WeedWise Program

Think twice before killing those thistles: Thistle Identification

Fewleaf thistle (Cirsium remotifoilum), a native thistle

Thistles are one of the most common weed problems that property owners deal with in Clackamas County.  Many of the thistles we encounter are invasive and can grow to dominate a property.  But did you know there are at least four thistles that are not only non-invasive, but native to western Oregon?  These native thistles are not only well-behaved, but are beneficial to the health of our working lands and natural areas.

Native Thistles are Important!

Native thistle provide important habitat and food sources for native fauna. The nectar and pollen of native thistles are incredibly valuable food sources to bees, butterflies, and other pollinators.  Many insects feed on the leaves, stems, flowers and seeds, while some songbirds also feed on thistle seeds. These nectar sources help support pollinators year-round, and can help to increase yields for many valuable crops.

The presence of native thistles also makes it harder for the aggressive non-native thistles to invade an area.  Native thistles help to support healthy populations of beneficial insects that will also consume non-native thistles. Our native thistles also remain in balance with other native plants and do not aggressively displace other plants.  Despite their benefits, native thistles are either knowingly or unknowingly killed because it is assumed they will become a big problem, or simply because they have spines, or are considered “weeds”.  Because of this, many native thistles are in danger of being completely eradicated from our area.


The Problem with Non-Native Thistles

Milk thistle (Silybum marianum) infestation in Clackamas County

On the other hand, non-native thistles can overrun an area and displace native plants, reduce agricultural yield, and create problems for grazing animals when they infest a field or pasture. They also cost a lot of money and time to control.  One of the struggles with controlling thistles is that it can be difficult to distinguish between them.  We are here to help!


General Thistle Characteristics

Thistles are members of the sunflower family (Asteraceae) and can be annual, biennal, or perennial.  True thistles have spines along the leaf margins and their flower heads are generally pink-purple and surrounded by bracts that are typically spiny. Below are some questions to help you determine whether your thistle is native or not:

If the answer to all of these questions is “NO”, it is likely you have a native thistle!

Native Thistles

Non-native Invasive Thistles

Want to dig deeper?

There are several other species that we did not discuss here, either because they aren’t known to be in western Oregon, or because they aren’t true thistles… and this post is already long enough! Follow the links below for more information about these “thistle-like” counterparts .