So “What’s the big deal?” you may ask.  The big deal is that this weed has become one of the most difficult weeds to control for our neighbors to the south and west.  Populations of waterhemp that are resistant to ALS herbicides are now widespread.  In some states, resistant waterhemp plants to both ALS and triazine herbicides have been reported.  Couple this kind of herbicide tolerance with the fact that waterhemp emerges later in the season than most other weed species and we need to start talking about a major deviation from most weed management strategies currently being used.  An Iowa State University study showed that initial waterhemp seedling emergence occurred two weeks later than velvetleaf and total waterhemp emergence reached 50% about one month later than velvetleaf.
 

Waterhemp was thriving in areas of this Fond du Lac County corn field.	Waterhemp plants are easy to spot as they begin to out-stretch soybean plants.

    At this point in the growing season, it will be difficult to spot waterhemp problems in corn fields from the front seat of the pick-up.  However, they can easily be seen in soybeans as the weed stretches beyond the crop canopy.  Waterhemp is about the same size as the pigweeds we’re more familiar with.  The primary difference between waterhemp and redroot or smooth pigweed is that waterhemp has no hairs on either its leaves or stems.  Waterhemp leaves tend to be narrower and the seedhead is less coarse and somewhat thinner.  The native habitats of waterhemp are flood plains and other wet, low-lying areas.  Thus, wet areas of fields, soils with poor drainage, and fields bordering marsh areas are prime candidates for initial invasion by waterhemp.  If  you have any field where weed control is good with the exception of plants that look like pigweeds, it will be well worth an investigation to make sure it is not waterhemp.

If waterhemp is found…..

    If waterhemp is confirmed in a field, there are few effective herbicide options at this point in time (perhaps a ropewick applicator with Roundup may be feasible in some soybean situations).  Where plants are merely scattered or few, begin to employ the hand-pulling crews.  If waterhemp populations are too numerous to hand-pull but confined to a relatively small acreage, it may be  worthwhile to destroy the area with tillage to keep the weed from going to seed.  Wherever waterhemp plants are noted, at the very least harvest these fields last if possible and clean-off harvest equipment before moving-on to the next field.  I would like to think that we can stop the spread of waterhemp but the best we can probably hope for is to slow its spread and keep it out of some fields or farms.

Why waterhemp now?

    The question arises as to why we have found ourselves with a waterhemp problem now when it hasn’t been in the past.  Several reasons come to mind.  First, and probably most importantly, the “one-pass” ALS post-emergence herbicide programs that many farmers use play right into the hands of waterhemp proliferation.  Not only is resistance built-up within a few years, but there is little or no residual activity with these products giving no support for the late-emergence pattern of waterhemp.  There are several herbicides that have excellent activity on waterhemp, but no single one will provide consistent, full-season control.  Pre-emergence herbicides don’t provide long enough full-season control.  Most post-emergence products (even non-ALS types) are usually made prior to the emergence of most waterhemp plants.  Experience from states like Iowa shows that an integrated weed control program will work the best.  This means some type of two-pass herbicide program, using cultivation, and/or rotating crops wherever feasible.  We’ll talk about specific options at a later date.

In summary…..

    Take time now to confirm that those pigweed escapes are not actually waterhemp plants.  If  possible, eliminate these plants before they go to seed.  Harvest these fields last or clean-off harvest equipment before moving-on to the next field.  Finally, take time this winter to plan an effective weed control strategy for future years.