Prickly sowthistle and annual sowthistle are quite similar in appearance.  The basal lobes of prickly sowthistle (left) curl under and look like an ear. With annual sowthistle these lobes do not curl under to the extent that prickly sowthistle do. Its foliage clasps the stem, and is arranged alternately along the stem.

 

 

                           Flowers occur in clusters and bloom from early summer through fall. Seed heads are pure white and somewhat spherical.

 

 

Perennial sow thistle: the comes-back-every-year thistle. 
Perennial sow thistle ranks among the top 15 most abundant weeds in western Canada, and is also known as creeping sow thistle, field sow thistle, field milk thistle, gutweed, swine thistle and marsh thistle. It is a tall creeping perennial with horizontal underground roots that can go as deep as 5 to 10 feet.

Perennial sow thistle reproduces vegetatively from the root, which is easily broken and gives rise to new buds (a root section with a well developed bud less than 1 inch in length can reproduce). This accounts for the persistence of perennial sow thistle under cultivation. It also reproduces by seed. An individual plant can produce up to 4,000 seeds that may lie dormant in the ground for several years.

Perennial sow thistle seeds are easily carried by the wind and also cling to animals, machinery and clothing. Plants are frequently found in patches in regions of the prairies that receive higher precipitation. The weed prefers moist, fertile soils and is more of a problem in the dark brown and black soils than in the brown soil zone.

Even a mild infestation can drastically reduce crop yield. A North Dakota study showed that a density of 15 plants per square yard reduced yields by 14% in spring wheat. In addition, the presence of sow thistle in a crop can slow harvest. Green matter in the grain can increase drying costs and dockage. In-crop weed control, as part of an integrated weed management program, can help prevent losses.

Annual sow thistle: the newly-seeded-where-you-least-expect-it thistle. 
Annual sow thistle has two forms: annual sow thistle (Sonchus asper) and spiny annual sow thistle (Sonchus oleraceus). Unlike perennial sow thistle, both forms of annual sow thistle have a taproot, and reproduce only by seed. Both forms are an erect, annual or winter annual plant with stems 8" to 5 feet high, roughly pentagonal in cross section. Spiny annual sow thistle differs from annual sow thistle in that leaf margins are flatter and weakly spiny, and seeds are wider.

Both annual forms have generally been well controlled by tillage and existing herbicides. However, they can also grow as winter annuals, making them more of a problem when tillage is reduced, or when pulse and oilseed stubbles are not tilled in the fall. The development of herbicide resistance has become an issue with spiny annual sow thistle.

Like most members of the Compositae plant family (which includes dandelion), sow thistles have varying germination success depending on the seed position on the soil surface. Flat-sided seeds like those of the sow thistle have an advantage, because the attachment scar, which is the area most permeable to water, contacts the soil when the seed is horizontal.

Light also helps annual sow thistle germination. In conventional tillage farming, many annual sow thistle seeds are buried and do not germinate well. But in reduced tillage, where seeds are at the surface, the increased light helps stimulate germination.

Control Options 
To effectively control sow thistle, growers must first distinguish between annual and perennial forms. Herbicides registered for control of one form may not control the other. Tillage helps control annual sow thistle, and is generally more effective against perennial sow thistle than Canada thistle. However, the deep penetrating root system of perennial sow thistle requires frequent deep tillage, which is undesirable due to cost and risk of soil erosion.

Pre-harvest glyphosate is an option, but it is generally less effective on perennial sow thistle than on other perennials, such as Canada thistle. Dow AgroSciences offers Vantage and Vantage Plus glyphosate options for sow thistle control.

In-crop herbicide treatment may be the best option for controlling both annual and perennial sow thistle. However, spraying in-crop herbicides too early can reduce perennial sow thistle control because shoots emerge over a long period of time (longer than Canada thistle). Delaying herbicide application until close to the end of the recommended application window allows the maximum number of thistle shoots to emerge and be controlled.

Some herbicide control options for perennial and/or annual sow thistle include:

·    Curtail M* and Prestige* are labeled in cereals for post-emergent control of both annual and perennial sow thistle. 

·    Dow AgroSciences products containing the new active ingredient florasulam are effective for post-emergent control of annual and perennial sow thistle. 

·    Clopyralid, the active ingredient in Lontrel*, provides top growth control of perennial sow thistle. 

·    Dow AgroSciences has also recently received registration for Eclipse, a product developed to broaden the weed control spectrum for Roundup Ready® canola. Eclipse controls perennial sow thistle (along with dandelion, wild buckwheat and Canada thistle) better than a straight glyphosate application.