Weed Control for Ontario Sweet Potatoes

Weeds not only provide cover for diseases, insects but also compete for moisture, nutrients, sunlight and growing space needed by crop plant. Integrated Weed Management uses all available weed control strategies in the best possible way to manage weed population. Such strategies include cultural, mechanical and chemical methods of weed control. Anyone who has grown sweet potatoes knows that weed control is a very labour intensive process, with lots of time spent cultivating, both before and after planting. Some Ontario sweet potato growers are able to successfully manage weeds through the season exclusively by cultivating and hoeing. For those growers who chose to use chemicals, three herbicides are currently registered for use on sweet potatoes in Ontario.

Command 360 ME (Clomazone) is registered for use on sweet potatoes and it should be applied post-transplant, but prior to weed emergence, for control of many annual grasses and broadleaf weeds including velvetleaf, lamb's-quarters, lady's thumb, eastern black nightshade, barnyardgrass, green foxtail, yellow foxtail. Command can only be applied to sweet potatoes once per season and should not be applied within 95 days of sweet potato harvest. When applied at recommended rates, Command 360 ME will provide season long weed control. This product is broken down primarily through the activity of soil microbes, and may leave some residues that could affect subsequent crops. The product label lists re-cropping intervals for many horticultural crops after Command. Of particular concern to many sweet potato growers is the potential impact of a spring application of Command on rye cover crops planted in the fall after harvest. There were reports of bleaching and other injuries to fall rye planted after sweet potatoes last season, so keep this in mind if you plan to put in a cover crop this fall.

Dacthal W-75 (Chlorthal Dimethyl) is registered for weed control in Ontario sweet potatoes. Dacthal W-75 should be applied directly over transplants, prior to weed emergence, or as a layby application for up to 6 weeks post-transplant (any weeds present should be weeded and cultivated prior to application of Dacthal). Sensitive weeds include annual grasses and certain broadleaf weeds (lamb's-quarters, purslane, common chickweed, redroot pigweed, carpetweed and groundcherry).

Poast Ultra (Sethoxydim) plus Merge is the only post emergence weed control product registered on sweet potatoes in Ontario. This does not control sedges or broadleaf weeds and does not provide residual weed control. It can be applied for control of actively growing grasses. The time required for complete control is normally 7 to 21 days following treatment, depending on growing conditions and crop competition. Wait until the majority of grasses have emerged, because it does not control grasses that emerge after application. It should not be applied within 30 days of harvest. For rates and further details, refer to product labels or check OMAFRA Publication 75, Guide to Weed Control.

Regardless of the method of weed control you choose, it's a good idea to look out for and keep track of what weeds you're seeing in your fields - in a nutshell to scout for weeds just as you would for insects and diseases too.

Questions about sweet potato pests and their control can be directed to Melanie Filotas, OMAFRA's Integrated Pest Management Specialist, Specialty Crops at 519-426-4434 (email: melanie.filotas@ontario.ca).