Introduced from Europe in alfalfa seed in the late 1800
A perennial plant that has a branching grey-green stock

Diffuse & spotted napweed
(Centaurea diffusa, Centaurea stoebe )

Quick Identification

Diffuse knapweed:

Hairy, grayish-green leaves on many branches growing from a single upright stem
Small white flowers

Spotted knapweed:

Bright purple flowers
Lower bracts are black tipped giving it a spotted appearance
Several stems grow from the base of the plant and are a darker green than diffuse knapweed

Hybrids:

Diffuse and spotted knapweed have cross bred to produce plants that have a mix of characteristics

Impact

Diffuse knapweed is an extremely aggressive weed that inhibits the plants growing nearby by secreting an allelopathic chemical. So far 7 different types of biological control insects have been released.

Spotted knapweed is a highly aggressive species that invades moist, rich grasslands, disturbed areas and road edges. Preventing further expansion of this weed is critical to maintaining the forage base for wildlife.

Management

Hand pulling is effective if the root is removed. Bio control released in the Boundary has been effective in reducing diffuse knapweed populations but is not effective control for spotted knapweed or hybrid plants. Effective herbicides are available.

Flower
Diffuse knapweed has small white flowers that emerge from a green-grey bracket with tan tips. Spotted knapweed has bright purple flowers that emerge from darker green brackets with black tips which give the spotted appearance.

Leaves & Stems
In the first year diffuse and spotted knapweed grow in rosettes. In the second year they grow branching stems with bristly hairs, and flowers. Diffuse knapweed branches from a single stem while spotted knapweed grows several branching stems that have purple stripes.

Roots
Grows from a tap root and a few lateral roots

Reproduction & Dispersal
Primarily from seeds and sometimes by from lateral roots.

Seeds
Diffuse: Light brown to black and without bristled

Spotted: Seeds are bristled

Preferred Habitat
Both species prefer open habitat with well-drained soil. Spotted knapweed grows in moister soil than diffuse, but is less tolerant of shade.

Interesting Facts
Knapweed has been used medically to treat bruises and swelling.

Introduced from Europe in alfalfa seed in the late 1800

A perennial plant that has a branching grey-green stock

Diffuse & spotted napweed (Centaurea diffusa, Centaurea stoebe )

Quick Identification

Diffuse knapweed:

Hairy, grayish-green leaves on many branches growing from a single upright stem

Small white flowers

​

Spotted knapweed:

Bright purple flowers

Lower bracts are black tipped giving it a spotted appearance

Several stems grow from the base of the plant and are a darker green than diffuse knapweed

​

Hybrids:

Diffuse and spotted knapweed have cross bred to produce plants that have a mix of characteristics

Impact

Management

Flower Diffuse knapweed has small white flowers that emerge from a green-grey bracket with tan tips. Spotted knapweed has bright purple flowers that emerge from darker green brackets with black tips which give the spotted appearance.

Leaves & Stems In the first year diffuse and spotted knapweed grow in rosettes. In the second year they grow branching stems with bristly hairs, and flowers. Diffuse knapweed branches from a single stem while spotted knapweed grows several branching stems that have purple stripes.

Roots Grows from a tap root and a few lateral roots

Reproduction & Dispersal Primarily from seeds and sometimes by from lateral roots.

Seeds Diffuse: Light brown to black and without bristled Spotted: Seeds are bristled

Preferred Habitat Both species prefer open habitat with well-drained soil. Spotted knapweed grows in moister soil than diffuse, but is less tolerant of shade.

Interesting Facts Knapweed has been used medically to treat bruises and swelling.