Watershed's Most Wanted: Invasive Weeds

Over the summer, Outreach Assistants Dylan and Dorothy have been helping out at many weed pulls across the watershed and have learned a lot about the detrimental effects that invasive species can have in an ecosystem. So far 196 bags of weeds have been removed this summer, including 160 bags coming from the awesome volunteers at the Blueweed Blitz. Below are some of the weeds that our Dynamic Duo has most commonly encountered this summer.

Knapweed (Centaurea spp.)


ID: White or purple flowers depending on species. The flower head is conically-shaped with triangle floral bracts; each stem end has a flower. Leaves look to be a gray-green due to being covered in fine hairs; they also have an alternate growth pattern on the stem.

IMPACTS ON THE WATERSHED: Spotted knapweed produces a chemical called catechin, which spreads throughout the immediate soil and prevents any plants in the area  from establishing. Because knapweed communities are much less dense than native plant communities, they don't hold the soil together as well; extra runoff and erosion can occur around knapweed communities, leading to increased sediment in waterways.

REMOVAL: The plant must be pulled from the soil in its entirety, because it can regenerate from a root fragment. The weed must also be pulled before it goes to seed and spreads some of the 140,000 seeds it produces. Although livestock will graze knapweed, the seeds will stay viable in their droppings and spread further. There are chemical sprays as well as 12 biological control agents including moths, flies, weevils, and a rust.


Leafy Spurge (Euphorbia esula)


ID: Green-yellow heart-shaped floral leaves lacking petals. Flowers look a little like an upside-down umbrella or dish and saucer. Smooth, hairless leaves in alternate or spiral arrangement.

IMPACTS ON WATERSHED: Leafy spurge contains a toxic latex sap that can cause skin irritation in humans and has been known to kill cattle. Leafy spurge is also a fierce competitor, often choking out areas of native plants.  

REMOVAL: Hand-pulling and mowing are ineffective on leafy spurge due to its large, complex root system. It also contains a toxic compound that irritates human skin; however sheep and goats are not affected by it and therefore will graze on leafy spurge. The land can be regularly cultivated for 2-3 years to destroy the plant's root system and keep it from coming back. There are various chemicals that can be used to control leafy spurge. Seven species of insects are used as bio-controls for leafy spurge; these include 5 beetles and 2 moths, which consume either the roots, shoots, or leaves of the plant.


Blueweed (Echium vulgare)


ID: Purple or blue flowers branching off of the tall, erect stem. Long, rounded leaves appear on the lower parts of the stalk. The entire plant is covered in fine bristles, including the stalkless first-year rosettes. Its root is a large black taproot that can measure up to 2 feet in length. The plant appears to have many small black spots along the stalk, which are swollen bristle bases.

IMPACTS ON WATERSHED: Blueweed contains a compound that makes it poisonous to cattle and horses; while livestock rarely graze on this plant, it may be consumed if other forage plants in the area are in poor health. Blueweed’s large tap root allows it to out-compete native vegetation, especially in nutrient-poor and disturbed soils.

REMOVAL: To effectively remove blueweed, you must dig out the tap root and remove the entire plant. It can not be grazed due to poisonous alkaloids, and mowing will not kill the plant and may only restrict seed production. The chemical Mecroprop-p (with 2,4D and Dicamba) is used to spray blueweed, especially the difficult-to-pull rosettes.


Outreach Assistant Dylan is happy to have pulled up these two GIANT blueweed plants!

Outreach Assistant Dylan is happy to have pulled up these two GIANT blueweed plants!

Hawkweed (Hieracium spp.)


ID: Yellow dandelion-like terminal flowers on a tall leafless stem, with a rosette at the base of the plant containing narrow, hairy leaves.

IMPACTS ON WATERSHED: Hawkweed is a very aggressive competitor in the plant community because it has multiple forms of reproduction including seed, runners, and root buds. This high-speed spreading plant can quickly form mats that choke out native plants and lower biodiversity.  

REMOVAL: Small communities of hawkweed can be dug out yearly by hand, ensuring that both the roots and the runners get pulled along with the plant. Regular mowing or grazing can decrease spread by seed; however, mowing can encourage spread by the runners and can actually cause the plant complex to grow more. The most effective way to control hawkweed, once it has established, is by selectively spraying the plant communities with pesticide.  


Toadflax (Linaria spp.)


ID: The stem of toadflax species has numerous long, narrow leaves and bright yellow flowers with an orange spot on the lower lip. The flowers are arranged very close to the stem in a dense linear cluster. The flowers have an unpleasant odour when in bloom.

IMPACTS ON WATERSHED: Toadflax plants are known for their colony-producing creeping roots. Toadflax roots can be almost 4 m long, aiding the weed in choking out native plants. Colonies of a single species like toadflax lower the biodiversity of the ecosystem, which can have adverse effects on plant and wildlife populations.

REMOVAL: Hand pulling can be effective if maintained for up to 10 years. Seeds can be viable for up to 8 years in the soil. The small stem-mining weevil Mecinus janthius is used as a biocontrol for toadflax. Toadflax can also be controlled with a large array of different chemicals and pesticides.



Keep invasive species out of your yard (and our watershed) by planting native species - check out OWC's newest edition of “50 Best Plants for Prairie Urban Gardens in Southern Alberta,” available at our office for a minimum $2 donation.

You can also do your part to keep invasive species in check by volunteering at an upcoming weed pull, such as the August 15 Sartoris Road weedpull with Castle Crown Wilderness Coalition.

The “Dream Team” from the 16th annual Blueweed Blitz. We pulled 30 bags of blueweed in our area in 5 hours, winning us the Bent Back Award.

The “Dream Team” from the 16th annual Blueweed Blitz. We pulled 30 bags of blueweed in our area in 5 hours, winning us the Bent Back Award.