Canada thistle

Description:

Perennial thistle native to Eurasia. Flowers from July - August in the northeast. Highly invasive. Link to flowers: http://en.wikipedia.org/wiki/File:Cirsiu...

Habitat:

Facultative, but most injurious in wetlands.

Notes:

Canada thistle seeds prolifically and relies on wind dispersion for spread. It also spreads by rhizomes, adding another element of difficulty to control. This makes digging an ineffective method and cutting the flower heads a stop-gap measure at best. The best method I have found is spraying triclopyr at flowering time, while spraying the basal rosettes seems to have very little effect. Unfortunately, there is a very short window between flower bud set and seed maturation. This makes timing the herbicide application critical.