Nature’s Notebook

Connecting People with Nature to Benefit Our Changing Planet

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Photo for species Cirsium_arvense

Although Cirsium arvense is native to southeastern Europe and eastern regions of the Mediterranean, it has spread to most of the temperate areas of the world. It was accidentally introduced into North America during the 17th century as a contaminant in crop seed. Today, in cooler areas of North America, the plant is especially troublesome and hard to keep under control.

Photo Credit:
© T.F. Niehaus. Courtesy of Smithsonian Institution, Dept. of Systematic Biology, Botany.

Cirsium arvense

Canada thistle, creeping thistle, field thistle, Californian thistle
What does this species look like?
What does this species look like?: 

Canada thistle is an erect, perennial, herbaceous plant growing 1 to 5 feet tall, with some branching along slightly hairy stems. Male and female flowers occur separately on different plants. Tiny, rose-purple, lavender or white flowers are tightly clustered into flowerheads appearing like a single flower at the top of the branches. Plants are often connected by underground roots and form large patches. These connected plants will typically produce only one sex of flowers.

Canada thistle prefers deep, well-aerated, cool soils, and is less common in light, dry soils. It grows best in disturbed areas (pastures, old fields, waste places, fence rows, roadsides, and along railroads). It is occasionally found in wet areas where water levels fluctuate (stream banks, ditches), and can invade sedge meadows and wet-mesic grasslands. It also grows along lakeshores, seashores, sand dunes, and open sandy areas.

Where is this species found?
States & Provinces: 
AB, AK, AL, AR, AZ, BC, CA, CO, CT, DC, DE, IA, ID, IL, IN, KS, KY, MA, MB, MD, ME, MI, MN, MO, MT, NB, NC, ND, NE, NH, NJ, NL, NM, NS, NT, NV, NY, OH, ON, OR, PA, PE, QC, RI, SD, SK, TN, UT, VA, VT, WA, WI, WV, WY, YT
Special Considerations for Observing

If drought seems to be the cause of leaf senescence for a plant, please make a comment about it for that observation.

Which phenophases should I observe?
Leaves

Do you see...?

Initial growth
New growth of the plant is visible after a period of no growth (winter or drought), either from above-ground buds with green tips, or new green or white shoots breaking through the soil surface. Growth is considered "initial" on each bud or shoot until the first leaf has fully unfolded. For seedlings, "initial" growth includes the presence of the one or two small, round or elongated leaves (cotyledons) before the first true leaf has unfolded.

Leaves
One or more live, fully unfolded leaves are visible on the plant. For seedlings, consider only true leaves and do not count the one or two small, round or elongated leaves (cotyledons) that are found on the stem almost immediately after the seedling germinates. Do not include fully dried or dead leaves.

Flowers

Do you see...?

Flowers or flower buds
One or more fresh open or unopened flowers or flower buds are visible on the plant. Include flower buds or inflorescences that are swelling or expanding, but do not include those that are tightly closed and not actively growing (dormant). Also do not include wilted or dried flowers.

How many flowers and flower buds are present? For species in which individual flowers are clustered in flower heads, spikes or catkins (inflorescences), simply estimate the number of flower heads, spikes or catkins and not the number of individual flowers.

Less than 3;3 to 10;11 to 100;101 to 1,000;More than 1,000

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Open flowers
One or more open, fresh flowers are visible on the plant. Flowers are considered "open" when the reproductive parts (male stamens or female pistils) are visible between or within unfolded or open flower parts (petals, floral tubes or sepals). Do not include wilted or dried flowers.

What percentage of all fresh flowers (buds plus unopened plus open) on the plant are open? For species in which individual flowers are clustered in flower heads, spikes or catkins (inflorescences), estimate the percentage of all individual flowers that are open.

Less than 5%;5-24%;25-49%;50-74%;75-94%;95% or more

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Fruits

Do you see...?

Fruits
One or more fruits are visible on the plant. For Cirsium arvense, the fruit is very tiny and seed-like and is crowded into a spent flower head. The seed-like fruit has a tuft of white fluff, and changes from whitish-yellow or yellow-green to tan, light yellow or light brown and drops or is blown from the plant after the spent flower head reopens. Do not include empty flower heads that have already dropped all of their fruits.

How many fruits are present?

Less than 3;3 to 10;11 to 100;101 to 1,000;More than 1,000

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Ripe fruits
One or more ripe fruits are visible on the plant. For Cirsium arvense, a fruit is considered ripe when the spent flower head has reopened and the white fluff is visible. Do not include empty flower heads that have already dropped all of their fruits.

What percentage of all fruits (unripe plus ripe) on the plant are ripe?

Less than 5%;5-24%;25-49%;50-74%;75-94%;95% or more

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Recent fruit or seed drop
One or more mature fruits or seeds have dropped or been removed from the plant since your last visit. Do not include obviously immature fruits that have dropped before ripening, such as in a heavy rain or wind, or empty fruits that had long ago dropped all of their seeds but remained on the plant.

How many mature fruits have dropped seeds or have completely dropped or been removed from the plant since your last visit?

Less than 3;3 to 10;11 to 100;101 to 1,000;More than 1,000

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