Scout Now for those Problem Weeds: Pigweeds, Ragweeds, and Other Weeds....Oh My!

Posted: June 4, 2013

Take time to look over your fields to see if any weeds are becoming a problem.
Test your knowledge.  Which one is redroot pigweed, waterhemp, and Palmer amaranth?  Answer in next week’s newsletter (Image by W. Curran).

Test your knowledge. Which one is redroot pigweed, waterhemp, and Palmer amaranth? Answer in next week’s newsletter (Image by W. Curran).

In general, most of our Pennsylvania farmers have been pretty fortunate about the weeds that they battle each year; that is, they could be a lot worse (you might not agree with this).  Most of our farmers don’t routinely deal with weeds like common cocklebur, giant ragweed, the annual morning-glories, and the real problem pigweeds (waterhemp and Palmer amaranth).  These are weeds that our neighbors to the west and south may be routinely dealing with and they typically require a higher level of management.  This is the time of year we should be scouting those corn and soybean fields looking for problems, and particularly new ones. 

Let’s first talk about the pigweeds.  We have been discussing our concern about waterhemp and Palmer amaranth which have become problems in areas of the Midwest and South.  Both have multiple resistance to herbicides; Palmer amaranth in particular has really changed the way cotton and soybean are grown in the South with more and more tillage taking place.  The pigweeds are fairly hard to distinguish from one another, particularly as young plants.  There are several good publications available (see the following links) to help distinguish these different Amaranth species, but often they need to become more mature and perhaps even be flowering to help with definitive identification.  We are currently growing some pigweeds in our greenhouse on campus to help familiarize ourselves with waterhemp and Palmer amaranth and to use for outreach educational purposes.  Our experience thus far is less than clear at least when they are small and vegetative.  Maybe larger infestations would be more obvious, although we don’t wish that on anyone!  In any case, the key here is that if you encounter an herbicide performance issue with a pigweed, pay close attention and let’s make sure we know what we are dealing with.  To finish up the pigweed discussion, here’s a quiz for you.  See accompanying image of our greenhouse plants and write down which photo is redroot pigweed, waterhemp, and Palmer amaranth (A, B, or C).  We’ll let you know next week the correct answer.    Sorry, no prizes this time, only your personal satisfaction (or dissatisfaction).

Helpful pigweed identification tools:

The second weed we would like to mention is giant ragweed.  Common ragweed is familiar in many of our agricultural fields and is the smaller cousin to giant ragweed.  We noticed a new challenge taking hold this week in a no-till soybean field that Bill passes each day to and from work.  The field margins were infested with giant ragweed and this is a new problem on this farm (see image below).

  giant ragweed








Image 2.  Giant ragweed getting a foothold in no-till soybeans, Centre County PA (Image by W. Curran).

Giant ragweed is a common plant in our region, but not a common field crop weed, although there are some farms that have had trouble with this weed for a number of years.  Interestingly, this species is oneof the worst weeds in the state of Ohio and is quite common in other areas of the Midwest.  It seems to be a particular problem for soybean growers and herbicide resistance to glyphosate (Group 9) and ALS herbicides (Group 2) is an increasing occurrence with this species.  Research conducted by colleagues at Ohio State University (Schutte et al., 2012) showed distinct differences between what they called “successional” or wild type giant ragweed and “agricultural” types.  The “successional” giant ragweed are those that we see along the margins of fields, hedgerows, forest edges, etc. and more occasionally in crop fields.  These populations tend to have a short germination period that begins early in the spring and they are historically less difficult to manage with tillage and herbicides. This is likely the type of population that we observed in the no-till soybean field on the commute to work.  In contrast, the “agricultural” populations appear to be selected for over time with tillage and herbicides and have different seed dormancy characteristics that give them prolonged germination with emergence later into the summer.  This characteristic makes these populations much more difficult to manage and particularly if they also have herbicide resistance.   The bottom line is to pay attention to this species when you first notice it and don’t let it get a foothold in your agricultural fields.  We don’t want the “agricultural” type to become the norm.

Contact Information

William S. Curran
  • Professor of Weed Science
Phone: 814-863-1014
Dwight Lingenfelter
  • Program Development Specialist
Phone: 814-865-2242