integrated systems of weed management in organic Vidalia® onion.  Treatments were a factorial arrangement of summer solarization, cultivation, and herbicides appropriate for use in certified organic production systems.  Plots were solarized with clear plastic during the summer months prior to transplanting onion, with the mulch removed in October.  Cultivation treatments used a tine weeder, with cultivation twice at 2-wk intervals, four times at 2-wk intervals, and non-cultivated.  Herbicides were clove oil plus vinegar, clove oil plus a petroleum oil insecticide used as an adjuvant, and a non-treated control.  Onion were transplanted in December of 2007 and 2008, with cultivation and herbicide application events occurring the following January and February both seasons.  Onion were harvested and graded in April each year.  Sub-samples of onion bulbs were collected from each plot and stored in a controlled atmosphere storage facility at the University of Georgia Tifton Campus.  After 120 days, the sub-samples were rated for fungal and bacterial diseases that developed during storage.  Weeds present in the trials were cutleaf eveningprimrose and swinecress.  Preplant solarization during the previous summer did not control the cool-season weeds present in these trials, along with no effect on onion yield.  It is theorized that dormant, cool-season weed seed are immune from solarization during the previous summer.  Cultivation with a tine weeder twice at two-week intervals was equally effective in controlling cutleaf eveningprimrose and swinecress as four cultivations, with both cultivation regimes increasing onion yield compared to the non-cultivated control.  Cultivation with a tine weeder neither damaged nor bruised onion bulbs sufficiently to increase incidence of diseases in stored onion.  Clove oil herbicide treatments provided minimal weed control and had no effect on onion yield.  Previous experience with clove oil showed contact activity on warm-season dicot weeds, under summer conditions.  This was not the case with cool-season weeds under winter conditions, despite efforts to apply the clove oil herbicide treatments during periods of mild temperature in January and February.  These data show that cultivation with a tine weeder effectively controlled weeds in transplanted onion and protected yield without the need for handweeding.  Cultivation with the tine weeder did not damage the onion bulbs and there was no increase in diseases of stored onion.  Cultivation with a tine weeder can be immediately adapted by organic growers for use in transplanted onion.