Spotted knapweed

Integrated Taxanomic Information System (ITIS)

Taxonomy• Family: Asteracea – Aster

family• Genus: Centaurea L. –

knapweed• Species: Centaurea

biebersteinii DC. • Synonyms:

• Acosta maculosa auct. non Holub

• Centaurea maculosa auct. non Lam.

• Centaurea stoebe L. ssp. micranthos (Gugler) Hayek

• Common name: Spotted knapweed

Species description

• Biennial or short-lived perennial with deep tap root.

• Juvenile plants and over-wintering adults are in the rosette form.

• Small plants have one unbranched stem with one flower head, large plants have multiple stems and as many as 100 flower heads.

• Mature plants grow to height of between 15 & 122 cm (6” – 4’).

Species description

• Flower heads are 5 to 10 mm long and can occur singly or in clusters at branch tips.

• Each head bears stiff, black tipped bracts giving the plant its “spotted” appearance.

Species description

• Seeds are 2.5 mm long.• Seeds are oval, black or

brown, with pale vertical lines.

• Each seed has a short bristly pappus about half the length of the seed.

Biology

• Pollinated primarily by flies and bees; self compatible.• Each plant usually produces between 400 and 900

seeds; seed production is highly dependant on annual precipitation.

• Plants in irrigated fields produced as many as 25,263 seeds.

• Rarely produces rhizomes; population growth dependent almost solely on seed.

Biology

• Three seed-germination types produced by parent plant:

1) Non-dormant seed germinates regardless of light.

2) Seed with dormancy set by far red light.

3) Dormant seed that does not germinate until buried with soil.

• Dormant seeds readily accrue in seed bank; seeds viable as long as 8 years.

• Germinate over a range of soil depths, water content, and temperature.

Biology

• Seeds germinate between late March and June of each year. Earliest plants to germinate have the highest survival.

• Seedlings have tap root as long as 80 cm after two months of growth.

• Seedling mortality is highly variable from year to year (precipitation), but averages about 12%.

Biology

• Juvenile and mature plants spend winter as a rosette.

• Rosette mortality has been reported as high as 48% and was attributed to root grazing by weevils and underground rodents.

• Adult spotted knapweed flower 1-3 years over lifetime.

• Plants as old as 12 years have been documented.

Geographic Range: Native

• Central Europe and east to central Russia, Caucasia, and western Siberia.

• Found in elevations ranging from 578 to 3,040 m.

• Found in precipitation zones from 20 to 200 cm annually.

Spotted knapweed in Romania

Geographic Range: North America

• Introduced as contaminants in alfalfa and soil used as ship ballast.

• First recorded in Victoria, British Columbia in 1893.

• Since spread to virtually every Canadian province and nearly every state in the US.

Geographic Range: United States

• Limited to San Juan Islands, WA, until 1920.• 1920-1960: spread to 20 counties in Pacific Northwest.• 1960-1980: spread to 48 counties.• 1980-present: spread to nearly every state including

every county in Washington, Idaho, Montana, and Wyoming.

• Nevada:

Preferred habitats: everywhere!

What makes it invasive?

1) Vacant niche2) Allelopathy

3) Atmospheric CO2 / global climate change4) Resource availability / Competition5) Microevolutionary change6) Escape from biotic constraints??7) Biodiversity8) Disturbance / land use9) Anthropogenic

What makes it invasive?Allelopathy

• According to Weir et al. (2003), a study as early as 1832 hypothesized that allelopathy could be responsible for spotted knapweeds success.

• Fletcher and Renney (1963) were first to extract substances with allelopathic potential from leaves, stem, and roots.

• Since 1963 numerouse studies have isolated chemicals such as cnicin, (±) –catechin, and numerous polyacetylenes (Stevens 1986, Kelsey and Locken 1987, Locken and Kelsey 1987, Bais et al. 2002 and 2003, and others).

What makes it invasive?Allelopathy

• Ridenour and Callaway (2001) showed that spotted knapweed has negative effects on biomass of roots and shoots of Festuca idahoensis which are ameliorated by activated charcoal.

What makes it invasive?Allelopathy

• Bais et al. (2003) reported that a mixture of (+)-catechin and (-)-catechin was present in root exudates of spotted knapweed.

• Further studies showed that (-)-catechin was solely responsible for phytotoxic activity, whereas (+)-catechin provided antimicrobial properties.

• (-)–catechin inhibits root growth and germination in field soils of two North American grasses (Festuca idahoensis and Koeleria micrantha), “by triggering a wave of reactive oxygen species initiated at the root meristem, which leads to a Ca2+ signaling cascade triggering genome-wide changes in gene expression and ultimately death of the root system”.

What makes it invasive?

1) Vacant niche2) Allelopathy3) Resource availability / Competition

4) Atmospheric CO2 / global climate change5) Microevolutionary change6) Escape from biotic constraints7) Biodiversity8) Disturbance / land use9) Anthropogenic

What makes it invasive? Competition / Resource availability

• Callaway et al. (2004) showed that the presence of arbuscular mycorrhizae can significantly alter the competitive balance between spotted knapweed and native grasses / forbs.

+ +

- -

AMF helps (+) SK or hinders (-) SK

What makes it invasive?Competition / Resource availability

• Numerous studies have determined how spotted knapweed and various other native species respond to manipulations of limiting resources such as nitrogen, phosphorus, and water (Story et al. 1989, Herron et al. 2001, Blicker et al. 2002, Olson et al. 2003, Blicker et al. 2003, Sperber et al. 2003, and Hook et al. 2004).

• In a glasshouse studies, Blicker et al. (2002, 2003) studied water & N use of spotted knapweed in comparison with three native grasses.

– Spotted knapweed did not have the greatest WUE.– Spotted knapweed did not use the most water.– Spotted knapweed did not have the greatest NUE.– Spotted knapweed did not use the most nitrogen.

What makes it invasive?Competition / Resource availability

• Herron et al. (2001) hypothesized that removal of nitrogen and phosphorus from soil would shift competitive balance to native bluebunch wheatgrass. – Without nutrient manipulation, spotted knapweed had

higher N uptake and biomass.– When N was removed from soil, the competitive

balance shifted to bluebunch wheatgrass.– Phosphorus removal had no effect.

What makes it invasive?Competition / Resource availability

• Spotted knapweed is probably a better competitor for nitrogen when N levels are high.

• Probably not a better competitor for phosphorus.• Does not use water efficiently; deep tap root• Arbuscular mycorrhizae can have a positive or negative

effect on competition between spotted knapweed and native species.

• Callaway and Ridenour (2004) hypothesized that there could be a connection between allelopathy and competition for resources.

What makes it invasive?Competition / Resource availability

What makes it invasive?

1) Vacant niche2) Allelopathy3) Resource availability / Competition

4) Atmospheric CO2 / global climate change5) Microevolutionary change6) Escape from biotic constraints7) Biodiversity8) Disturbance / land use9) Anthropogenic

What makes it invasive? Atmospheric CO2 / global climate change

• Ziska (2003) showed that leaf area significantly increased with each step increase in CO2.

• Leaf, stem, and root weight, as well as overall biomass also increased significantly with each step increase in CO2.

Pre-industrial

Current

Future

What makes it invasive?

1) Vacant niche2) Allelopathy3) Resource availability / Competition

4) Atmospheric CO2 / global climate change5) Microevolutionary change6) Escape from biotic constraints7) Biodiversity8) Disturbance / land use9) Anthropogenic

What makes it invasive?Biodiversity

• Ortega and Pearson (2005) studied bluebunch wheatgrass grassland types with spotted knapweed present at high and low densities in western Montana.

• “impacts of strong invaders, such as C. maculosa, on native diversity may be evident even at low levels of invasion… biotic resistance may not be a significant force in the real world”.

What makes it invasive?

1) Vacant niche2) Allelopathy3) Resource availability / Competition

4) Atmospheric CO2 / global climate change5) Microevolutionary change6) Escape from biotic constraints7) Biodiversity8) Disturbance / land use9) Anthropogenic

What makes it invasive?Disturbance / Land Use

• Spotted knapweed densities are highly correlated with the degree of soil disturbance.

• Because spotted knapweed reproduces entirely by seed, disturbed soil allows for rapid establishment and spread.

What makes it invasive?

1) Vacant niche2) Allelopathy3) Resource availability / Competition

4) Atmospheric CO2 / global climate change5) Microevolutionary change6) Escape from biotic constraints7) Biodiversity8) Disturbance / land use9) Anthropogenic

Impacts

• Ecological– Spotted knapweed monocultures eliminate

native species.

– Can severely reduce wildlife forage.– Alters biogeochemical cycling by increasing

surface water runoff and stream sediment yield.

Impacts

• Economic– Reduction in quality and yield of livestock

forage.– Does have some nutritional value early in the

spring and is used by sheep, goats, and cattle.

– Bucher (1984) claimed a loss of $156 million per year due to a 63% decline in forage in Montana.

– Reduces desirability of recreational areas.

Impacts

• Social– Rural economy and lifestyle can be severely

affected by large infestations.– Recreation opportunities can be limited by

large infestations.

Management

• Inventory and mapping are critical to proper management.

• Remote sensing has been effective tool for mapping spotted knapweed.

Management

• Prevention– Prevention of spotted knapweed to adjacent

rangelands is most cost effective method.– Seed dispersal can be limited by driving on

designated roads.– Grazing livestock in newly establishing areas

early in spring.– Minimizing all soil disturbances; spotted

knapweed is usually first invader on disturbed sites.

Control: Biological Control

• Numerous natural enemies have been approved as biological control agents.

Bio-control Agents

Sphenoptera spp. (root beetle) Urophora spp. (seedhead fly)

Urophora spp. (seedhead fly)

Agapeta spp. (root boring moth) Cyphocleonus spp.

(root boring/gall weevil)

Control: Biological Control

• Numerous natural enemies have been approved as biological control agents.

• Marginal success at best for all agents.

• Seed production has been dramatically decreased in some cases, but not enough to significantly decrease infestation.

Control

• Burning• Single low intensity fire may elicit positive response of

spotted knapweed.• Repeated annual summer burns before seeds

develop has reduced populations 5-90%

• Grazing• Spotted knapweed can tolerate severe defoliation

related to grazing.• Repeated annual grazing in the spring during the

rosette stage can reduce populations.

Control

• Herbicides• Picloram, clopyralid, dicamba, and 2,4-D provide

effective control on spotted knapweed. Timing for best control varies depending on herbicide.

• Picloram provided nearly 100% control of spotted knapweed over 8 year span.

• Mowing• Long-term mowing effects not known.• Longest study of three years concluded that it

provided partial control if done at the flowering stage.

Eradication

• Little hope of eradication throughout all of introduced range.

• Best course of action is for regional land managers to form a comprehensive management plan that includes mapping, prevention methods, and multiple control methods.

Summary

• Key biological characteristics– Allelopathic– Usually out-competes natives for nitrogen– Thrives in disturbed sites

• Control strategies– Grazing/fire to reduce seed production– Chemicals can work; picloram had best results– Manipulate soil N to low levels where natives out-

compete