Thistles are flowering plants that belong to the Asteraceae family. There are over 200 different species of thistles found around the world. Thistles grow wild in many landscapes and are considered invasive weeds in some regions. When it comes to the color of thistle flowers, there is some variety between species.
The Most Common Thistle Flower Colors
Though thistles come in many different shapes, sizes, and species, most common thistle flowers are purple in color. The purple color comes from anthocyanin pigments in the flower petals. Some of the most widespread thistle species that feature purple flowers include:
- Scotch thistle (Onopordum acanthium)
- Spear thistle (Cirsium vulgare)
- Bull thistle (Cirsium vulgare)
- Canada thistle (Cirsium arvense)
- Russian thistle (Salsola tragus)
- Milk thistle (Silybum marianum)
- Sow thistle (Sonchus oleraceus)
The vivid purple color of thistle flowers comes from the presence of anthocyanin pigments. These plant pigments have important functions beyond just giving the flowers their visual appearance. Anthocyanins act as antioxidants that can protect the plant from environmental stressors like drought, high light, and disease. The purple color likely helps attract pollinating insects to the flowers.
Other Thistle Flower Colors
Though purple is undeniably the most common, there are some thistle species that display other flower colors:
- Pink – Examples include dwarf thistle (Cirsium acaule) and Grundy thistle (Onopordum illyricum). The pink color is also due to anthocyanins.
- Yellow – Examples include bristly thistle (Carduus pycnocephalus) and winged thistle (Carduus tenuiflorus). The yellow comes from carotenoid pigments.
- White – Examples include Italian thistle (Carduus pycnocephalus) and slenderflower thistle (Carduus tenuiflorus). The white color comes from a lack of pigments.
- Red – Examples include Cornell thistle (Cirsium canescens) and Flodman thistle (Circium flodmanii). The red comes from anthocyanins.
So while purple thistle blossoms are the most iconic, several species display other hues. Now let’s go into more detail on some of the most common thistle species and the color of their flowers.
Scotch Thistle Flowers
Scotch thistle (Onopordum acanthium) features vibrant purple flower heads. Each flower head contains hundreds of tiny individual tubular florets. The spiny stems and leaves have a silver-green color. Scotch thistle is native to Europe and Asia but has become an invasive species in suitable temperate climates like Australia and North America.
Spear Thistle Flowers
Spear thistle (Cirsium vulgare) is one of the most widespread thistle species. It has colonized large swaths of Europe, Asia, and North America. The spear-shaped leaves give the plant its common name. The flowers are deep magenta-purple in color, comprised of many small disk florets. Spear thistle flowers from July to September in the northern hemisphere.
Bull Thistle Flowers
Bull thistle (Cirsium vulgare) is a biennial thistle species with an erect stem and multi-branched, spherical pink-to-purple flower heads. The sharp spines on the leaves and stems give bull thistle its common name. Each flower head contains 100-500 tiny tubular disk florets. Bull thistle grows as a weed throughout temperate regions worldwide.
Canada Thistle Flowers
Canada thistle (Cirsium arvense) is an aggressive, creeping thistle native to southeastern Europe and the eastern Mediterranean. It has spread as an invasive weed throughout the upper Midwest and western mountains of North America. The pink-to-purple flowers are less than an inch wide and borne in small clusters. Canada thistle spreads rapidly via horizontal roots that can extend 12 feet deep and 15 feet across.
Russian Thistle Flowers
Russian thistle (Salsola tragus), also known as tumbleweed, is an invasive plant that colonized the American west in the late 1800s. It forms spherical bushes up to 3 feet high covered in spiny, green leaves. Clusters of tiny pinkish-purple flowers develop at the ends of branches. When the plant dies, the round bush breaks off and tumbles across the landscape, scattering seeds as the wind pushes it.
Milk Thistle Flowers
Milk thistle (Silybum marianum) is a medicinal plant grown for its edible seeds and leaves. It produces large pink-to-purple flower heads containing tubular florets. The mottled white and green leaves have a milky-white colored vein pattern, giving rise to the common name milk thistle. Extracts from the seeds are used to support liver health.
Sow Thistle Flowers
Sow thistle (Sonchus oleraceus) is an annual, herbaceous weed native to Europe and western Asia. Young leaves are edible when cooked and have a nutty flavor. The plant produces many bright yellow flowers, each containing over 100 florets. The seeds have a white, fluffy pappus which aids in wind dispersal.
Dwarf Thistle Flowers
Dwarf thistle (Cirsium acaule) is a perennial that only reaches about 6 inches in height. It has very small solitary pink to purple flower heads, each containing around 100 disk florets. The hairy leaves form a basal rosette. Dwarf thistle grows in meadows and prairies in much of North America.
Grundy Thistle Flowers
Grundy thistle (Onopordum illyricum) is a species native to Europe and northern Africa and naturalized in Australia. It is a biennial that forms large grayish-green leaves in a basal rosette during the first year. The second year, it bolts into a 6 foot tall flower stalk covered in narrow, elongated thistle heads. The flowers are pink to purple in color.
Bristly Thistle Flowers
Bristly thistle (Carduus pycnocephalus) is a weedy species of Mediterranean origin that has spread to Australia, North America, South America, and South Africa. It has an erect stem and large, oval-shaped leaves that clasp the stem. The flower heads are comprised of tubular yellow or white florets. Bristly thistle flowers during the summer.
Winged Thistle Flowers
Winged thistle (Carduus tenuiflorus) is native to regions bordering the Mediterranean but has become an aggressive invasive weed in Australia, North America, and South America. Plants produce numerous small flower heads containing yellow tubular florets. The stems and leaves have white hairs and spiny wings, giving rise to the common name winged thistle.
Italian Thistle Flowers
Italian thistle (Carduus pycnocephalus) is native to the Mediterranean region. It has spreading patches of spiny leaves and green stems with white wings. The solitary flower heads are oval in shape and contain white tubular florets. Italian thistle has been introduced as a seed contaminant to Australia, North America, and South America.
Slenderflower Thistle Flowers
Slenderflower thistle (Carduus tenuiflorus) is native to Eurasia but has naturalized in North America and Australia. The upright stems produce many small oval flower heads, each with 25-50 white tubular florets. The stems and leaves have distinctive spiny, green wings. Slenderflower thistle flowers from early to late summer.
Cornell Thistle Flowers
Cornell thistle (Cirsium canescens) is a North American native perennial found in prairies and glades from Arkansas to Wisconsin. It produces oval-shaped pink to purple flower heads, each with around 100 disk florets. Leaves are covered in fine white hairs, giving the foliage a whitish appearance.
Flodman Thistle Flowers
Flodman thistle (Circium flodmanii) is a prairie wildflower native to the central United States. It has thin, spiny leaves and stems up to 5 feet tall. Showy rose-purple flowers develop solitary or in groups of 2-5. Flodman thistle flowers from July to September and provides an important nectar source for butterflies and bees.
How Flower Color Varies Between Thistle Species
While purple is the most common thistle flower color, not all thistles conform. Flower color is primarily determined by the presence and combination of pigments produced in the plant. Here are some reasons thistle flowers display different hues:
- Anthocyanins produce blue, purple, and pink colors. All thistles produce anthocyanins, but some species make more than others.
- Carotenoids produce yellow and orange tones. Some thistles lack carotenoids in their flowers completely.
- Flavonoids produce white flowers when all other pigments are absent.
- Flower color can also be influenced by soil pH and sun exposure during growth.
- Genetic differences between thistle species drives variations in color.
While the classic thistle flower is purple, evolution and genetics have led several species down alternative color pathways. This diversity provides visual interest in the landscapes where these wildflowers grow.
Conclusion
Thistles encompass over 200 species, most known for their vivid purple flower heads. The purple color comes from anthocyanin pigments in the florets. A few thistle species display pink, yellow, white, or red blossoms instead, resulting from different pigment levels and combinations. Some of the most common thistles with purple flowers include Scotch thistle, spear thistle, bull thistle, Canada thistle, Russian thistle, and milk thistle. Examples of thistles with other flower colors include dwarf thistle (pink), sow thistle (yellow), bristly thistle (yellow), and slenderflower thistle (white). While purple thistle blossoms are the most iconic, the genus displays delightful diversity when it comes to flower color.
