Spotted knapweed 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.
Spotted knapweed is an erect, biennial or perennial, herbaceous plant eventually growing 8 to 48 inches tall. Plants remain in an initial rosette stage for 1 to 4 years; it is rare for flowers to appear in the same year as the plant emerges. Its purple, pink or white flowers are tiny, with 25 to 40 flowers tightly clustered into flowerheads that appear as a single "flower" at the ends of the branches. Generally, each flower contains both male and female parts, although some flowers are sterile.
Spotted knapweed grows on open, disturbed sites, travel corridors, industrial sites, agricultural fields, and pastures, as well as dryland range, timbered range, Ponderosa pine and Douglas fir forests, and along river and riparian areas, preferring coarse, gravelly and sandy soils.
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.
Centaurea biebersteinii is native to eastern Europe and is thought to have been introduced to North America in the late 1800s as a contaminant in alfalfa (Medicago sativa) seed or possibly in a ship's ballast. Its flowers are pollinated by insects, many of which are bees, but it is thought that wind can also facilitate pollination for the plant.
Gardens with this plant