By Tyler Sabelli, CCA-ON, CCS, 4R NMS
Crop Sales Specialist,
AGRIS Co-operative - Leamington and Wanstead Farmers Co-operative

As we begin starting up the combines and getting ready for the 2020 fall harvest season, one problem weed in particular we need to watch out for is waterhemp. Producers have done an excellent job managing other glyphosate-resistant weeds on their farms such as Canada Fleabane and Giant Ragweed, but knocking on the front door is a weed called common waterhemp (Amaranthus Rudis). Today, we have resistance to Group 2, 5, 9 and 14 biotypes in southwestern Ontario. We are seeing it show up in many new fields over the last couple of years as we’re out scouting, as well as an increase presence from growers on social media throughout the territory. Growers should be concerned about this weed as it will impact crop yield and will change how we approach our herbicide management program. The biggest challenge is to correctly identify it, as it could resemble its close cousin redroot pigweed (Amaranthus retroflexus). It’s also important that we understand its life cycle, where it is today and how it can move from one field to the next.

Identification

Properly being able to identify waterhemp is key. There is a lot of good literature and resources available in more detail, however the following characteristics will give you a good idea on where to begin.

• Summer annual species – can grow up to 6’ tall
• Leaves - hairless, oval-shaped with a waxy appearance, mature leaves will have a wavy margin
• Stems – hairless, waxy appearance, can vary in colour from green to burgundy
• Flowers – dioecious, male and female are different plants. Seed head will be long with clusters, seed will appear green, red to purple on female plants.

The seed can be difficult to spot with the naked eye, but you will see this plant well above the soybean canopy this Fall if present, confirm it is hairless and has a waxy appearance.

In order for growers to effectively manage spreading glyphosate-resistant waterhemp, it is important to understand how it can move from field to field. Unlike the resilient Canada fleabane where viable seed is spread through the air, waterhemp seed is primarily moved around by equipment including planters, drills, tillage implements and combines. As producers prepare for fall harvest, it is important they prioritize which fields are combined first; fields where waterhemp is present should be combined last to reduce moving seed to other areas. 

It may also be necessary to clean equipment between fields as a further precaution to eliminate the chance of spreading. 

It is also known that birds may transport viable seed as well so routine scouting is necessary. 

Waterhemp will predominately germinate in the spring, but we are seeing more and more germinating mid-summer and into the early fall. This weed can become a problem quickly as even a young plant germinated in September can produce viable seed.

As we approach the harvest season, pay close attention to what weeds have escaped your current herbicide programs. It is important to know how to correctly identify waterhemp to minimize the impact on your operation. 

Know how it potentially moves around so you can manage accordingly. It is crucial to make good notes this fall in the combine as to where it may be present so appropriate herbicide programs can be put into place next spring. 

In addition, it may be beneficial to consider some fall herbicide applications on fallow fields or wheat stubble fields that may not have a cover crop to help minimize the chance for waterhemp to reproduce. 

As always, please contact your local AGRIS or Wanstead crop sales specialist to discuss your best options for how to manage this challenging weed in the future.

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Resources:

Mike Cowbrough – OMAFRA  

Common Waterhemp – Division of Plant Sciences U. of Missouri

Photos: Peter Smith