Get to Know Common North American Thistle Species

By Kelly Miller

For most gardeners, a thistle isn’t a welcome sight. Spiky and hairy, non-native invasive thistle species often pop up where they’re not wanted. They spread aggressively, with deep roots that are tough to pull up.

However, many thistle species are native to North America. These species provide significant food sources for birds and pollinators.

Thistles deserve our appreciation, along with a watchful eye. Here’s all you need to know, including lists of invasive and native varieties, and tips for managing both types.

All About Thistles

Most thistle species belong to the Cirsium or Carduus genera within the Asteraceae family (commonly known as the aster or daisy family).

Traits common in thistles include:

• Composite flowers. A thistle flower head consists of dozens of tiny, tightly packed flowers that form a spiky cluster.
• Spikes. Thistle leaves have spikes around their margins.
• Prickles. Prickles occur on the stems and leaf surfaces.
• Biennial life cycles. Some thistles are perennials, but most common thistle species are biennials (with two-year life cycles).

Thistles are resilient plants that grow in poor conditions. They’re usually regarded as weeds. However, they’re an essential food source for wildlife. This is especially true for thistles growing in their native range.

Many small birds feed on protein-rich thistle seeds in fall and winter. Meanwhile, thistle flowers attract a variety of bees and butterflies. For example, in some regions, thistles are the most essential flowers for monarch butterflies as they migrate south toward Mexico.

Which Thistles Are Invasive?

In North America, the most common and widespread thistle species are non-native invasives. These include:

Musk Thistle, Nodding Thistle (Carduus nutans)

Musk thistle is a durable, spiny biennial plant that was introduced to North America in the mid-1800s. It is native to parts of Europe, Asia, and North Africa and is considered an invasive weed in the United States where it grows wild in overgrazed pastures, along roadsides, ditches, waste areas, and disturbed sunny locations.

Bull Thistle, Spear Thistle (Cirsium vulgare)

Bull thistle prefers full sun and disturbed soils, commonly appearing in gardens, roadsides, and unkempt areas on agricultural land. Reproduces by seed. Pulling up young shoots by hand is an effective control for minor infestations.

Canada Thistle (Cirsium arvense)

Canada thistle is native to Europe and Asia, not Canada, although it’s now widespread  and invasive throughout much of North America. Canada thistle is a perennial weed with spiny, lance-shaped foliage and wispy pink or purple flower heads. Canada thistle spreads aggressively through its roots. Other common names include: creeping thistle and field thistle.

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Controlling Unwanted Thistles

Non-native thistle species are common throughout most of North America and are considered invasive in many regions. They thrive in sunny areas and grow easily in disturbed soils, such as roadsides. They often appear in the least-tended parts of home landscapes.

Thistles spread aggressively. Each plant produces more than 1,000 seeds. These spread via the wind and have high germination rates. Buried seeds are viable for several years. Thus, early detection is helpful. Pulling up young plants before they produce seeds prevents reproduction.

Before attempting control, it’s good to know which species you’re dealing with. Canada thistle produces new shoots from its roots, making it difficult to eradicate without herbicides. Other thistles are more easily managed with organic control.

An established population is tricky to kill. Pulling up the plants often fails to capture the entire root system. This can result in breaking up roots and turning one plant into several, worsening the problem. Thistles are also difficult to handle, due to their sharp edges. Take proper precautions to protect your skin.

As a long-term approach, repeatedly mowing thistles can eventually starve the root system and prevent seed production. It’s also helpful to maintain healthy soil and encourage the growth of desirable groundcover, since these weeds thrive in poor soils and open space.

For major infestations of Canada thistle, chemical treatment is the commonly recommended control. 

Which Thistles Are Natives in North America?

It can be difficult to tell native and invasive thistles apart, since they have similar appearances. However, natives usually don’t spread aggressively. They’re also more beneficial for native pollinators.

In general, invasive thistles stand out due to their bright flowers. The flowers on natives tend to have duller hues. Many natives also have leaves with white, woolly undersides. However, each species has its own traits, and many natives can be easily confused with invasives. Thus, it’s best to identify the specific species you’re dealing with before determining its status, and whether or not you want it growing on your property.

Here are nine thistles native to North America. These species needn’t be treated as weeds, and are worthy residents of a native plant, pollinator, or meadow garden.

Cobweb Thistle (Cirsium occidentale)

Cobweb thistle is native to western regions of North America. It gets its common name from the fine, cobwebby fibers that coat its leaves and flowerheads. Cobweb thistle is a biennial plant. It forms a low rosette of foliage the first year, then in the second year tall spikes of flowers are produced. Flowers range from white to shades of purple to pink to red.

Carolina Thistle, Soft Thistle (Cirsium carolinianum)

Carolina thistle is a North American native found growing wild in woodlands, along roads, and open fields throughout the central and southern United States. It is a biennial plant that forms a low rosette of foliage the first year, then in the second year tall, branched stems of flowers are produced. The blooms are held on long, spindly stems and may be found in a range of colors from white to shades of purple.

Wavyleaf Thistle (Cirsium undulatum)

Wavyleaf thistle is a common native plant found over a very large range from central North America across the Great Plains to the Pacific coast. It’s an interesting-looking plant with a basal rosette of foliage that has spiny, hairy, wavy leaves. Tall, branched stems of flowers appear from summer into fall. The blooms may be lavender-purple to pink, or occasionally white.

Pasture Thistle, Fragrant Thistle (Cirsium pumilum)

Pasture thistle is a durable native plant found in open prairies, fields, along roadsides, and sandy areas in the northeastern and central United States into Ontario, Canada. It is a biennial, producing a rosette of basal foliage for a year or two then the following year producing tall stems of purple flowers that will go to seed. The flowers appear over a long season in summer.

Flodman’s Thistle, Prairie Thistle (Cirsium flodmanii)

Flodman’s thistle is a common native plant found over a large region of the northern United States and throughout Canada. Flodman’s thistle is most notable for the downy, almost cobweb-like hairs that cover the stems and leaf undersides. Any spines on the plant are relatively soft. Purple blooms are produced over a long season from mid to late summer. This is an especially important plant for supporting native bees.

Tall Thistle, Roadside Thistle (Cirsium altissimum)

Tall thistle is a native plant that is commonly found growing in eastern and central North America along roads, woodlands, and in neglected areas. This is a biennial plant that forms a low rosette of foliage the first year. Purple flowers are produced the following year on a tall, branched stalk that can get as tall as 10’ (3m) under optimal conditions.

Bristle Thistle, Yellow Thistle, Horrid Thistle (Cirsium horridulum)

Bristle thistle is a North American native plant found in coastal regions of the Eastern and Southern United States where it grows in marshes, along roadsides, and in meadows and pasture lands. This is a biennial that grows a low rosette of deeply cut, spiny foliage the first year. Flowers are produced the following year on a tall, thick, branched stalk and blooms can range in color from yellow, to purplish-pink, to white.

Swamp Thistle (Cirsium muticum)

Swamp thistle is a native plant found growing in wet meadows, swamps, marshes, and woodlands throughout Central and Eastern North America. This is technically a biennial, so allowing it to reseed itself will help you continue getting new plants. Swamp thistle produces tall, multibranched stems of violet-pink flowers from summer into fall. You can collect the seed or allow it to disperse naturally with the wind. Swamp thistle has fewer spines than most other species of thistle.

Field Thistle (Cirsium discolor)

Field thistle is native to a wide region of eastern and central North America. It tolerates a range of soil conditions and, as its name suggests, it’s common to find it growing in open fields such as prairies, pastures, and along roadsides or the edge of forested areas. Field thistle produces lavender-pink blooms from summer into fall. Bees, beetles and butterflies are attracted to the field thistle flowers for its nectar. It is also a host plant for Painted Lady butterflies.

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Thistles and Native Plant Gardening

Native thistles are important plants in American ecosystems, including grasslands, woodlands, and wetlands. Their seeds provide cold-weather nourishment for songbirds like indigo buntings, chickadees, and goldfinches, while their nectar is a favorite of bumblebees. Thistle foliage, meanwhile, is a key food source for caterpillars.

Thistles suffer from a bad reputation, due to the aggressive spread of non-native varieties. However, native thistles are less common, and much less aggressive. In fact, some species have become very rare.

Fortunately, native thistles are gaining appreciation. For example, in 2019, the State of Iowa removed the noxious weed status of all native thistles. 

In a native plant garden or a protected wild area, native thistles should be made welcome. If you’re interested in adding thistles to your landscape, the six species shown above are some of the most widely available options. For an extended list of native thistle species, and best practices for conservation plantings, see this Native Thistles Guide from the Xerces Society.

More Ideas for a Wildlife-Friendly Garden

Thistles aren’t well-liked among our kind, but wild animals appreciate them. They have their place in nature, and might deserve consideration for your native garden, too. Find more ideas for developing a wildlife-friendly home landscape here: Tips for Rewilding Your Landscape.