Canada thistle is a USA-NPN calibration plant species. Calibration species have broad distributions and are ecologically or economically important. The NPN integrates observations on calibration species to get "the big picture" of plant responses to climate across the nation. In addition, this species is potentially invasive. Observations on its phenology will provide valuable information toward understanding its potential for spread and for its control. The NPN does not promote planting or cultivation of this or any invasive plant.
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.
You should observe...
Here are the phenophases you should observe about this plant.
New growth of the plant is visible, 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.
In at least one location on the plant, a fully unfolded leaf is visible. For seedlings, consider only true leaves and do not count the cotyledons (one or two small, round leaves) that are found on the stem almost immediately after the seedling emerges.
One or more fresh flowers or flower heads (inflorescences) are visible on the plant. Flower heads include many small flowers that usually do not open all at once. Do not include wilted or dried flowers that remain on the plant, or heads whose flowers have all wilted or dried.
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 unfolded or open flower parts. Do not include wilted or dried flowers that remain on the plant.
One or more fresh fruits are visible on the plant.
One or more ripe fruits are visible on the plant.
|Recent fruit drop
One or more fresh 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.
If drought seems to be the cause of leaf senescence for a plant, please make a comment about it for that observation.
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.
Gardens with this plant