Beyond the Prickles: Distinguish Native from Invasive Thistles Before Enacting

By Micah Most

This time of year there are windrows to bale, county fairs and rodeos to attend, pastures to check and livestock breeding decisions to make. The last thing anyone wants to see as they hustle around their working lands are the characteristic purple and white plumes marking the beginning of a potential thistle infestation. 

Thistles are in the sunflower family (Asteraceae), with gray or green foliage and flowering heads ranging from pale white and lavender to reddish purple and violet. They often have spines or prickles on leaf margins and around flower heads.

Most species growing on the High Plains have a biennial growth cycle. 

In the first year, plants grow as short, vegetative rosettes of leaves which capture and store energy from the sun. In the second year, energy stores fuel growth of an upright stalk and flowers resulting in seed production. 

A few notable species, including the invasive Canada thistle and the native Wyoming thistle, follow a perennial growth cycle, with mature plants growing back each year from the same root base. These species also reproduce from seeds.

Native versus

non-native thistle

Wyoming is home to a number of native and non-native thistles. Knowing which plants are harmless and which are noxious, can save time and money by eliminating unnecessary interventions. 

There are four non-native thistles designated as noxious weeds by the state of Wyoming.

These are Canada thistle (Cirsium arvense), musk thistle (Cardus nutans), plumeless thistle (Carduus acanthoides) and Scotch thistle (Onopordum acanthium). All are notable for their ability to outcompete native plants.

Not all thistles are noxious or invasive. In fact, many of Wyoming’s native thistles provide key habitat for pollinators and winter food for songbirds. For this reason, positive identification is important before enacting management. 

Native species have beneficial contributions to the ecosystem and are not a risk for outcompeting other plants in their community.

For comprehensive information on how to distinguish native and non-native thistles, check out the “Wyoming Thistle Field Guide: Native and Non-Native” online at bit.ly/wythistle. 

One challenge of managing noxious thistles is their ability to produce large quantities of highly mobile seeds. Individual thistle seeds have a plume of hairs, called a pappus, which allows the wind to disperse them over large distances. 

EDRR and IPM

Effective management interventions take place before plants have released seed.

Early detection and rapid response (EDRR) is a concept in weed management involving identifying a weed and intervening quickly before it becomes established at a damaging level.

This simply means paying attention to surroundings while checking fence, haying and going about day-to-day business. Land managers should note any changes and seek to identify unfamiliar plants. 

When invasive thistle species are present and a management tool is needed, there are many options available. Integrated pest management (IPM) is a system describing these options, which can be used alone or in tandem.

Cultural control methods include preventative measures to reduce introduction of noxious species, like cleaning boots and gear when moving from field to field. Since invasive thistles readily disperse by the wind, it can be difficult to prevent their establishment outright.

Physical or mechanical control works well for isolated patches of thistles. Using a shovel or hoe to chop out isolated plants before they set seed is an effective control method for biennial species. 

With Canada thistle, this can actually increase the spread by splitting up rhizomes, so another control method may be necessary. 

Biological controls are living species which prey on the target organism. 

Musk thistles are damaged by the feeding activity of the musk thistle rosette weevil. These insects develop in the immature rosette of the thistles and stunt their growth. 

Biological control of Canada thistle has been attempted with the Canada thistle gall fly and a stem mining weevil, but with limited success. More recently, rust fungus control of Canada thistle has been studied with promising stand reduction results when inoculum is applied during the rosette stage.

Chemical control of invasive thistles using herbicides that kill on contact is an option when those listed above are not suitable. Land managers should work with their local weed and pest district when chemical control of invasive thistles is desired. 

Cost share options are available in most counties. 

Noxious thistle species can quickly overtake an environment and are expensive to eradicate once established. On the other hand, native thistle species are not invasive and provide a wide array of ecosystem benefits. 

Knowing the difference can save land managers time and money during the growing season.

When invasive thistles are present, using the principles of IPM will help ensure the intervention plan selected is appropriate for the situation.

Micah Most is the University of Wyoming Extension agriculture and natural resources educator serving Johnson County. He can be reached at mmost@uwyo.edu or 307-684-7522.