Cirsium

Lore & Background

Cirsium thistles are known for their effusive flower heads, usually purple, rose or pink, also yellow or white. The radially symmetrical disc flowers are at the end of the branches and are visited by many kinds of insects, featuring a generalised pollination syndrome. They have erect stems, with a characteristic enlarged base of the flower which is often spiny. The leaves are alternate, spiny in many but not all species, and in some species can be slightly to densely hairy. Extensions from the leaf base down the stem, called wings, can be lacking (Cirsium arvense), conspicuous (Cirsium vulgare), or inconspicuous. They can spread by seed, and also by rhizomes below the surface (Cirsium arvense). The seeds have a tuft of hair, or pappus, which can carry them far by wind. Many species are considered weeds, typically by agricultural interests. Cirsium vulgare (spear thistle) is listed in the United States (where as a non-native invasive species it has been renamed 'bull thistle') as a noxious weed in nine states. Some species in particular are cultivated in gardens and wildflower plantings for their aesthetic value and/or to support pollinators such as bees and butterflies. Some species dubbed weeds by various interest groups can also provide these benefits. Cirsium vulgare, for instance, ranked in the top 10 for nectar production in a UK plants survey conducted by the AgriLand project which is supported by the UK Insect Pollinators Initiative. Not only does it provide abundant nectar, it provides seeds for birds, such as the European goldfinch Carduelis carduelis, and supports the larvae of the Painted Lady butterfly Vanessa cardui. Some ecological organizations, such as the Xerces Society, have attempted to raise awareness of the benefits of thistles, to counteract the general agricultural and home garden labeling of thistles as unwanted weeds. The monarch butterfly (Danaus plexippus), for instance, was highlighted as relying upon thistles such as tall thistle (Cirsium altissimum) as nectar sources during its migration. Some prairie and wildflower seed production companies in the United States supply bulk seed of native North American thistle species for wildlife habitat restoration, although availability tends to be low. Thistles are particularly valued by bumblebees for their high nectar production.

Reader's Guide

Cirsium thistles hold significance both as agricultural weeds and as valuable ecological resources. Many species are considered noxious weeds, particularly Cirsium vulgare, which is listed as a noxious weed in nine U.S. states. However, these same plants provide substantial benefits to pollinators and wildlife. Cirsium vulgare ranked in the top 10 for nectar production in a UK survey and was third in nectar sugar production per floral unit in another British study. The seeds attract small finches such as the American goldfinch and European goldfinch, and the larvae of the Painted Lady butterfly feed on the plants. The monarch butterfly relies on thistles like Cirsium altissimum as nectar sources during migration. Organizations such as the Xerces Society have worked to raise awareness of these benefits. Some species are cultivated in gardens and wildflower plantings for aesthetic value and pollinator support. Certain species have been traditionally used as food in southern Europe, Japan, India, and Korea. The name 'Cirsium' is the Greek word for thistle, kirsos, likely derived from 'swollen vein'.

Did You Know?

• Cirsium thistles are distinguished from other thistle genera by having a seed with a pappus of feathered hairs on their achenes.
• Cirsium vulgare ranked in the top 10 for nectar production in a UK plants survey supported by the UK Insect Pollinators Initiative.
• The monarch butterfly relies upon thistles such as tall thistle (Cirsium altissimum) as nectar sources during its migration.
• Cirsium oleraceum is cultivated as a food source in Japan and India, and Cirsium setidens is used as a vegetable in Korean cuisine.