Centaurea stoebe ssp. micranthos (Spotted knapweed)

Centaurea stoebe L. ssp. micranthos (Gugler) Hayek

Acosta maculosa auct. non (Lam.) Holub

Centaurea biebersteinii DC.

Centaurea maculosa auct. non Lam.

Spotted knapweed

Kingdom	Plantae	Plants, but not fungi, lichens, or algae
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

About plant names...

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 24" (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 21.

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:

Centaurea stoebe ssp. micranthos on the USDA Forest Service's Fire Effects Information Database

Centaurea stoebe ssp. micranthos on the Plant Conservation Alliance’s Alien Plant Working Group Least Wanted List

Centaurea stoebe ssp. micranthos on Forestry Images

Centaurea stoebe ssp. micranthos at the USDA National Invasive Species Information Center

Centaurea stoebe ssp. micranthos at cabi.org’s Invasive Species Compendium

Centaurea stoebe ssp. micranthos on Invasive.org, Center for Invasive Species and Ecosystem Health (PDF)

Centaurea stoebe ssp. micranthos on forestupdate.frec.vt.edu

Centaurea stoebe ssp. micranthos on SEINet—the Southwest Environmental Information Network

References:

Old, Richard, 1200 Weeds of North America DVD, XID Services, Inc., 2012

Uva, Richard H.; Neal, Joseph C.; DiTomaso, Joseph M., Weeds of the Northeast, Comstock Publishing Associates, 1997, p. 126

Newcomb, Lawrence, Morrison, Gordon (Illus.), Newcomb’s Wildflower Guide, Little, Brown and Company, 1977, p. 234

Peterson, Roger Tory, McKenny, Margaret, Peterson Field Guides: A Field Guide to Wildflowers of Northeastern and North Central North America, Houghton Mifflin, 1968, p. 92, 306

1 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire
≈ 3½ × 2½" (8.6 × 6.7 cm)

2 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire
≈ 4 × 3½" (11 × 8.9 cm)

3 · 7/19/2017 · West Virginia
≈ 3½ × 5" (9.3 × 13 cm)

4 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 11 × 7" (28 × 18 cm)

5 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts
≈ 16 × 12" (41 × 30 cm)

6 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 8 × 6" (20 × 14 cm)

7 · 7/19/2017 · West Virginia
≈ 4 × 3½" (10 × 9.2 cm)

8 · 7/19/2017 · West Virginia
≈ 3½ × 2½" (9.7 × 7.3 cm)

Centaurea stoebe ssp. micranthos description by Thomas H. Kent, last updated 28 Dec 2020.

9 · The white variant is unusual. · 7/17/2014 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 8 × 5" (19 × 12 cm)

10 · 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)

11 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts
≈ 2 × 3" (5.4 × 7.5 cm)

12 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 9 × 6" (23 × 16 cm)

13 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 6 × 4" (16 × 10 cm)

14 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire
≈ 5 × 3½" (13 × 8.8 cm)

15 · The white variant is unusual. · 7/17/2014 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 8 × 6" (20 × 14 cm)

16 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire
≈ 2½ × 2" (7.2 × 5.8 cm)

17 · 7/21/2010 · Nashua River Rail Trail, Ayer, Massachusetts
≈ 5 × 8" (13 × 19 cm)

18 · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts
≈ 10 × 7" (25 × 16 cm)

19 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire
≈ 5 × 3½" (13 × 8.8 cm)

20 · 7/30/2012 · Tom and Susan’s, Pepperell, Massachusetts
≈ 3½ × 2½" (9.8 × 6.6 cm) ID is uncertain

21 · 12/12/2020 · Tom and Susan’s, Pepperell, Massachusetts ID is uncertain

22 · 8/6/2009 · Nashua River Rail Trail, Groton, Massachusetts
≈ 5 × 4" (13 × 10 cm)

Range:

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