Weed Watch 2022

Experts discuss the challenges of invasive and noxious weeds, along with risk mitigation.

By Kristen Lutz

Every weed is invasive … every single one of them,” says Patrick Lynch, member of the Ontario Certified Crop Advisor Association and Better Farming agronomist.

Although weed control can be managed with herbicides and other management practices, their presence is an ongoing challenge. Invasive and noxious weeds not only pose risks to producers but also create several environmental concerns.

Better Farming spoke with experts about which weeds to look out for this spring and which risk mitigation techniques can be used to combat this ever-propagating problem.

Invasive weeds

“Every area has weeds that are invasive, and (types) will vary by geography,” says Lynch.

If you spot something you’re not already familiar with, Mike Cowbrough, weed specialist for field crops and chief inspector of the Weed Control Act for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), suggests investigating what Ontarians are reporting as plants of concern at www.eddmaps.org/Ontario/distribution. There are over 200 species posted, divided into categories and searchable by region.

Purple Loosestrife Weed
    Purple Loosestrife - Ken Allison photo

Species such as purple loosestrife, common buckthorn and Japanese knotweed are troublesome in rural Ontario. They’re common in ditches and grow along fence lines, but they also have the potential to creep into grazing systems.

Japanese Knotweed

    Japanese Knotweed - Invasive Species Centre photo

“Invasive species are important to recognize, as they alter the natural environment, habitats, disrupt ecosystems, and threaten biodiversity. They displace native plants by competing for nutrients, water and space,” says Danie Glanc, a farm policy analyst with the Ontario Federation of Agriculture.

Phragmites Weeds
    Phragmites - Lauren Bell photo

Cowbrough explains that varieties like dog-strangling vine and phragmites have become more of a concern to areas surrounding pastures and cropland.

“Phragmites have been called one of Ontario’s worst invasive species,” says Glanc. “They’re found mostly in wetlands, outcompete native plants, destroy habitats of native aquatic species, and block drainage ditches. Seeds of phragmites are easily dispersed by wind, giving it prolific ability to spread and invade other moist habitats.”

Threats to farmland

Species such as wild chervil and fleabane will impact farmland directly. They germinate easily and can cause direct competition with crops, reducing yields and can be found throughout grazing land.

Lynch adds that water hemp and palmer amaranth are two species that have recently crept their way onto Ontario’s croplands. Both of these species have a very strong capability to overtake fields, however, of these two, water hemp poses the most imminent problem. Water hemp emerges throughout the growing season and is becoming more resistant to herbicides.

Overview of Farm with Weeds

    Burdock - Patrick Lynch, Ken Allison, Lauren Bell, Invasive Species Centre & Ken Towle photo

Invasive species are able to occupy new regions through adaptation, such as the spread of lightweight seeds via wind travel, as previously mentioned by Glanc.

“Some of these species are coming with migratory birds,” Lynch says. “Birds will fly south, eat the seeds, fly north and drop the weed seeds wherever they stop to do feeding. Every grower needs to be aware of the species in their area and how they are going to be moving.”

“Invasive weeds establish themselves, persist, and spread widely in areas outside the plant’s native range. These plants cause serious ecological, economic and social effects,” says Glanc.

Managing weeds is big business. According to the Invasive Species Centre, municipalities and conservation authorities spend an estimated $50.8 million per year on invasive species (plant, insect and aquatic) in Ontario alone. It indicates municipalities spend an annual average of $2.8 million on phragmite mitigation alone, and wild parsnip at just over $1 million. Environment Canada’s data from 2010 estimated the annual economic impact of invasive plants on Canadian agriculture was $2.2 billion.

Notorious & noxious

Noxious weeds can have a direct impact on crops. Common barberry, found on fence lines and in roadside ditches, is considered an alternate host to oat rust.

“The barberries would start the rust, and then it went from the barberries to the oats. So, if you got rid of the alternate host then you would get rid of this strain of rust,” Lynch says, explaining an earlier program designed to eradicate common barberries in Ontario.

Currently, 25 species are considered noxious weeds in Ontario.

Noxious weeds can also be known as nuisance weeds and include native, non-native, introduced and invasive species. The OMAFRA Weed Control Act states that noxious weeds can have three characteristics: difficult to manage on agricultural land and will reduce yield and the quality of a crop, negatively affect the health and wellbeing of livestock, or poses a risk to the health and wellbeing of agricultural workers.

Producers can view the complete list of 25 noxious species in Ontario on OMAFRA’s website. Some noxious weeds are fairly common, including poison ivy.

Poison hemlock is another invader. “It looks like a small bamboo plant and kids love to cut it and use it sort of like a blowgun,” says Lynch. “They put it up to their lips and (then) they get a very bad rash. It’s similar to poison ivy, except that with poison hemlock the irritation is worse.”

Threats to workers and livestock

Not all noxious weeds cause skin rashes. Some, such as leafy spurge, cause more extensive damage and can even be toxic to humans and livestock.

Lynch explains that leafy spurge often grows on the edge of fields and can accidentally be harvested along with forage and fed to livestock.

Giant Hogweed
    Giant Hogweed - Ken Towle photo

Other species can pose toxicity risks. “The sap from giant hogweed and wild parsnip are toxic to skin and can cause human health concerns,” explains Glanc. “Wild parsnip is toxic to cattle, horses and sheep, and is known to reduce weight gain and fertility in the livestock that consumes it.”

Ragweed is also considered a noxious weed according to Weed Control Act because it poses a health risk to agricultural workers.

“Everybody has it: it’s in the city, it’s in the country, it’s all over the place and it’s only there because it’s a hard weed to control. Also, the pollen is a health hazard for people that are allergic to it,” says Lynch. “It’s a bad weed as much as it is an allergen. It’s a massive seed producer and it’s hard to control.”

The Weed Control Act of Ontario exists to detail and deal with noxious weeds. “There are three main intents of its legislation: one, to reduce the infestation of noxious weeds that impact the industries of agriculture and horticulture. Two, to reduce the plant diseases by eliminating plant disease hosts such as common barberry and European buckthorn. And three, to reduce health hazards to livestock and agricultural workers caused by poisonous plants,” says Cowbrough.

Negative effects & climate change

To add to their list of harmful effects, invasive and noxious weeds can damage native and natural environments – including precious insect species.

“The impact on native ecosystems is severe and potentially irreversible,” cautions Glanc. “Invasive plants can threaten many endangered or at-risk species, and even cause their extinction. For example, dog-strangling vine threatens the monarch butterfly, which is a species at risk in Ontario. Eggs from the Monarch are laid on the plant, but the larvae are unable to complete their lifecycle and do not survive,” says Glanc.

Although weeds may not be directly affected by climate change, the more frequent extreme weather conditions and higher presence of carbon dioxide can result in a higher transmission and growth of invasive and noxious species.

“Warmer temperatures can create environments that allow certain species to become established that otherwise would not. The most recent example is of Palmer amaranth and the potential for establishment under different climate change models,” explains Cowbrough.

“Climate change has the potential to accelerate the introduction and spread of invasive plants. More frequent extreme weather events such as droughts and floods can stress native species, creating opportunities for invasive species movement and establishment,” says Glanc.

Management techniques

Luckily, management techniques for weed control have proven to be effective in reducing the presence, and therefore, the transmission of invasive and noxious species.

Field being sprayed with herbicides
    sircco/iStock/Getty Images Plus photo

“Depending on the plant in question, there are herbicides that specifically target certain species. However, managing invasive plants is complex and requires a multi-faceted approach,” explains Glanc. “There is no one-size-fits-all solution. Using a combination of methods often offers maximum impact and control of invasive plants. Furthermore, when using herbicides, be sure to follow all legislation and regulatory requirements.”

“Significant progress has been made in the last 20 years to evaluate the sensitivity of certain invasive/noxious weeds to herbicides. Dr. Francois Tardif at the University of Guelph has spearheaded a lot of that work,” Cowbrough says.

Other management practices besides herbicide application can also be effective.

“The best weed control is a combination of methods which would include tillage, herbicide treatment, crop rotation and alternate crops,” says Lynch. “But you have to know your weed and you have to know its limitations and weaknesses.

“Glyphosate-resistant fleabane is a terrible weed across Ontario, but there’s a lot of good research to show if you plant cereal rye after winter wheat you don’t have near as much fleabane the next spring,” he continues. “This is because cereal rye has an allelopathic effect that inhibits fleabane germination and growth.”

Cowbrough states that recently “the risk of introduction through machinery,” has become significant.

Lynch agrees. “We will have a few weeds in the corner of the field and then we’ll take the combine and spread them through 100 to 200 acres.”

He mentions a potential opportunity for a weed separator on the back of a combine. “The combines did this all the time when I was a kid; there was a separation system on the combine that collected the seeds and put them in a separate bin. You’d empty the combine, and you’d empty the weed seeds and destroy them.”

However, there’s no infrastructure to attach a weed seed separator on new combines. Instead, producers should be diligent during harvest season and pay close attention to where they’re starting their combine and the potential weeds they could be spreading throughout their fields.

Fortunately, research on invasive species is ongoing and new products to control these weeds continue to come to market. Cowbrough mentions professor Tardif is investigating new herbicides and biological control agents for the more persistent and harmful weeds.

“The basics are important. Regular scouting to identify new species allows you to take action when there are only tens to hundreds of introduced plants … as opposed to thousands and hundreds of thousands if left alone,” reminds Cowbrough. BF

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