Knowing When to Look for Weeds

April 21, 2000
Enclosed in this week's Bulletin is an 8 1/2" x 11" glossy bulletin entitled Weed Emergence Sequences. This bulletin will help focus attention on when different weed species emerge throughout the growing season and the duration of their emergence. This bulletin can also be used as an identification guide of 16 common weed species in the Midwest. Each weed species is characterized by an emergence date relative to growing degree-days as well as the length of time (weeks) that each of these species emerges.

You may ask, "Why is it important to know when these different weed species emerge?" Knowing when a species emerges and the length of time that it emerges helps you make good decisions on how to manage that particular weed species. A good example is waterhemp. In looking at the Weed Emergence Sequences bulletin we see that waterhemp is one of the later-emerging species. It is characterized to emerge after corn emergence and when there are more than 350 growing degree-days (GDD). It can also be characterized as emerging over a prolonged period of time (8 to 10 weeks). Knowing these characteristics about waterhemp can help refine management decisions. With its later emergence date, applying a soil-applied herbicide closer to planting can often extend the control of waterhemp later into the growing season compared with applying the same herbicide several weeks prior to planting. Also, understanding that waterhemp emerges over a prolonged period during the season provides an opportunity to select a sequential program to improve overall control. Frequently, a timely postemergence herbicide application and/or cultivation helps control later-emerging waterhemp that may have escaped the residual activity of a soil-applied herbicide. Another good example is eastern black nightshade. Nightshade is a weed that many producers often don't realize they have until the soybeans start to mature and drop leaves. Knowing when to look for this weed species may help producers plan a timely postemergence herbicide application to control this troublesome species.

So where are we currently in regards to weed emergence in Illinois? Over the past couple of weeks we have seen the emergence of many of the Group 1 weed species, including giant ragweed, common lambsquarters, and Pennsylvania smartweed. As corn planting is beginning in earnest, we need to take into consideration that many of these species will be present at planting and should be considered when planning burndown strategies for no-till fields. For burndown efficacy ratings, refer to Table 8 in issue no. 2 of the Bulletin. We have also started to see the emergence of a number of the Group 2 weed species, including common ragweed and giant foxtail, in certain parts of the state. So during planting over the next couple of weeks, keep your eyes open to what weeds have emerged and follow along with the Bulletin. Knowing what weeds are present and what weeds will soon emerge will help you make good decisions on how to manage a number of these different species.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague