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Waterhemp is Flowering (07/21/21)


I was taking notes last week and observed flowering structures on waterhemp plants. Why is flowering important? Because female waterhemp plants produce a tremendous amount of seed and seed remains viable from 4 to 6 years. It has been reported that waterhemp in an open canopy can produce 200,000 to 500,000 seeds per plant.

Researchers at Iowa State University and the University of Illinois examined the number of days after flowering for waterhemp to produce viable seed. In the experiment, waterhemp with multiple branches were pollinated for 24 hours. Branches from pollinated waterhemp plants were harvested and exposed to either warm (86F) or cold (-4F) conditions for 48 hours and then moved to room temperature. Researchers measuring seed germination as an indicator of seed viability determined that seeds from branches exposed to warm treatment were viable 7 to 9 days after pollination whereas seeds from branches exposed to cold treatment were viable 11 days after pollination.

Is it too late to spray waterhemp? We think so. Applying herbicide to waterhemp at the flowering stage is not a recommended practice since herbicides do not reduce waterhemp seed production. We recommend hand-weeding scattered flowering waterhemp, especially large plants in open areas of the field or waterhemp near field borders. Bag and carry waterhemp plants from the field if you are unsure how long plants have been flowering.


Tom Peters

 Extension Sugarbeet Agronomist

NDSU & U of MN