Acosta maculosa auct. non (Lam.) Holub
Centaurea biebersteinii DC.
Centaurea maculosa auct. non Lam.
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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. 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:
The USDA Forest Service's Fire Effects Information Database The Plant Conservation Alliance’s Alien Plant Working Group Least Wanted List The USDA National Invasive Species Information Center Cabi.org’s Invasive Species Compendium Invasive.org, Center for Invasive Species and Ecosystem Health (PDF) SEINet—the Southwest Environmental Information Network References:
Old, Richard, 1200 Weeds of North America DVD, XID Services, Inc., 2012
1 · 9/14/2023 · Nature Conservancy Access Trail, Sprague Pond, Phippsburg, Maine · ≈ 4½ × 7″ (12 × 18 cm)
2 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire · ≈ 5 × 3½″ (13 × 8.7 cm)
3 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire · ≈ 6 × 4″ (15 × 10 cm)
4 · 7/19/2017 · West Virginia · ≈ 4 × 6″ (10 × 15 cm)
5 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
6 · 9/9/2014 · Bemis Road, Pepperell, Massachusetts · ≈ 9 × 6″ (22 × 15 cm)
7 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts · ≈ 1½ × 1′ (48 × 32 cm)
8 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
9 · 7/19/2017 · West Virginia · ≈ 5 × 3½″ (13 × 9.2 cm)
10 · 7/19/2017 · West Virginia · ≈ 5 × 3″ (12 × 8.4 cm) Centaurea stoebe ssp. micranthos description by Thomas H. Kent, last updated 14 Sep 2023.
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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 · ≈ 11 × 7″ (28 × 18 cm)
13 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts · ≈ 7 × 4″ (16 × 11 cm)
14 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
15 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
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 · ≈ 8 × 6″ (20 × 14 cm)
18 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire · ≈ 3½ × 5″ (8.7 × 13 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 · ≈ 5 × 3″ (12 × 8.4 cm)
21 · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
22 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire · ≈ 5 × 3½″ (13 × 8.8 cm)
23 · 7/30/2012 · Tom and Susan’s, Pepperell, Massachusetts · ≈ 3½ × 2½″ (9.8 × 6.6 cm)
24 · 7/29/2023 · Tom and Susan’s, Pepperell, Massachusetts
25 · 8/6/2009 · Nashua River Rail Trail, Groton, Massachusetts · ≈ 7 × 4½″ (17 × 11 cm)
26 · 9/9/2014 · Bemis Road, Pepperell, Massachusetts · ≈ 9 × 6″ (23 × 16 cm) Range:
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ID is uncertain 
