Thistle

Weed Identification Guide
Types of Weeds

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

For example, successful biological control Musk Thistle, a problem plant in the Southeast, has been documented by released two types of weevils, the rosette weevil and the thistle head weevil.

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

A variety of butterfly species nectar on thistle, especially fritillary butterflies.

Larvae of the Painted Lady Butterfly are especially fond of Canada Thistle. Unfortunately their diet is not sufficient to keep the plant in check for more than half of season, as the the ceterpillars develop.

Thistle