The weed specialists found that giant ragweed was the first weed to appear in late March or early April. Two to seven days later sunflower emerged, followed over the next six weeks by velvetleaf, woolly cupgrass, giant foxtail, waterhemp, morningglory, cocklebur and fall panicum. Emergence was measured throughout the growing season. But peak emergence varies greatly among species. Soil temperature determines the peak for early-emerging weeds, while rainfall drives the peak for species that emerge later in the season.
Some weeds have a tight emergence pattern, while others are more prolonged.
Variability in emergence patterns only complicates your herbicide application decisions, admits Hartzler. It may also make certain weed-management programs unsuitable. It's simply asking too much of a single herbicide application to control weeds from late March into July. Software predicts emergence Frank Forcella, research agronomist for the USDA Agricultural Research Service, agrees. He has taken Hartzler's research one step further by developing a software program to help farmers forecast weed seedling emergence and growth.