Steinkamp’s strategy worked well in 2012, despite just 0.15 in. of June rainfall for herbicide activation — with one exception. “One field gave us fits, and that was a field where we deviated from the plan.”

About a month after planting, the field looked clean enough to skip the postemerge application, “but that wasn’t the case.” By the time Steinkamp realized his mistake, “the Palmer was knee-high” — much too big to kill with any herbicide. So out came the hoes again.

Postemerge herbicides work best when Palmer plants are no more than 3 inches tall, Sprague says. That makes application timing a challenge, because the weed grows so fast. In 2012, Palmer amaranth in Sprague’s research plots “grew from 3 to 7 inches in fewer than 5 days.”

postemergence herbicide application options to control Palmer amaranth

The good news is that “in the Midwest, we can control Palmer amaranth in soybeans and corn with the herbicides we have available today,” Steinkamp says — “as long as we have a good plan and stick to it.”