Centaurea stoebe L. ssp. micranthos (Gugler) Hayek Acosta maculosa auct. non (Lam.) Holub Centaurea biebersteinii DC. Centaurea maculosa auct. non Lam. 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 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 1 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire 2 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire 3 · 7/19/2017 · West Virginia 4 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts 5 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts 6 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts 7 · 7/19/2017 · West Virginia 8 · 7/19/2017 · West Virginia Centaurea stoebe ssp. micranthos description by Thomas H. Kent, last updated 28 Dec 2020. © FloraFinder.org. All rights reserved. |
9 · The white variant is unusual. · 7/17/2014 · Amos Kendall Conservation Land, Dunstable, Massachusetts 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 11 · 8/4/2009 · Near Nashoba Hospital, Ayer, Massachusetts 12 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts 13 · The white variant is unusual. · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts 14 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire 15 · The white variant is unusual. · 7/17/2014 · Amos Kendall Conservation Land, Dunstable, Massachusetts 16 · 7/31/2018 · Andres Art Institute, Big Bear Mountain, Brookline, New Hampshire 17 · 7/21/2010 · Nashua River Rail Trail, Ayer, Massachusetts 18 · 8/7/2013 · Amos Kendall Conservation Land, Dunstable, Massachusetts 19 · 7/28/2013 · Andres Institute of Art, Big Bear Mountain, Brookline, New Hampshire 20 · 7/30/2012 · Tom and Susan’s, Pepperell, Massachusetts 21 · 12/12/2020 · Tom and Susan’s, Pepperell, Massachusetts 22 · 8/6/2009 · Nashua River Rail Trail, Groton, Massachusetts Range:
|