- 1). Notice the size of the group of thistles you want to identify. Thistle species that are invasive to Colorado can grow to over six feet tall, with many plants clustered in a dense grouping. Native species, such as the Fringed and Elk Thistles, usually have shorter stems or no stem at all and appear in small groups, with one to four plants in each group. The Scotch Thistle is an invasive species and can grow up to eight feet tall and in dense groupings that are barriers to livestock.
- 2). Identify the color of the flower. All thistle flowers have a fluffy, white appearance at the end of the flowering season. This is an indication of the seed distribution for the plant and should not be confused with the actual color of the flower itself. Flower petals of the thistle plant are thin and slightly waxy, numerous and usually colored. White or whitish flowers are generally an indication of a native species and appear on the Fringed, Mountain, Elk and Prairie Thistle plants. Yellow flowers appear on the native Parry's Thistle, as well as the invasive Yellow Starthistle. Invasive thistle plants such as the Canada, Scotch, Globe and Musk Thistle all produce purple flowers.
- 3). Identify the area in which the thistle plant is growing. Thistle plants growing above the timber line are generally native species. Soil conditions in the area also determine the type of thistle that is able to grow there. Invasive thistle species can generally tolerate harsher soil conditions, such as saltier soil or soil that has been leached of nutrients that cannot support native flora.
- 4). View the leaves of the thistle plant in question. Look at the general number of spines on each leaf, as well as whether the leaf has a smooth appearance or is covered with hair-like structures. The Scotch Thistle has leaves that distinguish it from other thistle plants found in Colorado by the covering of hair-like structures, while the Canada Thistle has a smooth upper leaf appearance. The musk thistle also has a smooth appearance to the upper side of the leaf, but has a visibly lighter mid-rib running along the length. Leaves of the Mountain Thistle have many spines and a long, thin appearance.
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