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In BC, weeds are generally defined as non-native, introduced or alien plants to North America. We primarily categorize them as Noxious and Invasive. In BC, most invasive weeds are legislated noxious under the Weed Control Act, either provincially or within one or more regional districts. However, there are also many species of weeds that are not legislated yet still can have serious impact on agricultural or natural areas.

dscn0865 Environment

In natural ecosystems, weeds displace or destroy native plant populations. Many rare and endangered native plants are also at risk from extinction from non-native plant invasion.

By removing the natural wildlife weeds also reduce wildlife habitat.

Many Red and Blue listed vertebrates are dependent on healthy weed-free grasslands for their survival.

Shallow root systems cause increased erosion, causing higher surface runoff, which increases stream sediment and reduces water quality.

Good condition bluebunch wheatgrass/rough fescue range in Kamloops produces 493 kg/ha available forage. That range infested with knapweed produces 62 kg/ha of forage, an 87% reduction.



Weeds outcompete with existing forage and are often unpalatable to livestock, reducing overall levels of grazing

They also compet

e with agricultural crops and reduce overall crop yield and quality

Weeds require costly, long-term strategies to control and manage their spread Weeds can be host to insects and diseases in crops.

copy of sulphur cinq okib hwy 97

The effects of weed infestations are widespread costing ranchers, farmers, conservation organizations, utility companies, governments and the general public millions of dollars each year to control.


copy of sulphur cinq okib hwy 97 Forestry

Dense infestations of weeds increase the risk of wildfire because they are a source of fuel as they mature.

Noxious weed infestations compete with new tree seedlings for soil nutrients, light and moisture. This results in increased costs for silviculture.

Spotted & diffuse knapweed (2 provincial noxious weeds) infested an estimated 30,000 ha of uncultivated lands in 1972. Today the figure is 91,000 ha. A 6% increase per year. Resulting annual losses in beef production alone exceed $2.5 million. Values for environmental deterioration, reduction in grassland recreational value and quality of wildlife habitat and property values is undetermined.


Recreation and Tourism

Weeds destroy the natural beauty biodiversity of the landscapes.

They can limit access for recreationalists (i.e. dense infestations of marsh plume thistle)

marsh plume thistle2 
Threats to Health

Some weeds can be toxic or cause skin irritations to livestock and humans, for example dalmation toadflax and leafy spurge.

Some cause seasonal allergies and hay fever

Hoary Alyssum and Hound's Tongue can be toxic to horses and livestock.


dalmation toadflax

To learn about poisonous plants, check out Stock Poisoning Plants of Western Canada, a manual produced by Agriculture and Agri-Food Canada. It describes the plants that most frequently cause poisoning and injury to livestock in Western Canada, as well as a number of plants that are potentially dangerous. Species found in both native rangelands and cultivated croplands are included. Click here to download this manual in PDF format.

In Canada, a 1991 national survey of 58 commodities estimated the value of lost production, due to weeds, was $984 million.  Two thirds of that total occurred in Western Canada.


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The Invasive Species Council of BC has been working towards increased research of the impacts of invasive plants in British Columbia. To download a PDF version of their 2009 Economic Impacts Report click here.

In B.C. crop losses are estimated at well over $50 million annually. Costs incurred for controlling weeds is unknown

In BC, it was estimated that there are approximately 1.1 million hectares of open grassland range, and undetermined open forested range susceptible to knapweed invasion.

In 1982, it was estimated, if knapweed occupied these grassland the loss to annual farm gate beef value alone would be $41 million. That figure for 2001 would increase to $76 million annually.

 The Urgent Threat: Statistics from the United States

Total annual losses in North Dakota on grazingland and wildland from leafy spurge was valued at $87.3 million. Current leafy spurge infestations cause a reduction of over 1000 jobs per year in North Dakota.

Overall, noxious weeds cause an estimated 27 billion in losses to crop, pasture, and forest production in the US each year.

Research on western U.S. federal lands estimated 2.5 million acres infested with invasive weeds in 1985. In 1994, 8.5 million acres were infested. A 14% increase per year. This 14% equates to 4600 acres per day being infested on western Federal lands

In Montana, spotted knapweed was introduced in 1920, today it infests 4.7 million acres.

In California, yellow starthistle was introduced in 1869, by 7.9 million acres were infested. Yellow starthistle currently occupies nearly a half million acres in Washington and Idaho states.
In Montana, studies showed surface water runoff and stream sediment yields were 56% and 192% higher respectively, in spotted knapweed dominated sites compared to native perennial grasslands

Montana studies show spotted knapweed infested range reduce Rocky Mountain elk use by 98% compared to bluebunch wheatgrass dominated sites. In North Dakota, deer use of leafy spurge infested areas was 70% less than non-infested sites.
Manitoba research found non-native plant invasion produces significant changes in species composition of birds.

Respectfully submitted by: David Ralph, A.Sc.T., Provincial Weed Technologist and Edi Torrans, Southern Interior Weed Management Committee