Annual Sow Thistle

Invasive:

Prickly Sow-Thistle (Sonchus asper)

This spiny weed grows avidly among disturbed soils, dotting roadsides, and waste disposal sites. Although its yellow flowers may cause confusion, Prickly Sow-Thistle’s large, toothed basal leaves and annual root formation act as key distinguishing features.

 

 

 

 

Perennial Sow Thistle (Sonchus arvensis)

Unlike its annual counterpart, Perennial Sow Thistle has an extensive system of creeping roots and shoots (rhizomes) that aids its distribution and makes it more difficult to remove.

 

 

 

 

 

Wall Lettuce (Lactuca muralis)

This delicate, perennial weed’s fibrous root system and ivy-like tipped leaves are key distinguishing features.

Originating from Europe, this prickly weed is a member of the Aster Family.

Annual Sow Thistle thrives in fertile, moist soils and full sun. Often times, you can spot this thorny invasive growing along roadsides, logged areas, marshlands, agricultural fields, rangelands, gravel pits, and even gardens!

As its name might imply, this troublesome plant is an annual, dying off entirely each fall and creating new seedlings in the spring. It spreads exclusively by seed, eliminating any chance of vegetative spreading.

Annual Sow Thistle’s main mode of transportation is the wind, which easily catches its light pappus (cluster of hairs on the seeds) and carries the seeds to new areas. There, they can germinate at any point throughout the growing season, and remain viable in the soil for years. It’s important to note, though, that these lightweight seeds can also spread by hitching a ride on humans, animals, and water flow.

Annual Sow Thistle is currently under STRATEGIC CONTROL throughout the Sea to Sky Corridor, so the best approach to controlling its spread is by PREVENTION.

 

Learn to identify Annual Sow Thistle: use the images presented in this profile page to learn how to identify this thorny invasive!

What to do if you spot it: You can report any Annual Sow Thistle sighting by clicking here.

 

DO:

• Regularly monitor properties for weed infestations.
• Plant weed-free crop seed for gardens and crop fields.
• Ensure soil and gravel are uncontaminated before transport.
• Check wildflower mixes to ensure that they do not contain Annual Sow Thistle.
• Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.

 

DO NOT:

• Do not mow weeds that have gone to seed set.
• Do not unload, park or store equipment or vehicles in infested areas; remove plant material from any equipment, vehicles, or clothing used in such areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
• Do not plant Annual Sow Thistle in a garden, no matter how well-contained its enclosure may seem.
• Do not move soil that has been contaminated with Annual Sow Thistle.

 

Control

Mechanical

Hand-pull small infestations; for larger infestations, mowing repeatedly before seed set will help control Annual Sow Thistle’s spread, but mowing height must be lower than 20 cm to prevent regrowth.

In order to ensure eradication, the taproot must be removed, as re-sprouting can occur if left in the soil.

Chemical

Before flowers bloom, selective herbicides can be applied to prevent seed production; meanwhile foliar applications of MCPA and 2,4-D offer effective post-emergence control.

We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Biological

Research is ongoing in Australia about a rust fungus (Miyagia pseudosphaeria) and an eriophyid mite species. Although not yet approved for release in BC, a gall midge species (Cystiphora sonchi) also shows potential.

Cultural

Annual Sow Thistle is palatable for cattle and sheet, and intensive grazing can suppress infestations on some sites by preventing seed set. Once the grazing has weakened the plants, other control methods can be more effectively used.