Weed Emergence Timing in Delaware

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Many environmental factors influence when weeds start and stop germination, and these environmental cues occur on the micro-scale. As a result, it requires large data sets to accurately predicted emergence. Our data was collected in the same field at the Carvel Center in Georgetown, DE and may not accurately predict all sites, but does provide a general understanding of weed emergence.  We have been monitoring the emergence of twelve common summer annual weeds since 2018. We have tilled half the plots in mid-April and the others are not tilled. The plots were counted every 10 to 14 days from early April until the first of September to calculate cumulative emergence. The table below is the average date that 50% and 75% emergence has been achieved. Those species that germinate later in the summer or have a longer germination period often require a postemergence herbicide application that includes a residual herbicide. In our list, this includes annual morning glory, common cocklebur, Palmer amaranth, and jimsonweed. In general, tillage had little impact on when germination begins. However, there was a longer period of germination in no-till compared to conventional tillage.

Annual morning glory in our area is difficult to control and a big reason is the germination pattern. It emerges later than most species and has one of the longest germination periods.

Better understanding of weed germination can help improve overall weed control, particularly knowing which species may emerge later. However, the take home lesson is not to wait to control these late emerging species, rather knowing that longer-season crops that do not shade the ground may need additional cultivation or longer lasting herbicide for full-season control.

Days between 25 and 75% emergence is calculated to evaluate how long germination occurs for the species.