Image 1. Horseweed seedlings. Image by Heather Karsten taken at the Penn State Agronomy Research Farm, Centre County PA on 5/8/17.
The last 10 days of cool wet weather is perfect for promoting spring germination of horseweed/marestail in those no-till fields that are about to be planted to soybean. This is very similar to what happened last year in May when we had about a two-week cool wet period which appeared to have triggered this winter/summer annual into a more prolonged germination and emergence period in the spring than in some other years (Image 1).
In 2016, we had horseweed/marestail emerging through the month of May and in many cases after spring burndown treatments had been applied. Fields that did not receive a residual herbicide along with the burndown, had problems with lack of control and few options for postemergence management.
This year is following a similar pattern, so be sure to scout. If you are dealing with fields with a history of marestail, adjust those herbicide programs accordingly. For details on management considerations for marestail, refer to a recent article we posted a couple weeks ago entitled "Manage Horseweed/Marestail Before You Plant Soybeans."