Issue No. 20, Article 3/August 24, 2012
From Pollination to Seed Maturation of Waterhemp
This time of year provides an opportunity to assess the success of waterhemp in thwarting our attempts to control it with various herbicides. In many fields across the state, soybean fields in particular, infestations of waterhemp towering above the crop canopy reflect how challenging successful management can be. The amount of seed produced by female waterhemp plants can vary greatly; published research has demonstrated that individual female waterhemp plants growing in noncompetitive conditions are capable of producing in excess of one million seeds per plant, although the actual amount of seeds produced when waterhemp is growing under competitive conditions is less. Regardless, the capacity to produce large amounts of seed helps facilitate the rapid spread of waterhemp infestations, especially when the seeds are scattered by harvesting and tillage equipment.
Mature waterhemp plants infesting a soybean field.
The dark green areas of seedling waterhemp indicate the path of harvesting equipment during the previous harvest.
We are often asked if applying a herbicide to large waterhemp plants would reduce the amount of seed they produce. We do not recommend this practice, for myriad reasons. One in particular is that the interval between when female waterhemp flowers are pollinated and the seeds become mature is shorter than some might realize.
Research conducted at the University of Illinois by Michael Bell, a weed science graduate student under the direction of Dr. Pat Tranel, examined how long it took female waterhemp plants to produce viable seed after flowers were pollinated. Female waterhemp plants were pollinated for 24 hours, then separated from the male plants. Two branches were harvested from female plants at various intervals after pollination; these branches were placed under either warm (86°F or 30°C) or cold (-4°F or -20°C) conditions for 48 hours, then stored at room temperature until all harvests were complete. Germination tests were then conducted to determine how soon after pollination the seeds became viable.
Seeds that were stored for 48 hours under the warm conditions became viable just 7 to 9 days after pollination, compared with 11 days for the cold-stored seeds. The color of the seed coat changed from white to black in about 12 days.
These results illustrate how quickly female waterhemp plants can produce viable seed after pollination. It's unlikely that all female plants in a field are pollinated at an identical time, so total seed production in any particular field likely occurs over several weeks during late summer, even though maturation of individual seeds can occur relatively quickly. So if you have a field with only a few scattered mature waterhemp plants and you decide to physically remove them, you might want to take along a plastic garbage bag to carry out any female plants with seed, as the seed might already be mature.--Aaron Hager
Reference: Bell, Michael S., and Patrick J. Tranel. 2010. Time requirement from pollination to seed maturation in waterhemp (Amaranthus tuberculatus). Weed Science 58(2):167-173.