Now is the time to find new weed infestations in fields. To be successful, however, weeds must be identified when they are seedlings. There are numerous sources of information help identify seedling weeds, however, Iowa State University Extension publications such as IPM-44 and IPM-46 are excellent resources about two important weeds, woolly cupgrass and common waterhemp. Another good resource for pictures of weed seedlings is ISU Extension's Weed Management home page at http://extension.agron.iastate.edu/extweeds/ 
Why do new weeds need to be identified now? The best time to identify new weed infestations is at the seedling stage, despite the inherent difficulty of seedling identification. Fields are readily accessible and with planting just finished, growers are more likely to closely inspect individual fields. More importantly, if a new weed infestation is discovered, a seedling typically is easier to control than an adult plant. Growers have more options available and more time in which to accomplish control, and seedling weeds respond to mechanical control better than larger weeds.
Where are new weeds found? A new weed infestation generally is found in a limited area, thus management can be accomplished without great expense. These areas usually are near field entrances, along fence rows and terraces, and in ditches or where waterways connect to other fields. The most common sources of new weed species are harvesting equipment, tillage implements, hay, and runoff water from other fields. Inspect these areas early in the season, and identify seedling weeds.
If you are having problems with identification, samples can be sent to Weed Science Extension, 2104 Agronomy Hall, Iowa State University, Ames, IA 50011. Wrap samples in newspaper or paper towels. Do not wet the paper or towels because samples will rot quickly. Samples should be mailed early in the week.
This article originally appeared on page 74 of the IC-478(10) -- May 26, 1997 issue.