Perennial Sow Thistle

Sonchus arvensis

Description 4

Despite the name, sow thistles more closely resemble giant dandelions than they do the true thistles. Perennial Sow Thistle is considered a noxious weed throughout most of its range including BC, Alberta and Alaska. This is an invasive species able to spread long distances by wind-blown seed. It is perennial with rhizomes that can grow to 10 feet and are very difficult, if not impossible, to remove by hand unless caught at a very early stage of development. It can grow up to 2 m. It is particularly a problem for agricultural producers. In natural areas it has been known to invade beaches and lake shores.

Range in Yukon 4

Primarily in the Whitehorse area as far north as Carmacks but has been reported from Destruction Bay, Johnson’s Crossing and the Kotaneelee gas plant. In recent years it has begun to spread rapidly along highway corridors.

Similar Species 4

Prickly sow thistle (Sonchus asper) is known only from Haines Junction, Whitehorse and the La Biche River. This is an annual plant with a short taproot. Narrow-leaved Hawksbeard (Crepis tectorum) is another annual plant with short taproots. It occurs throughout Yukon.

Ecological Impact 4

Perennial Sow Thistle may modify or retard the successional establishment of native species. At high densities in can dramatically reduce water resources and possibly decrease native plant diversity.

Control 4

Because the root system is usually well developed, it is almost impossible to dig out. Therefore perennial plants require depletion of nutrient reserves in the root system, prevention of seed production and prevention of dispersal. If roots are cut or broken off new plants may sprout. Mowing or cutting to reduce root reserves and seed production should be done a few times a season for several years. When hand pulling use a shovel and remove as much of the root as possible. When cut, flowering plants will continue to produce seeds. Plant material should be put in a clear plastic bag and placed in the sun until the roots and seeds are cooked.

For more information visit: Yukon Invasive Species Council

Sources and Credits

  1. (c) Frank Mayfield, some rights reserved (CC BY-SA), http://www.flickr.com/photos/33397993@N05/5320745981
  2. (c) Donald Hobern, some rights reserved (CC BY), https://www.flickr.com/photos/dhobern/19924016036/
  3. (c) --Tico--, some rights reserved (CC BY-NC-ND), https://www.flickr.com/photos/tico_bassie/4000853000/
  4. (c) L.G. Johanson, some rights reserved (CC BY-SA)

More Info

Range Map

iNaturalist.ca Map

Plant Daisy Family (Asteraceae)
Color yellow