Find Native Thistles
Our native thistles (Cirsium sp.) are important components of our landscape. Unfortunately, they are often all mistaken for invasive, weedy plants. Our native thistles can be distinguished from the only two invasive thistles in our area by occurring individually (versus rhizomatous as in Canada thistle, Cirsium arvense) and lacking rigid hairs on the upper leaf surface (as in Bull thistle, Cirsium vulgare). All thistles are easily recognized by their large, prickly stature and flowers arranged in heads.
The new Head Curator of Natural History Collections at Denver Botanic Gardens, Jennifer Ackerfield, is passionate about thistles. Her research has even identified at least two new species of native thistles “hidden” in plain site in Colorado. Help DBG document the abundance and distribution of all native thistles by photographing as many plants as possible in the month of July. Your findings will show up on the July EcoQuest iNaturalist project as well as the overall Denver EcoFlora Project.
Cirsium undulatum, or wavy leaf thistle, is a common native thistle in the metro area:
WHAT IS AN ECOQUEST?
EcoQuests, part of the Denver EcoFlora project, challenge citizens to become citizen scientists and observe, study, and conserve the native plants of the City via iNaturalist, an easy-to-use mobile app.
HOW DO I GET STARTED?
HOW DO I PARTICIPATE GIVEN COVID-19?
As a participant, it is up to you how much or how little you take part. Please stay close to home and maintain a social distance of 6 ft. from people. Wear a cloth mask in public areas. Respect closures and avoid places of high use. Do only what feels safe for you and your family and is in accordance with your local regulations.
WHAT IS THE GOAL?
The EcoFlora project is designed to meaningfully connect citizens with biodiversity, and to assemble novel observations and data on the metro area’s flora to better inform policy decisions and conservation strategies.
Comments
Great challenge, Jennifer! :) Hope that some folks go out and observe those thistles!
Thanks, Sam! I can't wait to see how many thistle species will be found!
Ooo very excited about this one, but could use some clarification re: "occurring individually" vs the invasives, as I'm super new at this. The thistles in the back right portion of the photograph appear clustered together, but look like the C. undulatums in the foreground. How can I distinguish rhizomatous groupings from closely spaced individuals? Thanks!
Seems like Onopordum acanthium, although not a Cirsium, is becoming a huge invasive problem. You are only looking at Cirsium, though?
Great question, @meleavery. When you see Canada thistle, it typically is growing in a mass. This is because it has deep rhizomes, and it is also really good at outcompeting anything in its path. There is a thistle guide to Colorado (soon to be outdated thanks to my research, but the best thing we have at the moment!): https://summitcountyco.gov/DocumentCenter/View/11988/Thistle_brochure_good
I think this will help answer all your questions!
Tom - I'm only looking at Cirsium, but yes Onopordum is becoming quite prevalent. I've seen more this year than ever before.
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