Thistles are often considered weeds and many species are invasive and non-native to the US. Species such as the bull thistle are recognized by various states for inclusion in noxious weed laws. However, not all thistle species are considered threatening, and distinguishing them is important to controlling them. With proper identification, herbicides can be applied more effectively.
Cirsium vulgare, commonly known as bull thistle and common thistle, has long leaves that are tipped with a rigid spine. The leaves have hairs on their upper and lower surfaces. The flowers are purple and spiky with fine hairs. At the base of the flowers are spiny bracts. It is recognized as a noxious weed in Arkansas, Iowa, Maryland, Minnesota, Oregon, New Mexico, Pennsylvania and Colorado. It is present all over the US and in many parts of Canada.
Scotch thistle's scientific name is Onopordum acanthium. Its large, broad leaves are coarse and spiny with a tint of blue or gray that comes from the hairs that line its foliage. Spine-tipped and red-purple in color, its flowers are 1 to 2 inches wide and can bloom either singly or in groups of up to five. Scotch thistle is considered a noxious weed in Arizona, Nevada, California, Colorado, Idaho, Connecticut, Missouri, New Mexico, Oklahoma, Arkansas, Oregon, Utah, Washington and Wyoming. It grows in most areas of the US.
Carduus nutans is also called musk thistle and nodding thistle. It has single pink or purple flowers on tall stems and, like the bull thistle, it has several spiny bracts. The flowers can grow up to 2 inches around. The leaves are prickly and largely hairless. It's considered a noxious weed in 25 different states including Arkansas, Ohio, Utah and Kentucky. It grows in most states, as well as most of southern Canada.
Cirsium altissimum, or tall thistle, is variable in its leaf structure. Its leaves can be lobed or unlobed, but all have spines along the margins that point toward the leaf's tip. The undersides of the leaves have thick white hairs. The pink or purple flowers are about an inch around. Only two states, Arkansas and Iowa, consider it a noxious weed and it grows throughout the mid and eastern US, excluding the northern reaches of New England.