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Invasion of Exotic Plants

We share a complex, interconnected planet with hundreds of thousands of plants. Each of these could be considered a weed in one situation or another. From a national park perspective, weeds are exotic (non-native) plants that compete with native plants for moisture, food, light and living space. Some of these were (and still are) carried across the oceans, mainly from Eurasia, as unknown baggage. Others are distant relatives so altered by agriculture that they no longer have a place in natural environments. These newcomers and selectively altered invaders thrive in the absence of natural predators, parasites, competitors and disease. Most exotic species do well in disturbed landscapes. Once established, they spread and adversely affect native plants and animals.

This is a significant concern for the park, with its 958 known species of higher plants, some of which are extremely rare (168 provincially rare, 50 nationally rare and 35 found only in Waterton Park!). With flowers as spectacular as orchids; grasses as essential as fescue; and shrubs as nutritious as huckleberry and saskatoon; non-native plants have the potential to severely affect park habitat. Waterton Lakes National Park is also responsible to its neighbours to ensure that noxious species do not spread beyond its boundaries.

Since 1978, Parks Canada has actively worked to control the spread of non-native plants by removing them manually or by limited use of herbicides (where manual control hasn't been effective). Reseeding disturbed areas with native plants and maintaining healthy native plant communities by reducing human impact has also been successful. In sensitive areas, where these control methods are difficult to implement, park managers are trying biological control. This involves releasing carefully selected insects that feed only on the targeted species of plant, eg. spotted knapweed. Biological control is also being used in adjacent areas like British Columbia and Montana and these insects may make their way into the park.

To date (1995), eighty-three (83) exotic plant species have been found in Waterton Lakes National Park. Every year, more seeds and new exotic species enter the park on vehicles, clothing, footwear and pets. Others are carried in by wind and water. Some escape from flower gardens in and around the park. Some species, like timothy (Phleum pratense) and smooth brome (Bromus inermis), were deliberately planted here to feed horses and cows which were allowed to graze in the park until 1947. These are now so well established they are considered beyond control.

The following plants are of particular concern in Waterton Lakes National Park:

Spotted Knapweed (Centaurea maculosa)

This Eurasian invader can produce up to 25,000 seeds per plant. The seeds can remain dormant in the soil for at least seven years and are transported long distances by humans, animals and water. The over wintering rosettes (low-growing cluster of leaves) grow rapidly the following spring to a height of 30 to 100 cm (12 to 40 in), with many stems and purple flowers. They readily grow in disturbed soils and are well adapted to low fertility and semi-arid areas. Once established, their roots give off a chemical toxin which inhibits the germination and growth of other plants, allowing knapweed patches to expand. Wildlife do not eat them because of their bitter taste and high fiber content. These factors combine to accelerate the expansion of the plants. Some areas of British Columbia and Montana have become knapweed wastelands, shrinking wildlife habitat and domestic livestock range.

Blue Weed (Echium vulgare)

Spreading readily on disturbed sites and along roadsides, this prickly, unpalatable plant becomes a problem when it crowds out native species. It's a prolific seed producer with each seed able to remain dormant for several years. If conditions are unfavorable, it can continue to grow as a rosette for several years before flowering, setting seed and dying.

Field Thistle (Cirsium arvense)

This invader from Russia can produce up to 10,000 seeds per plant each year. The seeds are dispersed and transported by animals, humans and water; remaining viable for as much as 21 years! Seedlings establish on recently disturbed ground and spread rapidly by creeping underground stems (rhizomes), whose perimeters can expand as much as 6 meters (20 feet) per year. This thistle replaces native grasses and plants by crowding them out or by producing a growth-inhibiting chemical in its roots.

Bull Thistle (Cirsium vulgare)

Spread by seeds, this plant invades disturbed areas and roadsides. Once established, it crowds out native plants.

Dalmatian Toadflax (Linaria dalmatica) and Common Toadflax (Linaria vulgaris)

Native to Europe, they were first introduced to North America in 1758 and were later used as garden ornamentals, leading to their further spread. Colonizing waste places, toadflax thrives on gravelly or sandy soils. They are difficult to control because of their extensive root systems and tremendous production of winged seeds. High toadflax densities can reduce native plant seed yields by one-third. They contain a glycoside that is poisonous to livestock but its effect on wild ungulates is unknown.

Leafy Spurge (Euphorbia esula)

This deep-rooted perennial (living more than two years) spreads by seed and the expansion of lateral roots from the parent plant. A tough plant, it lives in a wide variety of habitats from deserts to subarctic sites. It can also tolerate flooding for up to 4 months! It produces flowers and long lasting seeds from April to mid-August. The mature plant is poisonous to livestock.

Klamathweed (Hypericum perforatum)

Also known as goatweed or St. John's-wort, this perennial reproduces by seed and by spreading roots or rhizomes. It can cause photosensitization in livestock, a condition in which patches of white or light-coloured skin become seriously sunburned under normal exposure to sunlight.

Hound's Tongue (Cynoglossum officinale) and Burdock (Arctium minus)

These spread by many hooked seeds or burs which are easily transported on fur or clothing, making them a nuisance plant of disturbed areas along roadsides and trails.

While the invasion of exotic plants is not a recent phenomenon, as people travel more and disturb more landscape, populations of non-native plants increase. It would be impossible and too costly to completely eliminate all non-native plants from the park. Each year, more seeds are brought into Waterton Park - aided by wind, water, animals, vehicles and human activity. Invasions of non-native plants will be an ongoing management concern.

Throughout the summer months you may see park employees involved in exotic plant control. You may see signs along the road saying 'weed control in progress'. Curious visitors asking questions about these signs provide us a great opportunity to talk about these invaders.


Achuff, Peter L., Arthur W. Bailey and Lawrence M. Brusnyk. 1990. Non-Native Plant Management In Western Region National and Historic Parks: Issue Analysis and Recommendations. D.A. Westworth & Associates Ltd. Edmonton, Alberta.

Alex, J.F. and C.M. Switzer. Ontario Weeds. Ontario Agricultural College, University of Guelph, Guelph, Ontario, Canada.

Coleman, Mary. 1994. Non-Native Plant Management Strategy For Waterton Lakes National Park. Warden Service, Waterton Lakes National Park, Waterton, Alberta.

Kuijt, Job. 1982. A Flora of Waterton Lakes National Park. The University of Alberta Press, Edmonton, Alberta, Canada.

Mulligan, A. Gerald. 1976. Common Weeds of Canada. McClelland and Steward with The Department of Agriculture and the Publishing Centre, Supply and Services, Canada.

Wilson, E.O. and Frances M. Peter. 1989. Biodiversity. National Academy Press, Washington, D.C.

Personal Communications

Peter L. Achuff. 1995. Vegetation Ecologist, Waterton Lakes National Park, Waterton, Alberta.
Mary Coleman. 1995. Warden Service, Waterton Lakes National Park, Waterton, Alberta.

If you would like more information please mail to:
Parks Canada 
Waterton Park, Alberta,
Canada. TOK 2MO
Phone 403-859-2224


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