Invasive Plant Management

Invasive plants are not native to an area or ecosystem, and their introduction causes or is likely to cause economic or ecological harm, or harm to human health.    

Riparian areas are particularly sensitive to the threat of invasive species. Their rich soil and high moisture content provide good conditions for many species, particularly invasive ones. In addition, seasonal flooding of these areas distributes non-native seeds along riparian shores. Riparian areas can also be damaged by herbicides typically used to control invasive species in areas located further upland. In addition, heavily sloped areas are difficult to mow or till for weed control.   

The presence of invasive plants in riparian areas reduces their health rating. It is often one the main reasons for getting the rating of “healthy with problems”. Invasive plant species introduce undesirable ecological disturbances to ecosystems. They usually spread very fast and occupy extensive areas with dense stands that are difficult to eradicate. In riparian areas, the use of chemical control is not recommended due to contamination risks, and possible adverse effects on water quality, fish and other aquatic life, amphibians, birds, and other wildlife. In a semi-arid climate as in the SEAWA watershed, riparian areas are often limited to soil moistened by the adjacent waterbody. Native vegetation that is dependent on this habitat gets overwhelmed by invasive plants. Riparian areas then inadvertently become invasive plant propagation areas, and their seeds disperse by wind and moving water to other areas. Many native wildlife species depend on native plants for food and habitat.

Information on invasive plants in Alberta can be found here:

Some invasive plants are considered weeds under law, and must be eradicated or controlled. Under the provincial Weed Control Act, two designations exist that impose a legal obligation to control an invasive plant. These designations are Noxious, and Prohibited Noxious. Here is a publication that outlines these designations, and offers information for identification of such plants:  Alberta invasive plant identification guide: prohibited noxious and noxious

A list of provincially regulated weeds can be found here:

Not all invasive plants are legally considered weeds at the provincial or national level. Invasive plants may be found locally (example: Southeastern Alberta) or within a specific eco-region (example: grasslands) or an ecosystem (example: riparian areas or wetlands or forests). Under the Alberta Weed Control Act, municipal governments have the ability to legally control weeds at a local level through bylaws and regulations. Municipalities should seek the help of volunteers and stewardship groups when issues are at the local level.

Click here for information about weeds in the City of Medicine Hat.

Interested in what makes a plant a weed? Click here:  

Controlling Invasive Species

SEAWA has been conducting applied research into cultural methods to control riparian invasive plants, as well as developing management techniques for the species outlined below.

Research Team:

Principal Investigator - Marilou Montemayor

Project Team Members:

  • 2021 - Alexi Nelson, Ben White, Ian Mahon, Sydney Taplin
  • 2020 - Ben White, Alexi Nelson, Hannah Sabatier, Chris Beck
  • 2019 - Brooklyn Neubeker, Amy Adams, Ben White
  • 2018 - Natasha Rogers, Seline Solis

Russian Olive

An extensive Russian olive infestation in Medicine Hat, AB

Russian olive trees (Elaeagnus angustifolia) are of foreign origin and were introduced to the prairies decades ago, for shelterbelt planting and ornamental purposes. Many people like these trees because they do not require much care and are aesthetically pleasing. Unfortunately, they have exhibited invasive characteristics when grown in places that have a semi-arid climate, alkaline soils, non-saline to moderately saline soils, and open grassland landscapes, such as in Southeastern AB, including the SEAWA watershed and Medicine Hat area. In places with a different climate and soils, they are not a concern.

Russian olive populations in the SEAWA watershed have expanded so much that they can now be considered locally invasive, and must be controlled. They can be seen established along streambanks, roadside ditches, road rights-of-way, around ponds, on coulees, shallow depressions, narrow crevices, in parks, residential yards, and any other spot where their fruits have been spread. They choke out native shrubs and trees particularly in the few green natural areas of the semi-arid grassland. Additional information on the negative impacts of Russian olive trees can be found here. 

Another excellent resource on Russian olive is available here.

Physical removal is an effective method to control Russian olive

SEAWA Recommendation:

We invite people to help control Russian olive by not planting it in the first place. In riparian areas, avoid the use of chemicals. Trim the branches of seedling and saplings using a pair of loppers, then uproot them using a garden shovel. Put back the disturbed soil evenly to prevent resprouting. If you cut down grown trees, wrap the stumps with heavy duty garbage bags to prevent resprouting. For more information on the results of this experiment, click here.  

A thorough description of the Russian olive tree is found here:

A scientific paper on this local issue has also been published:

C.M. Pearce and D.G. Smith. 2001. Plains cottonwood’s last stand: can it survive invasion of Russian olive onto the Milk River, Montana floodplain? Environmental Management 28(4):623-637.

Leafy Spurge

A significant leafy spurge infestation along Seven Persons Creek

Leafy spurge (Euphorbia esula) is a perennial, noxious weed that is native to much of Asia and Europe. It is difficult to eradicate due to its rhizomatous growth habit, allowing it to continuously re-sprout from its extensive root system that can grow eight metres deep and over four metres laterally per year. This aggressive spread contributes to the severe displacement of native plants in riparian areas and rangelands. In addition, the plant exudes a milky latex when broken which is unpalatable and irritable to cattle, resulting in a reduction of productive land for grazing and agricultural use.

A detailed description of leafy spurge is available here:

Since 2018, SEAWA has been conducting experiments on the control of leafy spurge without the use of chemicals. Currently, four experiments are in place. They are as follows:

  • Digging leafy spurge from an area and planting native grasses or wolf willow shrubs
  • Digging leafy spurge from an area and planting silver sagebrush
  • Covering leafy spurge with plywood for a season and planting sagebrush the following season
  • Monthly mowing of leafy spurge (May - August)

Recent leafy spurge experiment results are available below:

Leafy Spurge Cultural Control Results (2019 Season) (2.33 MB)

Further information on leafy spurge is available here:

Reed Canary Grass

Dense stands of reed canary grass along both sides of Seven Persons Creek

Native or not, and notwithstanding its other practical uses (forage for example), reed canary grass’ (Phalaris arundinacea) invasive characteristics make its dense and tall establishment in riparian areas undesirable. Its aggressive growth forces out other grasses and desirable vegetation, thus reducing plant diversity, and the diversity of wildlife reliant on these plants. Dense stands of reed canary grass use water that should be available to other riparian vegetation and can reduce the stream flow itself.

Reed canary grass growing as the understory to a stand of sandbar willows

Reed canary grass grows extensively through its roots and rhizomes, making it very competitive. However, these below-ground parts grow shallow, usually in the top 10-15 cm of the soil surface, making them easy to dig out. Planting a site with shrubs that grow close together, such as golden currant (Ribes aureum), is helpful for preventing regrowth because reed canary grass is not tolerant of shade, and needs lots of space and light to grow.

  For more information about reed canary grass, see the following resources:    

Bittersweet Nightshade

Bittersweet nightshade (Solanum dulcamara) is a vining weed related to tomatoes and potatoes. It grows in moist soils and can develop an extensive root system, while climbing native vegetation and choking it out. It can be identified by its distinctly shaped leaves, as well as its purple flowers, and clusters of small tomato-like fruits. When the ripe fruits are crushed they emit a strong, sickly-sweet odour. In the fall, the leaves change from a bright green to a deep purple.

Bittersweet nightshade is toxic to humans and animals. Always wear gloves if you choose to handle this plant, and do not ingest any part of it.

SEAWA is experimenting with non-chemical controls for this invasive plant.

Japanese Brome

Japanese brome (Bromus japonicus) is a noxious weed, related to other invasive species such as cheatgrass (downy brome). It is a medium to tall grass that develops large, distinct seed heads with long awns that often appear 'kinked'. It is a problem in agricultural and urban areas, as it prefers disturbed soils. It is very competitive, and can push out desirable native and introduced forages. The long awns of this grass make it unpalatable to livestock. Its primary growing season is spring to early summer, with little to no productivity from mid-summer on.

SEAWA is experimenting with non-chemical controls for this invasive plant.