Return of the Waterhemp

Crops and Soils Agent - Fond du Lac County
University of Wisconsin Extension

Similar to the Eveready battery, waterhemp keeps growing, and growing, and growing, and growing. Waterhemp, a close cousin to the more familiar redroot and smooth pigweed species, is back in 1999 after it's premier showing in Fond du Lac County last year. Now is a good time to scout fields and look for this weed before things get too far out of control. If fields have already been sprayed with a herbicide, be on the lookout for weed seedlings that have similar characteristics to pigweed but are largely unaffected by herbicide applications (especially if an ALS herbicide was used).

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 last week in this area Fond du Lac County corn field.	Waterhemp seedlings are similar to their pigweed cousins but typically have narrower leaves and no hairs on the leaf or stem surface.

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.

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.

Take time now to confirm that those pigweed escapes are not actually waterhemp plants. If possible, eliminate these plants before they go to seed. At this point, a cultivator may be the only options. Depending on the size of the corn or soybean crop, explore chemical options with your ag chemical dealer. 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.