Sow Thistle, Perennial

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Pest Information
Common Name: Sow Thistle, Perennial
Family Name: Asteraceae
Latin Name: Sonchus arvensis L.
Other Names: Creeping sow thistle, field sow thistle, field milk thistle, gutweed, swine thistle and marsh thistle.
Provincial Designation: Noxious
Seed Act Designation: Primary noxious
Life Cycle: Perennial
Mode of Spread: Seed, Vegetative

Detailed Information
Provincial Situation: Perennial sow thistle ranks among the top 20 most abundant weeds in western Canada. The relative abundance and ranking, along with frequency, density of this weed in various agricultural eco-regions of Alberta is listed in Table 1.
Table 1. Distribution of perennial sow-thistle in Alberta
Agricultural EcoregionRankFrequency Uniformity2Density3Relative Abundance
Mixed Grassland370.810.60.50.4
Moist Mixed Grassland204.590.61.9
Fescue Grassland-----
Aspen Parkland1218.
Boreal Transition1920.413.51.25.8
Peace Lowland1521.
Alberta (All regions)1912.

1 Frequency = Percent of fields in which perennial sow-thistle occurred
2 Uniformity = percent of quadrates in which perennial sow-thistle occurred
3 Density = Average number of perennial sow thistle in occurrence fields.
Source: Leeson et al (2005). Prairie Weed Surveys. Cereal, Oilseed and Pulse Crops 1970s to 2000s.
Description: Perennial sow thistle is deep-rooted perennial spread by wind-dispersed seeds and fleshy, white, horizontal creeping roots. One or few stems grow 0.6 to 1.5 m tall from a basal rosette of prickly leaves. The stems are hollow, erect, containing milky latex and branched at the top. Leaves are alternate with slightly toothed margins. Flower heads are found in clusters at the end of the stem branches. Flower heads are entirely rayed with yellow petals.
Growth and Development (Life Cycle): Emergence: Perennial sow-thistle is a shallow germinator. Most seedling emergence from depths of less than 0.5 cm. Emergence will not occur if the seeds are buried more than 3 cm deep. Seedling emerges in the spring growing from underground roots or germinating from seeds. Seedlings survival is best under high moisture conditions and where heavy residue exists, such as zero-tilled fields as compared to conventionally tilled fields.
Seed Production: A single plant can produce up to 4.000 seeds.
Seed germination: Seed germinate at the shallow depths, later in the spring, after soil temperature reaches 20 to 25 degree C. They continue to germinate from spring to fall when conditions are favorable.
Dispersal: The seed, attached to a pappus, is dispersed by wind (about 10 m in a 16 km/ hr wind) and hooked pappus tips catch in fabric and animal fur.
Where it Grows (Habitat & Ecology): Perennial sow thistle occurs throughout Alberta in cultivated fields, pastures, meadows, woodland, waste places, roadsides, gardens and occasionally in lawns.
Reproduction (Dispersal): Perennial sow-thistle seed germinates when the soil has warmed in the spring, usually mid-to late May in Alberta. The seedling grows slowly for about two weeks until the leaves are about 3 cm long and then forms a rosette. First year rosettes form vertical roots up to 2 m deep, produce vegetative buds from depths up to 50 cm and horizontal roots with a spread of 60-100 cm. Bolting usually occurs in the second year when the rosette has 12-15 leaves. When cut during cultivation, root pieces as small as 1 cm can produce a flowering plant within a year.
Economic Importance (Beneficial Aspects): Detrimental: It is a noxious weed in many provinces, including Alberta. Sow thistle appears to accumulate potassium and may compete strongly for this element with alfalfa, winter wheat, barley, canola and sugar beets, causing significant yield losses in these crops. Moreover, green matter in the grain can increase drying costs and dockage. Perennial sow thistle is also a host of several economically important plant pests.
Beneficial:Perennial sow thistle excellent feed qualities, making it acceptable livestock feed. It is also excellent feed for rabbits and other foraging animals. The plant may have potential for use in oil or hydrocarbon production, since most of the latex is composed of oil. The plant is also a source of pentacyclic triterpenes, which may have use in the pharmaceutical industry
Origin: Perennial sow thistle is native to Europe, western Asia, and Iceland. The first North American report was from Pennsylvania in 1814. It first spread from Pennsylvania to and along the coast before spreading inland. It was reported from Manitoba in 1919.
Flowering: Flowering timing: early July until frost.
Flower color: yellow
Yield Losses: Even a mild infestation can drastically reduce crop yield. Researcher at Agriculture Canada in Regina has shown that 20 sow-thistle rosette per square meter have a potential to reduce canola yield by 27%. Similarly, a North Dakota study has shown that a density of 15 plants per square meter can reduce spring wheat yield by 15% 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.
How to Control: Controlling perennial sow thistle requires a management strategy that reduces weed competition, prevent seed production and exhaust the root reserves on the established sites.
In direct seeding systems, tillage is of no value, because it contributes to soil erosion. Moreover, soil disturbance during seeding and/or fall fertilizing will break up the fleshy horizontal creeping roots. Each fragmented root pieces with buds have the ability to produce new plants. In intensive tillage systems, tillage may be used to control this weed. In order for tillage to be effective, it is necessary to cultivate to a depth of 30 cm when the plants are in the seven- to nine-leaf stage followed by repeated cultivations at 2- to 4-week intervals throughout the summer. If tillage is not done right, it may increase the abundance of this weed due to spreading of rootstocks. The success of tillage depends on the depth of burial and degree of root breakage. If the root pieces are left on the soil surface, they will dry up, and if buried more than 30 cm deep, less than 10% will emerge.
Mowing:Mowing is less effective than tillage
Grazing:Grazing infested land effectively reduce stands because weed is palatable to sheep and cattle but it is not their preferred fodder.
Crop competition:Increase the competitive ability of crops:
  • Grow competitive crops such as perennial grasses
  • Drill fertilizer with seed or band fertilizer to make it more available to crop
  • Plant solid crop stands instead of row crops
  • Use higher seeding rate for planting
  • Plant tall varieties rather than semi-dwarf varieties

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    Last Modified: December 20, 2011

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