Have you ever seen pigweed flowering in the spring? In Corpus Christi, Texas, Texas A&M weed scientists observed mature seeds on Palmer amaranth by mid-March this year. This was followed by the the first siting of flower heads on Palmer plants on April 6 in College Station.
This observation highlights the likelihood that Palmer pigweed can add seed to the soil even prior to planting the summer crops in some areas of Texas, and these mature weeds have caused many challenges for obtaining adequate control with the preplant burndown herbicides. It is likely that pigweed seed production has occurred prior to row-crop planting in areas south of College Station in the Upper Gulf Coast, Coastal Bend and Rio Grande Valley regions due to much warmer temperatures. We could not rule out the possibility for pigweed seed production in areas north of College Station as crop planting is usually delayed by few weeks.
This observation points to the need for robust early-season weed management practices implemented prior to planting. Application of effective burn-down or pre-plant incorporated residual herbicides is critical even if plans are there to disk the field prior to crop planting. If herbicide resistance is suspected, the burndown herbicides must include herbicides for which the pigweeds are still susceptible and must be applied prior to pigweeds reaching the 4 to 6 inch growth stage. Continuous wet weather conditions can complicate herbicide application timings. Thus, applications must be made at the earliest possible window to achieve effective control of the pigweeds. If burndown applications did not provide sufficient pigweed control due to resistance, larger growth stages, or other reasons, consider disking the field prior to mature seed production if tillage is an option. Keep in mind that pigweeds only need about two weeks from flowering to mature seed production. Pigweed flowers mature from the bottom of the seedhead upward, so look for dark brown to black seeds at the base of the flower head (see Figure 3). It is imperative to be vigilant in preventing seed production in pigweeds, and the importance of monitoring and managing early-season pigweeds should not be overlooked.
It is unclear whether we are finding biotypes (or portion of a biotype) that have developed the ability to germinate and produce seed early in the season. Further, we are not sure what portion of these seeds can germinate immediately and produce seed again during late summer. Research will be conducted to answer these and other practically relevant questions.