Knapweeds belong to the genus Centaurea and are members of the Sunflower family (Asteraceae). This is a very large and diverse family of plants that includes dandelions, sunflowers, and daisies. Most knapweeds are non-native to North America. They were brought to North America following the immigrant trail from Europe and Asia. Together, these Eurasian knapweeds form a large complex of invasive species that are found throughout the United States and Canada. All told, 25 species of knapweeds occur in the two countries, predominantly as noxious rangeland weeds in the West. Six species are considered in this manual. Among the most troublesome are diffuse, spotted, and squarrose knapweeds. Lesser-known knapweeds (meadow, brown, and black) are closely related to the others and are included in this manual because they share similar biology and some of the same biological control agents.

Knapweeds are highly invasive weeds that are capable of forming large infestations under favorable conditions (Fig. 1). Knapweeds are distinguished by their bract shape, flower color, leaf shape, roots, seeds and branching habit. The taxonomic key can be used to identify the six knapweed species described in this manual. Sections following the key describe each of the species separately in greater detail to enable the user to identify each species in the field.

Figure 1. a. Spotted knapweed infestation in northern Idaho

Figure 1. b. Spotted knapweed rosette

All six knapweed species begin their lifecycle as seedlings that develop into prostrate rosettes of 5 to 12 lobed leaves (Fig. 1). Most species remain rosettes the first year. With the onset of warm, moist conditions the following spring, plants bloom on one to several branched, flowering stems. Plants have many seedheads that occur singly at the tips of branched stems.

Like other members of the sunflower family, the knapweed head, or capitulum, is an aggregation of small, individual flowers (Fig. 2). The individual flowers, or florets, are tightly clustered and anchored to a concave base, called the receptacle. The receptacle and florets are surrounded by an envelope of modified leaves, or bracts. Head size and bracts are important diagnostic characters for knapweeds.

As the head completes its development, the bracts separate to reveal the maturing florets, enabling pollination to occur. Seeds develop later in the season (knapweed seeds are also known as achenes). Seeds may have a tuft of whitish or tawny bristles at one end, called a pappus.

Insects used in knapweed biological control inflict damage to the plant in two places: the seedhead and the root. The plant is damaged by the larvae of these insects which feed in the head or root tissue, destroying it. Only the adult seedhead weevils eat foliage, otherwise adult insects generally don’t damage the plant.

Seed-feeding biocontrol agents attack the plant at specific stages of development: some attack the plant early, in the bud stage, and others attack later, when plants are in early to full bloom. Larvae eat and destroy seeds and receptacle tissue.

Root-boring biocontrol agents can attack the plant as soon as the root is large enough for the insect to feed. The root is composed of two key tissues: the root cortex and the central vascular tissue (Fig. 3). Both tissues are nutritious: the cortex tissue stores nutrients and the vascular tissue contains the channels in which nutrients and water move up and down the plant.

Figure 2. Knapweed capitulum showing placement of florets.

Figure 3. Key tissues in the knapweed root.

Key to the Knapweed Species

(Adapted from Roche and Roche 1993)

A1. Bracts that surround the flower head are spine-tipped, biennial or short-lived perennial

A2. Flower heads without spine-tipped bracts

Leaves: The basal leaves are up to 8 inches (20 cm) long, deeply lobed, and arranged in a rosette. Stem leaves, arranged alternately, are smaller and not lobed. Uppermost leaves are bract-like.

Stems: The stems are upright, stiff, and branched. Small plants usually have an unbranched stem and one flower head; large plants have a stem with many branches and can have over 100 flower heads.

Flowers: Flowering occurs from June to October. The 0.2 to 0.4 inch (5 to 10 mm) long flower heads occur singly or in clusters at the branch tips. Each head bears stiff bracts, which are black-tipped, giving the plant its ‘spotted’ appearance. Heads contain from 30 to 50 pink or purple colored flowers.

Seeds: Seeds are 0.1 inch (2.5 mm) long, oval, black or brown with pale, vertical lines. Each seed has a short, bristly pappus about half the length of the seed. Plants can produce up to 600 seeds, some of which can remain dormant for many years.

Figure 4. Spotted knapweed. a. Plant b. Seed c. Seedhead d. US distribution.

Leaves: The deeply lobed basal leaves are up to 4 inches (10 cm) long and 1 inch (2.5 cm) wide and arranged in a low-lying rosette. Lower stem leaves are alternate and divided into many lobes, whereas upper stem leaves are much smaller and have only a few slender lobes.

Stems: The single upright stem grows 6 to 24 inches (15 to 60 cm) in height and has numerous branches mostly on the upper half.

Flowers: Flowers are predominantly white, occasionally pink-purple. Heads are 0.5 inch (1.3 cm) long and covered with small, narrow bracts ending in sharp, rigid spines. The terminal spine is distinctly longer than the lateral, spreading spines. Flowering occurs from June to October.

Seeds: Seeds are 1/8 inch (5 mm) long, oblong, and dark brown. Seeds may have a pappus of short, pale bristles.

Habitat and Occurrence: Diffuse knapweed is wide-ranging, although it prefers habitats in the shrub-steppe zones and dry forest habitats. Though predominatly found in the Intermountain West, it is also found in the Midwest and the eastern U.S.

Figure 5. Diffuse knapweed. a. Plant b. Seed c. Seedhead d. US distribution.

Leaves: Rosettes of deeply lobed, gray-green leaves characterize squarrose knapweed.

Stems: The stems are upright, stiff, winged and branched. Small plants usually have an unbranched stem and one flower head; large plants have a stem with many branches and can have over 100 flower heads. Plants range in height from 6 to 24 inches (15 to 60 cm).

Flowers: Flowering occurs from July to September. Flower heads with 4 to 8 pink or purple flowers are borne singly or in pairs at the tips of branches. The seedheads are small and covered with spiny bracts having a long, recurved (backward pointing) terminal spine. The heads are deciduous, falling off the stems after the seeds mature.

Seeds: Squarrose knapweed seeds are pale to dark brown with pale vertical stripes and a short, white pappus. Only 3 to 4 seeds are produced per head, each measuring about 1/8 inch (5 mm) in length. Seeds are dispersed individually as they fall from the heads. Heads are transported when whole plants break off and tumble in the wind. Seeds disperse when whole heads break off from the stem and get lodged in the hair and fur of animals, much like cockleburs and burdock.

Figure 6. Squarrose knapweed. a. Plant b. Seed c. Seedhead d. US distribution.

Leaves: Basal leaves are up to 6 inches (15.2 cm) long, tapering at both ends and having the broadest part above the middle of the leaf. Stem leaves are lance-shaped, shallowly-lobed and stalkless.

Stems: There are usually few to several stems with many branches. Mature plants reach 3.5 feet (1.04 m) tall.

Flowers: Flowers are generally rose-purple in color, although white flowers occasionally occur. Flowering occurs from July to September. The heads are solitary at the ends of the upper branches. They are broadly oval and almost globe-shaped, 0.5 inch (1.3 cm) long. The bracts of meadow knapweed are light to dark brown, with a fringed margin.

Seeds: Meadow knapweed seeds are pale tan in color, plumeless, 1/8 inch (2 cm) long.

Habitat and Occurrence: Meadow knapweed prefers moister and cooler conditions than the other knapweeds. It occurs predominantly in coastal Washington and Oregon, but is also found in moister, cooler habitats of the interior, e.g. forest openings along rivers and streams.

Figure 7. Meadow knapweed. a. Plant b. Seed c. Seedhead d. US distribution.

Black knapweed is a perennial plant regrowing each year from a woody root crown (Fig. 8). It was introduced into the United States from the United Kingdom.

Leaves: Basal rosette leaves are broad, stalked, and shallowly lobed. Stem leaves are smaller and not lobed.

Stems: Stems are erect and branched near the middle, from 8 to 32 inches (20 to 80 cm) tall, the base of the stem is sometimes prostrate and rooting from the nodes.

Flowers: Flowering occurs from July to October. Flowers are rose colored. Heads occur solitary at the ends of the upper branches. They are broad and rounded, 0.5 inch (1.3 cm) tall and 1 inch (2.54 cm) wide. The bracts of black knapweed are dark brown to black, with a comb-like fringe on the margin.

Seeds: Black knapweed produces about 60 seeds per head. They are ivory with lengthwise stripes, and have a pale, short pappus.

Habitat and Occurrence: Like meadow knapweed, black knapweed occurs predominantly in coastal Washington and Oregon, and in other cooler regions of the inland Northwest.

Figure 8. Black knapweed. a. Plant b. Seed c. Seedhead d. US distribution.

Brown knapweed (Fig. 9) is a perennial that reproduces only by seeds. It is native to Europe.

Leaves: Basal leaves are up to 6 inches (15.2 cm) long, tapering at both ends with the broadest part above the middle of the leaf. Stem leaves are lance-shaped, shallowly-lobed and stalkless.

Flowers: Flowers are rose-purple in color, rearely white. Flowering occurs from July to October. Heads are solitary at the ends of the upper branches. They are broadly oval. The bracts of brown knapweed are light to dark brown, with a papery, translucent margin.

Seeds: Brown knapweed seeds are light brown, plumeless, 1/8 inch (2 cm) long. Each head produces about twelve seeds.

Habitat and Occurrence: Like meadow knapweed, brown knapweed prefers moister, cooler conditions than the other knapweed species. It occurs predominantly in coastal Washington and Oregon, although it is distributed both in the West and the East. It also occurs in British Columbia.

Figure 9. Brown knapweed. a. Plant b. Seed c. Seedhead d. US distribution.