Weed control in onions can be challenging. Onions have various challenges that make weed control difficult. They grow slowly early in the crop cycle and never achieve a good competitive crop canopy to suppress weeds. Also, given the high density plantings, it is not possible to effectively cultivate the beds. In addition, given the close spacing between plants, it is difficult to hoe out weeds without risking damage to the onion stand. In conventional onion production there are a number of materials that can provide excellent weed control (see Figure 1).  The timing of application of the materials can be tricky because some weeds may grow quickly and be difficult to control by the time the onions reach the 2^nd true leaf stage. That is why it is critical to plant the onions in a block with low weed pressure or to have an effective preemergent herbicide program.  Fortunately, onions have effective postemergent materials that can provide control of escaped weeds.

Yellow nutsedge is a particularly difficult to control weed in onion production. In 2012 we conducted a weed control trial on a site with a high population of yellow nutsedge, as well as other weeds such as purslane, lambsquarter, annual grasses, and mustards; the site provided an opportunity to evaluate the effectiveness and safety of various pre and post emergent materials for use on dry bulb onions. Materials that were tested included an untreated control and a standard program (Dacthal PRE followed by Goal Tender + Buctril POST). This standard treatment does not control yellow nutsedge, and so the standard treatment for controlling yellow nutsedge in onions was included: Dacthal, Goal and Buctril were followed by burning the yellow nutsedge with 7-7-0-7 acid fertilizer and Outlook. These materials were compared with Zeus (sulfentrazone, applied both PRE and POST) which is currently not registered for use on onions, but is of interest to us because it has activity in controlling yellow nutsedge.  

Table 1 shows the results on one weed evaluation date and the yield evaluation. There was a high population of weeds at this site and it can be seen that the standard Dacthal followed by Goal Tender + Buctril treatment provided excellent weed. The post application of Zeus at 3.0 oz/A reduced nutsedge to a significant degree, but was too phytotoxic and reduced the yield of onions.  However, the preemergent application of Zeus at 2.0 oz/A was safe to the onions and this use pattern will be further researched in this year's trials. The 7-7-0-7 acid fertilizer followed by Outlook provided the best control of all weeds including yellow nutsedge. The Outlook treatment provided an opportunity to measure the impact of yellow nutsedge on the yield of onions in this trial. The Outlook treatment yielded 7.8 tons/A more than the standard Dacthal followed by Goal Tender + Buctril weed control program.

223 hours per acre were needed to weed onions in the untreated control which is clearly uneconomical. All of the weed control strategies tested in this trial show a dramatic reduction in the time to hand weed onions. Economical onion production clearly depends on good weed control to keep costs down to safeguard yields. For complete results of this trial go to: http://cemonterey.ucanr.edu/Vegetable_Crops/Weed_Reports/