July 21, 2026

2026 Mid-Season Weed Survey Results

A summary of weed species that have been sent in for identification and herbicide resistance from May 1st to July 21, 2026).

Mike Cowbrough

Weed Management Specialist - Field Crops

Cesar Cappa

Weed Management Specialist - Horticulture Crops

Weed Management

The following is a preliminary summary of weed species from Ontario counties that have been submitted for molecular species identification and/or herbicide resistance from May 1st to July 17th, 2026. Thanks to Grain Farmers of Ontario, Ontario Bean Growers, Ontario Tomato Research Institute and S-CAP Protection and Risk Resilience Program. Details on how to submit samples can be found here.

Overall Resistance Detections


Weed Species	Confirmed Resistance Detected
Italian ryegrass	Group 9
Redroot pigweed	Groups 2, 5, 14
Common ragweed	Groups 2, 5, 14
Waterhemp	Groups 2, 9, 14
Giant foxtail	Group 2
Canada fleabane	Group 9
Lambsquarters	None detected (bioassay needed)
Common groundcherry	Identification only

Total populations submitted: 23

Resistant populations: 17 (74% test positivity)

Species with confirmed resistance: 6

Most common resistance pattern: Multiple resistance involving Group 2, often combined with Groups 5 and/or 14.

Italian (Annual) Ryegrass (Lolium multiflorum)

Lolium multiflorum - 3 populations submitted
County	Result
Perth	Glyphosate (Group 9) resistant
Oxford	No target-site glyphosate resistance detected; bioassay needed to determine non-target-site resistance
Middlesex	No target-site glyphosate resistance detected; bioassay needed to determine non-target-site resistance

Summary: 1 confirmed glyphosate-resistant population; 2 populations require bioassay testing.

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Redroot Pigweed (Amaranthus retroflexus)

Amaranthus retroflexus - 7 populations submitted
County	Result
Perth	Resistant to Groups 2, 5, and 14
Prescott & Russell	Resistant to Groups 2 and 14
Prescott & Russell	No target-site resistance detected (Groups 2, 5, 14)
Stormont, Dundas & Glengarry	Resistant to Groups 2, 5, and 14
Huron	Resistant to Groups 2 and 5
Huron	Resistant to Group 14 only
Prescott & Russell	Resistant to Groups 2 and 14

Summary:

2 populations resistant to Groups 2, 5, and 14
2 populations resistant to Groups 2 and 14
1 population resistant to Groups 2 and 5
1 population resistant to Group 14 only
1 population with no target-site resistance detected

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Common Ragweed (Ambrosia artemisiifolia)

Ambrosia artemisiifolia - 6 populations submitted
County	Result
Prescott & Russell	Resistant to Groups 2 and 14
Prescott & Russell	Resistant to Groups 2 and 14
Prescott & Russell	Resistant to Groups 2, 5, and 14
Prescott & Russell	Resistant to Groups 2, 5, and 14
Stormont, Dundas & Glengarry	Resistant to Groups 2 and 5
Kawartha Lakes	Resistant to Groups 2, 5, and 14

Summary:

3 populations resistant to Groups 2, 5, and 14
2 populations resistant to Groups 2 and 14
1 population resistant to Groups 2 and 5

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Waterhemp (Amaranthus tuberculatus)

Amaranthus tuberculatus - 2 populations submitted
County	Result
Oxford	Resistant to Groups 2, 9, and 14
Huron	Resistant to Groups 2 and 9

Summary: Both populations were resistant to Group 2 and glyphosate (Group 9); one population also carried Group 14 resistance.

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Lambsquarters (Chenopodium album)

Chenopodium album - 2 samples submitted
County	Result
Stormont, Dundas & Glengarry	No target-site Group 5 resistance detected; bioassay required
Prescott & Russell	No target-site Group 5 resistance detected; bioassay required

Summary: No target-site Group 5 resistance detected in either sample.

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Giant Foxtail (Setaria faberi)

Setaria faberi - 1 population submitted
County	Result
Stormont, Dundas & Glengarry	Resistant to Group 2

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Canada Fleabane (Conyza canadensis)

Conyza canadensis - 1 population submitted
County	Result
Stormont, Dundas & Glengarry	Glyphosate (Group 9) resistant

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Common Groundcherry (Physalis longifolia)

1 population submitted for identification
County	Result
Perth	Submitted as unknown grass; identified as Smooth Groundcherry

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Master List

Preliminary Molecular Testing Results for plant identification and target site resistance by Ontario county from May 1st to July 17th, 2026
Date	County	Query	Test Result
May 19th	Perth	Ryegrass (lolum) – confirm identification, suspect resistant to glyphosate (group 9)	Molecular ID: Lolium multiflorum (Italian/annual ryegrass) Positive: target site resistance to glyphosate
June 6th	Oxford	Ryegrass (lolum) – confirm identification, suspect resistant to glyphosate (group 9)	Molecular ID: Lolium multiflorum (Italian/annual ryegrass) Negative for target site resistance, a bioassay would confirm non target site resistance.
June 15th	Middlesex	Ryegrass (lolum) – confirm identification, suspect resistant to glyphosate (group 9)	Molecular ID: Lolium multiflorum (Italian/annual ryegrass) Negative for target site resistance, a bioassay would confirm non target site resistance.
June 26th	Perth	Species identification - unknown grass	Molecular ID: Physalis longifolia - Common groundcherry
June 26th	Perth	Redroot pigweed (Amaranthus retroflexus) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - target site resistance to groups 2, 5 and 14 herbicides.
June 26th	Prescott and Russell	Redroot pigweed (Amaranthus retroflexus) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2 and 14 herbicides. Target site resistance not detected for group 5 herbicides, a bioassay would confirm non target site resistance.
June 26th	Stormont, Dundas and Glengarry	Giant foxtail (Setaria faberi) - suspect resistance to group 2 herbicides.	Positive - resistant to group 2 herbicides.
June 26th	Stormont, Dundas and Glengarry	Lambsquarters (Chenopodium album) - suspect resistance to group 5 herbicides	Target site resistance not detected for group 5 herbicides, a bioassay would confirm non target site resistance.
June 26th	Prescott and Russell	Common ragweed (Ambrosia artemisiifolia) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2 and 14 herbicides. Target site resistance not detected for group 5 herbicides, a bioassay would confirm non target site resistance.
June 26th	Prescott and Russell	Lambsquarters (Chenopodium album) - suspect resistance to group 5 herbicides	Target site resistance not detected for group 5 herbicides, a bioassay would confirm non target site resistance.
June 26th	Prescott and Russell	Common ragweed (Ambrosia artemisiifolia) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2 and 14 herbicides. Target site resistance not detected for group 5 herbicides, a bioassay would confirm non target site resistance.
June 26th	Prescott and Russell	Redroot pigweed (Amaranthus retroflexus) - suspect resistance to groups 2,5 and 14 herbicides.	Target site resistance not detected for groups 2, 5 and 14 herbicides, a bioassay would confirm non target site resistance.
June 26th	Prescott and Russell	Common ragweed (Ambrosia artemisiifolia) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2, 5 and 14 herbicides.
June 26th	Stormont, Dundas and Glengarry	Canada fleabane (Conyza Canadensis) - suspect resistant to glyphosate (group 9)	Positive - resistant to glyphosate (group 9).
June 26th	Stormont, Dundas and Glengarry	Common ragweed (Ambrosia artemisiifolia) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2 and 5 herbicides. Target site resistance not detected for group 14 herbicides, a bioassay would confirm non target site resistance.
June 26th	Stormont, Dundas and Glengarry	Redroot pigweed (Amaranthus retroflexus) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2, 5 and 14 herbicides.
June 26th	Prescott and Russell	Common ragweed (Ambrosia artemisiifolia) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2, 5 and 14 herbicides.
June 26th	Oxford	Waterhemp (Amaranthus tuberculatus) - confirm ID, suspect resistance to groups 2,5,9 and 14 herbicides	Molecular ID: (Amaranthus tuberculatus - Waterhemp) Positive: target site resistance to groups 2, 9 and 14. Target site resistance not detected for group 5 herbicides, a bioassay would confirm non target site resistance.
June 26th	Kawartha Lakes	Common ragweed (Ambrosia artemisiifolia) - suspect resistance to groups 2,5 and 14 herbicides.	Positive - resistant to groups 2, 5 and 14 herbicides.
July 17th	Huron	Redroot pigweed (Amaranthus retroflexus) - confirm identification, suspect resistance to groups 2,5 and 14 herbicides.	Molecular ID - Redroot pigweed (Amaranthus retroflexus). Positive - resistant to groups 2 and 5 herbicides. Target site resistance not detected for group 14 herbicides, a bioassay would confirm non target site resistance.
July 17th	Huron	Redroot pigweed (Amaranthus retroflexus) - confirm identification, suspect resistance to groups 2,5 and 14 herbicides.	Molecular ID - Redroot pigweed (Amaranthus retroflexus). Positive - resistant to group 14 herbicides. Target site resistance not detected for groups 2 and 5 herbicides, a bioassay would confirm non target site resistance.
July 17th	Huron	Waterhemp (Amaranthus tuberculatus) - confirm ID, suspect resistance to groups 2,5,9 and 14 herbicides	Molecular ID: (Amaranthus tuberculatus - Waterhemp) Positive: target site resistance to groups 2 and 9. Target site resistance not detected for groups 5 and 14 herbicides, a bioassay would confirm non target site resistance.

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