The waterhemp towering above the crop canopy in many fields is clear evidence of how difficult it is to manage this weed species, said Aaron Hager, associate professor of weed sciences at the University of Illinois.
Female waterhemp plants growing in noncompetitive conditions can produce more than one million seeds per plant (fewer under competitive conditions). “The capacity to produce a large amount of seed helps facilitate the rapid spread of waterhemp infestations, especially when the seeds are scattered by harvesting and tillage equipment,” Hager said.
He added that applying herbicide to reproductive-stage waterhemp plants is usually not recommended because this practice does not significantly affect the amount of seed they produce. “One reason is that the interval between when the female waterhemp flowers are pollinated and when the seeds become mature is very short,” he explained.
U of I researchers examined how long it took female waterhemp plants to produce viable seed after the flowers were pollinated. In the study, female waterhemp plants were pollinated for 24 hours and then separated from the male plants. Branches from female plants were harvested at various intervals after pollination and placed under either warm (86 degrees F) or cold (-4 degrees F) conditions for 48 hours, then stored at room temperature until all harvests were complete.
The researchers then conducted germination tests to determine how soon after pollination the seeds became viable. They found that seeds that were stored for 48 hours under warm conditions became viable 7 to 9 days after pollination; seeds stored under the cold conditions became viable 11 days after pollination.
“The results confirm that female waterhemp plants can produce viable seed very soon after pollination,” Hager said.
Female plants in a particular field probably are not all pollinated at the same time. Thus, total seed production probably occurs over several weeks during late summer, even though individual seeds can mature relatively quickly. When removing scattered, mature waterhemp plants from a field, take along a plastic garbage bag to carry out any female plants with seed, as the seed might already be mature.