Centaurea biebersteinii DC.
Centaurea maculosa auct. non Lam.
Centaurea stoebe L. ssp. micranthos (Gugler) Hayek
Spotted knapweed
Spotted knapweed, native to eastern Europe, was introduced accidentally to North America in the early 1900s, as a contaminant in seeds for crops. It has since spread prolifically through much of the U.S. and Canada, where it competes with forage plants intended for livestock. It is considered an invasive species in many areas. Forage animals will eat it only as a last resort.
Plants: These attractively flowered but ungainly-looking plants average about 2′ (60 cm) in height and reach up to 5′ (1.5 m). They consist of 1-20 branched stems. Stems are dull green, covered with small rough hairs. Plants release catechin, a natural herbicide, slowing the growth rate of competing species, a trick called allelopathy.
Leaves: The leaves at the base of the plant are somewhat pale, forming a basal rosette with deeply lobed oval-shaped leaflets up to 8″ (20 cm) long, growing alternately from the main portion of the leaf. Thin stems with many branches form a tangle in the upper part of the plant, with very narrow leaflets less than 1″ (2.5 cm) in length.
Flowers: Pink to purple (rarely white) in color, about ¾″ (1.9 cm) in diameter, with each petal branching several times. The “spotted” in spotted knapweed is the brown tips on the bracts forming the urn-shaped structure beneath the flowers, for example, in Photo 24.
Fruits: Narrow, erect green pods contain seeds that are 1/16″ (2.5 mm) long, oval, and shiny black or brown with pale, vertical lines. A short pappus forms a micro-parachute at the tip of each seed.
Habitats: Dry areas including meadows, pastures, rocky areas, gravel sites, railroads, roadsides, vacant lots, and forest clearings.
Spotted knapweed flowers bear a superficial resemblance to those of ragged robin: both disheveled-looking, both similar in hue. If you look closely, though, knapweeds are composite flowers (like other members of the large aster family)—they are composed of ray flowers and disc flowers. Ragged robin flowers have petals, but no central disc.
Composite flowers, such as daisies, are typically composed of a central disc containing many small tubular disc flowers, surrounded by flat ray flowers. The ray flowers are often called petals, but each “petal” is actually a complete flower. Some composite flowers have only disc or ray flowers. “Composite” designates an aggregation of many small flowers that resembles a single flower, rather than two different types of flowers. The involucral bract is sometimes an important identifying feature—for example, this is where the spots are in spotted knapweed. Finally, the receptacle is where the parts of a flower head come together. 
Online References:
References:
Acosta maculosa auct. non (Lam.) Holub
Centaurea biebersteinii DC.
Centaurea maculosa auct. non Lam.
Centaurea biebersteinii DC.
Centaurea maculosa auct. non Lam.
1 · 9/14/2023 · Nature Conservancy Access Trail, Sprague Pond, Phippsburg, Maine · ≈ 4½ × 5″ (12 × 13 cm) 
2 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire · ≈ 3½ × 2½″ (8.9 × 6.9 cm) 
3 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire · ≈ 4 × 3″ (11 × 8.5 cm) 
4 · 7/19/2017 · West Virginia · ≈ 3½ × 5″ (9.4 × 13 cm) 
5 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 11 × 7″ (28 × 18 cm) 
6 · 9/9/2014 · Bemis Road, Pepperell, Massachusetts · ≈ 7 × 6″ (18 × 15 cm) 
7 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts · ≈ 16 × 12″ (41 × 30 cm) 
8 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 8 × 6″ (20 × 14 cm) 
9 · 7/19/2017 · West Virginia · ≈ 4 × 3½″ (10 × 9.2 cm) 
10 · 7/19/2017 · West Virginia · ≈ 4 × 3″ (10 × 8.4 cm) 
11 · The white variant is unusual. · 7/17/2014 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 8 × 5″ (19 × 12 cm) 
12 · That’s a clearwing moth, possibly Hemaris thysbe, not a hummingbird! · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire · ≈ 7 × 6″ (17 × 14 cm) 
13 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts · ≈ 2 × 3″ (5.4 × 7.5 cm) 
14 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 9 × 6″ (23 × 16 cm) 
15 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 6 × 4″ (16 × 10 cm) 
16 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire · ≈ 5 × 3½″ (13 × 8.8 cm) 
17 · The white variant is unusual. · 7/17/2014 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 6 × 4″ (15 × 10 cm) 
18 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire · ≈ 2½ × 2″ (7.2 × 5.8 cm) 
19 · 7/21/2010 · Nashua River Rail Trail, Ayer, Massachusetts · ≈ 8 × 5″ (19 × 13 cm) 
20 · 7/5/2021 · Curtis Farm Preserve, Harpswell, Maine · ≈ 4 × 3″ (10 × 7.6 cm) 
21 · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts · ≈ 10 × 7″ (25 × 16 cm) 
22 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire · ≈ 4 × 3½″ (11 × 8.8 cm) 
23 · 7/30/2012 · Tom and Susan’s, Pepperell, Massachusetts · ≈ 3½ × 2½″ (9.8 × 6.6 cm) 
ID is uncertain 
24 · 1/5/2026 · Tom and Susan’s, Pepperell, Massachusetts ID is uncertain 
25 · 8/6/2009 · Nashua River Rail Trail, Groton, Massachusetts · ≈ 5 × 4″ (13 × 10 cm) 
26 · 9/9/2014 · Bemis Road, Pepperell, Massachusetts · ≈ 9 × 6″ (23 × 16 cm) 
| Kingdom | Plantae | Plants, but not fungi, lichens, or algae (from Stearn's Botanical Latin) |
| Subkingdom | Tracheobionta | Vascular plants—plants with a “circulatory system” for delivering water and nutrients |
| Division | Magnoliophyta | Flowering plants, also known as angiosperms |
| Class | Magnoliopsida | Dicotyledons—plants with two initial seed leaves |
| Subclass | Asteridae | A large class that encompasses asters |
| Order | Asterales | Flowering plants with a central disk flower and surrounding petals, like daisies |
| Family | Asteraceae | The aster family, which also includes daisies and sunflowers; from the Greek ἀστήρ, “star,” for the star-shaped flowers |
| Genus | Centaurea | From the Latin and a reference to the Centaur Chiron who was supposed to have discovered the medicinal uses of a plant in Greece that came to be called Centaury |
| Species | stoebe | The PlantzAfrica website says that the name of the genus Stoebe is from the Greek stoibe, “stuffing, padding or heap.” It was apparently used for packing wine jars and making brooms and bedding. Umberto Quattrocchi gives the following, also for the genus Stoebe: “Greek steibein, stibo “to tread firmly,” stoibe “thorny burnet, a species of Poterium,” Latin stoebe, es for a plant, called also pheos (Plinius).” From David Hollombe: “stoibe, name used by Dioscorides for Poterium spinosum, also meaning a cushion or pad” |
| ssp. | micranthos | Small-flowered |
Centaurea stoebe ssp. micranthos description by Thomas H. Kent, not updated.
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