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Skaneateles Conservation Area/Invasive species/Centaurea

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Centaurea (knapweed) species

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Because the various knapweed species that are or might be present at the Skaneateles Conservation Area 9sca) are somewhat difficult to distinguish from one another, Centaurea will here generally be treated either all together or as just two groups:

Identification

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The genus Centaurea contains three species (and a hybrid) of invasive knapweeds that are most likely to be present at the SCA.

These species can often be distinguished by the involucre bracts, the scale-like structures surrounding the inflorescence, just below flower petals.[1]

  • Centaurea jacea (brown knapweed): Bracts are somewhat hairy, with broad, thin papery margins. Center of bract is dark brown.
  • Centaurea nigra (black knapweed): Bracts are oval-shaped with broad, comb-like, dark brown or black fringed margins. Fringe length up to 3 times as long as bract width.
  • Centaurea × moncktonii (meadow or Monckton's knapweed): Bracts are light to dark brown, roundish, with thin papery margin with fringes about equal width of bract center.
  • Centaurea stoebe ssp. micanthos (spotted knapweed): Bracts are black tipped, tip and upper margin have soft, spine-like fringe, Center spine shorter than others.
  1. ↑ Celestine Duncan (2018). "What’s the Difference Between Spotted, Brown, Black, and Meadow Knapweed?" TechLine Invasive Plant News Published November 2018; reviewed May, 2019.

Ecological impact

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Centaurea stoebe ssp. micranthos (spotted knapweed) is better adapted for extracting nutrients from the soil than are native species. Spotted knapweed alters the ecology of the ecosystem as it invades. Native species in areas where spotted knapweed occurs tend to have network root systems and as these species decline, their network root systems are replaced by the knapweed's taproot system. This taproot system alters the soil by lowering its water holding capability and increasing soil erosion.[1][2]

Spotted knapweed can significantly impact the herb layer by increasing its height and density. It often occurs in open dry sandy areas where the herb layer is low and/or sparse.

  1. ↑ Teresa Mauer, Mary J Russo, Margaret Evans (2002). Element Stewardship Abstract for Centaurea maculosa (Spotted Knapweed). The Nature Conservancy, Arlington. VA.
  2. ↑ M.J.Jordan, G.Moore & T.W.Weldy (2008). Centaurea stoebe ssp. micranthos. Invasiveness ranking system for non-native plants of New York. Unpublished. The Nature Conservancy, Cold Spring Harbor, NY; Brooklyn Botanic Garden, Brooklyn, NY; The Nature Conservancy, Albany, NY.

Biological characteristics and dispersal ability

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Centaurea stoebe ssp. micranthos (spotted knapweed) plants can produce up to 600 seeds or more with high viability and are self-compatible. Achenes are small, and long distance dispersal can either be passive (wind) or active (birds, rodents). Readily spread by vehicles and contaminated seed and hay supplies. Forms a dense layer above shorter vegetation, especially on low nutrient soils that generally support a low, sparse herb layer of native plant species[1]

Spotted knapweed is also reported to be allelopathic. It will germinate in existing vegetation, but the phytotoxin catechin produced by the plant inhibits germination of its own seeds, so they will germinate in areas that lack mature plants.[2][3]

  1. ↑ Teresa Mauer, Mary J Russo, Margaret Evans (2002). Element Stewardship Abstract for Centaurea maculosa (Spotted Knapweed). The Nature Conservancy, Arlington. VA.
  2. ↑ R.M.Callaway, H.P.Bias, T.L.Wier, L.Perry, W.M.Ridenour, J.M.Vivanco (2005) "Allelopathy and exotic plant invasion: from genes to communities: synopsis,updates, and implications." In Proceedings of the 4th World Congress on Allelopathy. (J.D.I.Harper, M An, H.Wu and J.H.Kent, eds.) Charles Stuart University, Wagga Wagga, NSW, Australia. August 2005. International Allelopathy Society. pp. 33-38.
  3. ↑ R.M.Callaway & J.M.Vivanco (2005). "Invasion of plants into native communities using the underground information superhighway." In Proceedings of the 4th World Congress on Allelopathy. (J.D.I.Harper, M An, H.Wu and J.H.Kent, eds.) Charles Stuart University, Wagga Wagga, NSW, Australia. August 2005. International Allelopathy Society. pp. 50-56. .

Ecological amplitude and distribution

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Large stands of spotted knapweed can occur in areas with few other invasive species already present. Suitable habitats include beaches, coastal dunes, cultivated fields, old fields, grasslands, shrublands, forests, woodlands, and roadsides, most of which are present to some extent at the SCA.

Spotted knapweed has been introduced to all northeastern states and provinces, and is present in all New York State PRISMs.

Difficulty of control

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Seeds remain viable for up to eight years. There is no evidence for more than 10 years. Plants will regrow from extensive underground root systems.[1]

  1. ↑ USDA-NRCS (2012) Plant Guide for Spotted Knapweed Centaurea steobe
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