Control of Canadian Thistle

From a site on "mint" production

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

It seems all you can really do is till frequently (no good for pasture), spray in the spring when the shoots are young and try to spot spray through the growing season to starve the creeping root.