When we see a plant and its prickly, we automatically assume it is a thistle and therefore it is a bad plant. The Oxford Dictionary defines a thistle as “A widely distributed herbaceous plant of the daisy family, which typically has a prickly stem and leaves and rounded heads of purple flowers.” There are 20 thistles native to Colorado and they are desirable plants. Thistles are in the Asteraceae, sunflower (daisy) family. So, resist the urge to pull every prickly plant you see on your property and only pull those that are truly invasive non-native plants.
Not every plant that is prickly is a thistle. So how do you identify a prickly plant that you come across to determine if it is native or non-native thistle or not even a thistle? Assuming that you don’t want to have to work through a dichotomous key (how botanists identify plants), there are some simple features that may indicate whether a thistle is either native or non-native. Native thistles normally grow singly or in small groups of 2 to 4 and not in massive stands like non-native thistles. Native thistles are usually shorter (<4’) and have white or pale colored flowers although don’t base your identification on the flower color alone. If you are above timberline, the thistle is most likely native. Invasive thistles tend to grow taller (4 to 12’) and in large, dense stands. Native thistle can either be biennials (two growing season life cycle) or perennials (they live longer than two years).
Other plants that may be confused with thistles are Prickly lettuce Lactuca serriola, Prickly poppy Argemone polyanthemos which has a white poppy flower, Prickly sow thistle, Sonchus asper, Yellow starthistle Centaurea solstitialis. With the exception of the prickly poppy, all of these are in the same Asteraceae, sunflower family as the thistles and are non-natives. The prickly poppy is in the poppy family Papaveraceae and is a native plant.
There are 5 non-native invasive noxious thistles – Bull thistle Cirsium vulgare, Musk thistle Carduus nutans, Scotch thistle Onopordum acanthium and Onopordum tauricum, Plumeless thistle Carduus acanthoides, Canada thistle Cirsium arvense or Breea arvensis. All of these thistles are biennials (two growing season growth cycle) with the exception of Canada thistle which is a perennial.
Biennial non-native thistles are easily managed by keeping them from going to seed. You can do this by pulling or undercutting the rosette (first growing season) or cutting off the flower stalk before it has a chance to produce seed (second growing season). Canada thistle is harder to manage as it lives multiple years and spreads by both seed and roots. The majority of the Canada thistle plant mass is in the roots so managing with the root system is the key to eradicating it. Trying to dig out the Canada thistle root system is next to impossible. If you don’t get every part of the root system, it resprouts. Some property owners try to manage Canada thistle by pulling or mowing it to keep it from having leaves so it can photosynthesize and store energy in the roots. This works for small infestations, but you have to be very persistent or this method does not work. Organic herbicides only burn the leaves on plants but do not translocate and kill the root system, so they are like pulling or mowing the plants. Synthetic herbicides do translocate and kill the root system and may be the best method to manage large Canada thistle infestations.
If you would like to learn more about Colorado’s thistles, Larimer County Weed District has a booklet on Colorado’s thistles both native and non-native, visit larimer.org/sites/default/files/uploads/2020/2019_thistle_guide_5.5_x_8_-_dec_30_1.pdf.
By Sharon Bokan, Colorado State University Extension. Sharon is the Small Acreage Coordinator at Colorado State University Extension Boulder County. For more information,
call 303.678.6176, e-mail firstname.lastname@example.org or visit ext.colostate.edu/boulder.