Controlling weeds in sweet potatoes and many crops is so important because they compete for nutrients, sunlight and water that can make sweet potato plants grow, according to Tara Smith, an Extension specialist with Louisiana State University. Also, too many weeds can affect quality and yield negatively.
Growers are wise to follow an “integrated approach” when focusing on weed management in sweet potatoes, according to the Ohio State University Vegetable Production Guide. Part of that integrated approach involves healthy growth, controlling existing and newly emerging weeds, and an effective herbicide control program. The guide notes that weeds are most troublesome early in the production season just after transplanting. So growers are encouraged to establish a preplanned approach or weed control program. That program contains the ability to properly identify the weed species present in fields; the importance of those species; their life cycle in each field; whether they are annuals or perennials since the control approach may be different; the soil characteristics that makes those weeds grow; whether the field is prone to soil erosion; the weed control tools available to combat any weeds of concern; and what crops you intend on raising in the future in that same field.
Smith advises watching out for perennial weeds such as johnsongrass, bermudagrass, alligator weed, and yellow and purple nutsedge, as these require yearly control. Cultivating in the fall and applying herbicides can help control these weeds, but Smith warned that growers should keep an eye out for any perennial weed regrowth.
Some other major weeds affecting sweet potatoes include pigweed, common cocklebur, common lambsquarter, common ragweed and Pennsylvania smartweed, according to Jonathan Schultheis, an Extension specialist in the Department of Horticultural Science at North Carolina State University (NCSU), and Kenneth Sorensen, an Extension specialist in the Department of Entomology at NCSU, who both contribute to http://ipmcenters.org.
Before growers plant and if weeds are actively growing, Smith said they can apply something like Paraquat as a preplant burndown. She recommends applying thoroughly with good ground coverage for effective control of weeds.
Growers can additionally apply a glyphosate as a preplant treatment when controlling alligator weeds, pigweeds, ground cherry and any grass species. For better glyphosate control, Smith recommends waiting seven days before cultivating.
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For chemical weed control, check with your local Extension agent to see which herbicides are available and are registered in your area. Smith said that properly identifying weed populations, soil type, the vigor of the sweet potato crop and weeds present, as well as environmental conditions can all play a role in how effective the herbicides will be. When choosing a herbicide, she said growers should keep in mind the possible crop injury (if any) that it can cause, as well as any preharvest application intervals. Read the label for proper rates and don’t exceed recommended rates.
Schultheis and Sorensen both said weeds are especially a concern in sweet potato plant beds because they cut down on the amount of plants and reduce plant weight. Herbicides can rescue annual grasses, and there is always hand weeding for the brave at heart. Broadleaf weeds, conversely, are more difficult to control.
In plant beds, Schultheis and Sorensen said growers can use Devrinol for preemergence control of annual grasses and small-seeded broadleaf weeds. For postemergence control of annual and perennial grasses in plant beds, growers can use Fusilade, Poast and Select.
Smith said some effective soil-applied herbicides include a clomazone, such as Command 3 ME; a flumioxazin, such as Valor SX; a s-metolachlor, such as Dual Magnum; and a carfentrazone-ethyl, such as Aim EC.
Outlook works well for annual grasses and small-seeded broadleaf weeds, according to Schultheis and Sorensen. For yellow and purple nutsedge control, they said Eptam is effective.
Low temperatures can cause sweet potato plants to grow slower than warmer temperatures, Smith said. Lower temperatures coupled with slower growth as well as excessive rainfall can cause herbicide injury.
She said that herbicides work best on weeds when the weeds are smaller and just growing. Control also is more effective when incorporating herbicides properly and receiving about a 1/2 inch or so of rainfall, or irrigating if necessary within two to seven days of application. Unfortunately, as mentioned earlier, excessive rainfall can reduce the effectiveness and cause leaching of the soil and herbicides.
Effective postemergence herbicides for annual grass and perennial grass control include a fluazifop, such as Fusilade; a sethoxydim, such as Poast; and a clethodim, such as Select, Smith said.
Growers also can cultivate in a timely manner and rotate with other crops to help eliminate or reduce weeds, she said. Choosing a field free of weeds or avoiding fields with a history of weed populations is recommended. She reminds growers that after sweet potato root systems are well established the vines can overshadow competing weeds and help reduce the weeds’ growth.
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