Thistle

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Thistle is a generic name given to a couple of plant genera in the family Asteraceae (sunflower family). Some thistle are native plants, some are introduced plants.

Most, if not all of the introduced thistle are considered invasive. The National Park Service, for example, lists five different thistles as invasive species that displaces native plants: Musk thistle; Italian thistle; Canada thistle; Bull thistle; Milk thistle.

Their roots can spread up to fifteen feet in diameter and six feet deep, making removal a challenge.

Identifying invasive species helps with eradication because different biological controls are suited to specific species.

The top picture shows the Canada Thistle, a species common across the United States.

© 2005-2007 Patricia A. Michaels