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14 POTATO GROWER | APRIL 2019 TOP 5 Considering Your Herbicide's Length of Residual Sourced from Syngenta Know More, Grow More blog As growers look for advantages over yield-robbing, tough-to-control weeds, there are several factors to consider that will dictate how long a herbicide's residual will last. For more information on weed resistance management and local herbicide recommendations, visit www.ResistanceFighter.com. 1 Amount of Moisture Rainfall or irrigation is critical to activate pre-emergence herbicides. Most herbicides require at least a half-inch of water after application. Try to plan herbicide applications around rain forecasts to help ensure a good activating rainfall for pre- emergence herbicides. It's important to scout fields after either rainfall or irrigation to ensure proper activation has occurred. If escaped weeds are present, make plans to treat these weeds as soon as possible to ensure they don't continue to develop. 2 Weed Type & Density It's critical to identify the problem weeds in your fields to ensure you're using the right products on the right fields, at the right rate and the right time. Large-seeded broadleaf weeds, like giant ragweed, cocklebur and morningglory, are tough to control because they germinate from deep within the soil profile. This makes it important to knock out these weeds early before they grow too large or multiply in your fields. 3 1 Weed & Crop Size Syngenta recommends spraying a post-emergence herbicide when both weeds and crops are small. The tallest weed in a field shouldn't grow taller than 4 inches. Keep in mind, it may only take a few days for weeds to surpass this height and grow too large for post-emergence herbicides to be completely effective. Post-emergence herbicides should also be applied before crops reach canopy, since canopy closure prevents herbicides from being able to penetrate the soil and reach the weeds.