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.


Species Tillage 50% 75% days btw 25 and 75%
Common ragweed No-till 10-May 4-Jun 45
Common ragweed Conv 30-Apr 10-May 20
Giant Foxtail No-till 30-Apr 25-May 35
Giant Foxtail Conv 30-Apr 25-May 35
Common lambsquarter No-till 15-May 4-Jun 35
Common lambsquarter Conv 10-May 30-May 40
Fall panicum No-till 15-May 14-Jun 45
Fall panicum Conv 25-May 30-May 30
Velvetleaf No-till 25-May 14-Jun 40
Velvetleaf Conv 15-May 4-Jun 30
Common cocklebur No-till 29-Jun 3-Aug 55
Common cocklebur Conv 30-May 19-Jun 40
Large crabgrass No-till 15-May 9-Jun 30
Large crabgrass Conv 25-May 4-Jun 25
Jimsonweed No-till 14-Jun 14-Jul 65
Jimsonweed Conv 20-May 4-Jun 25
E. B. nightshade No-till 4-Jun 24-Jun 40
E. B. nightshade Conv 14-Jun 19-Jun 20
Palmer amaranth No-till 14-Jun 9-Jul 55
Palmer amaranth Conv 9-Jun 24-Jun 25
Smooth pigweed No-till 9-Jun 19-Jun 20
Smooth pigweed Conv 30-May 19-Jun 25
Morningglory No-till 24-Jun 29-Jul 50
Morningglory Conv 14-Jun 29-Jul 55

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