Waterhemp is a weed species in the pigweed family. Although it looks very similar to common pigweed species, it is much more difficult to manage. The waterhemp population in Ontario has been slowly growing and spreading over the past few years, and is now found in many counties across the province. Waterhemp is becoming more common in the Dresden area as well. Waterhemp seeds are most commonly spread through water, birds, and equipment. If you suspect waterhemp is in your field, it is important to clean your equipment off well before leaving the field to avoid spreading seeds.
Plants in the pigweed family can be identified by their “pig nose” leaf tip. To differentiate species in the family, common pigweed species, including green and redroot pigweed, have hairy stems. Waterhemp is completely hairless. Below is a group of photos showing the difference in pigweed species.
Amaranthus visual comparison. Maggie Durnin. 2025
Unlike many common weeds that only emerge during a short window in the spring or fall, waterhemp can emerge throughout the entire growing season. This means having a strong herbicide program (including a PRE with residual properties) is important to successfully managing waterhemp in your fields.
Another challenging attribute is the genetic diversity within the species. Waterhemp is a dioecious species, meaning plants are either male or female, and both are needed in order to reproduce seeds. One waterhemp plant can produce 1 million seeds, so the population in a field can increase rapidly. These factors result in abundant genetic diversity, leading to herbicide resistance of multiple modes of action. Most waterhemp populations are at least Group 9 (glyphosate) resistant, but there are populations in Ontario and in our local counties that are also resistant to Groups 2, 5, 14, and 27. It is important that your herbicide program has multiple modes of action, so you have better chances of controlling the whole population.
The table below shows the number of known fields in Southern Ontario counties with herbicide resistant waterhemp populations. If you have waterhemp in your fields, getting the population tested for herbicide resistance can help you make informed decisions regarding your herbicide program.
Number of fields with resistant waterhemp by herbicide mode of action (WSSA group). Mike Cowbrough. 2025.
