Biology and Management of Canada Thistle

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Registered Herbicides

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General Description:

• grows upright with grooved stems 1-4 feet tall.
• deep and extensive root system with main roots running horizontally 15 or more feet.
• roots may extend a meter under the soil surface.
• alternate leaves clasp the stem (no petioles) are cut deeply into irregular lobes with spiny edges.
• Colony forming perennial that reproduces by seeds and root buds on spreading lateral roots.
• Seedling: Cotyledons are oval and somewhat fleshy. Germinating root buds do not have cotyledons. First true leaves are ovate with soft spines.
• Juvenile: Early spring growth appears as rosettes with irregularly lobed spiny tipped leaves. The degree of lobing, length of spines and the amount of pubescence varies between plant.
• Mature: Has upright stems which can be woody and grow 30cm to 2m tall. Stems are thin and green without spiny wings and are highly branched above. Leaves are alternate and lance shaped with irregular lobes, and spines. Mature plants have deep and extensive horizontal roots.
• Flower: Flowers are unisexual with male and female flowers on separate plants. Flowers are small heads (½ to ¾ inches) of pink to purple disk flowers, with spineless bracts.

Life cycle: perennial

• reproduces asexually from rhizamatous roots - any part of the root system may give rise to new plants.
• also reproduces from wind-blown seed which germinate late spring or early fall.
• brown seeds measure 1/8 inch, are long, generally flattened and curved, and finely grooved lengthwise.
• seeds may retain viability 4 + years in the soil.
• new seedlings develop into rosettes.
• purple flowers produced July - August.
• male and female flowers are on separate plants (dioecious) and seed production requires both male and female plants be present.

Habitat/ Distribution:

• a problem in both perennial and annual crops, rangeland, and minimum tillage systems.
• widely distributed throughout southern Canada and northern United States.
• native to southeastern Eurasia and northern  Africa - introduced as crop contaminant during 18th century.

Impact on Yield:

• Once established Canada thistle can outcompete and overtop mint from May until harvest.
• The smothering effect of 3-ft high Canada thistle over 1-ft high mint, combined with competition for water and nutrients, results in severe mint oil yield reductions.

• Competition from Canada thistle can result in an 80% reduction in mint oil yield and a loss of 33 lbs of mint oil per acre (Ian Heap).

Herbicide Control Notes:

• Basagran can be applied in summer without significant crop injury, but might not be as effective as fall application of Stinger.

Biological Control

Ceutorhynchus litura (crown/root weevil)
    Distribution within host range: limited sites
    Infestation of host: light (>10%)
   Control ability on seeds and/or plant density: Good
    Availability for redistribution: Limited
 
Larinus planus (seed head weevil)
    Distribution within host range:  limited sites
    Infestation of host:  heavy (> 70%)
    Control ability on seeds and/or plant density:  fair
    Availability for redistribution:  mass collections

Rhinocyllus conicus (seed head weevil)
    Distribution within host range:  widespread
    Infestation of host:  heavy (> 70%)
    Control ability on seeds and/or plant density: fair
    Availability for redistribution:  mass collections

Urophora cardui (stem gall fly)
    Distribution within host range: widespread
    Infestation of host:  heavy (> 70%)
    Control ability on seeds and/or plant density: fair
    Availability for redistribution: mass collections