Family:
Asteraceae (Sunflower)
Other Names:
spreading knapweed, tumble knapweed
USDA Code:
CEDI3
Legal Status:
Colorado Noxious Weed List B
Identification
Growth form:
Biennial or short-lived
perennial forb.
Flower:
Flower heads are broadly
urn-shaped, 0.6-0.8 inches tall, solitary or in clusters of 2-3 at
the ends of the branches. Floral bracts are yellowish with a
brownish margin, sometimes spotted, fringed on the sides, and
terminating in a slender bristle or spine. The heads contain two
types of flowers, ray flowers around the edges surrounding tubular
disk flowers. The ray flowers are white, rose-purple, to lavender.
Seeds/Fruit:
Seeds are light brown to
black.
Leaves:
Basal leaves are stalked and
divided into narrow, hairy segments. Stem leaves are smaller,
alternate, less divided, stalkless, and become bract-like near the
flower clusters.
Stems:
Stems are upright, 4-24 inches
tall, highly branched, angled, with short, stiff hairs on the
angles.
Roots:
Taproot.
Seedling:
Seedlings have finely
divided leaves that are covered with short hair.
Similar
Species
Exotics:
Diffuse knapweed may be
distinguished from other knapweeds by the terminal spine on the
floral bract.
Natives: None.
Impacts
Agricultural:
Diffuse knapweed
reduces the productivity of rangeland by displacing desirable forage
species.
Ecological:
Diffuse knapweed is a
pioneer species that can quickly invade disturbed and undisturbed
grassland, shrubland, and riparian communities. Once established,
diffuse knapweed outcompetes and reduces the quantity of desirable
native species such as perennial grasses. Diffuse knapweed contains
allelopathic chemicals, which can suppress competitive plant growth
and create single species stands (Watson and Renney 1974). The
densities of these stands can range from 1-500 plants/m2.
The replacement of native grassland with diffuse knapweed can reduce
biological activity and increase soil erosion (Sheley et al. 1997).
Human:
Some people develop a rash when handling the plants.
Habitat
and Distribution
General requirements:
Diffuse
knapweed is found on plains, rangelands, and forested benchlands. It
is generally found on light, dry, porous soils. Diffuse knapweed has
been observed in elevations up to 7,000 feet (Zimmerman 1997). It
prefers open habitats to shaded areas (Watson and Renney 1974).
Diffuse knapweed is not common on cultivated lands or irrigated
pasture because it cannot tolerate cultivation or excessive moisture
(Watson and Renney 1974).
Distribution:
Diffuse knapweed is now
common in the Front Range counties, and has been reported in
scattered infestations from both the east and west slope of
Colorado.
Historical:
Native to Eurasia.
Biology/Ecology
Life cycle:
Diffuse knapweed plants
first form low rosettes and may remain in this form for one to
several years depending on environmental conditions. Diffuse
knapweed plants that complete their juvenile growth by the fall
overwinter as rosettes and bolt in early spring (Watson and Renney
1974). Diffuse knapweed plants that have not finished the juvenile
stage by the end of fall remain as rosettes through the second year
and bolt during the third year. Flower buds are formed in early June
and flowering occurs in July and August (Watson and Renney 1974).
Mature seeds are formed by mid-August (Watson and Renney 1974).
Mode of reproduction:
Reproduces by
seeds.
Seed production:
A single diffuse
knapweed plant can produce up to 18,000 seeds (Harris and Cranston
1979) and a stand of diffuse knapweed can produce up to 40,000 seeds
per square meter (Watson and Renney 1974).
Seed bank:
Seeds may remain dormant
for several years.
Dispersal:
Seed dispersal for diffuse
knapweed is mainly by wind (Watson and Renney 1974). When the seed
capsule sways in the breeze or is disturbed, the seeds fall from the
small opening in top of the flower head and are distributed around
the parent plant (Watson and Renney 1974). However, most of the
involucres remain closed until the plant dries up, breaks off at
ground level and effectively becomes a tumbleweed, allowing seeds to
be individually dispersed over long distances (Zimmerman 1997).
Diffuse knapweed stalks readily lodge under vehicles, expanding
their long distance dispersal.
References
Beck, G. K. 1997. Natural resources
series, diffuse knapweed. Colorado State University Cooperative
Extension. Internet 05/05/98. Available: http://ozma.jefferson.co.us/dpt/openspac/weed/dfknapwd.htm
Fletcher,
R.A., and A.J. Renney.
1963. A growth inhibitor found in Centaurea spp. Canadian
Journal of Plant Science 43:475-481.
Harris, P., and R. Cranston. 1979. An
economic evaluation of control methods for diffuse and spotted
knapweed in western Canada. Canadian Journal of Plant Science
59:375-382.
Niefoff, J. E-mail message-broadcast.
Sent 9-12-97, 9:29 AM.
Roché,
B.E. and C.T. Roché. 1999.
Diffuse knapweed. In: R.L. Sheley and J.K. Petroff (eds.)
Biology and management of noxious rangeland weeds. Oregon State
University Press, Corvallis, pg. 217-230.
Schirman, R. 1981. Seed production
and spring seedling establishment of diffuse and spotted
knapweed. Journal of Range Management 34:45-47.
Sheley,
R.L., B.E. Olson, and L.L.
Larson. 1997. Effect of weed seed rate and grass defoliation
level on diffuse knapweed. Journal of Range Management 50:
39-43.
Watson,
A.K., and A.J. Renney. 1974.
The biology of Canadian weeds Centaurea diffusa and C.
maculosa. Canadian Journal of Plant Science 54:687-701.
Youtie, B. 1997. Weed control as the
first step in protecting and restoring native plant communities
on northeast Oregon natural areas. Conservation and Management
of Native Plants and Fungi. Native Plant Society of Oregon,
Corvallis, Oregon. pages 78-82.
Youtie, B. and J. Soll. 1994.
Non-chemical control of diffuse knapweed (Centaurea diffusa).
Knapweed Newsletter 8(3):2-3. Washington State University
Cooperative Extension, Pullman WA.
Zimmerman,
J.A.C. 1997. Ecology and
distribution of Centaurea diffusa Lam., Asteraceae. USGS
Biological Resources Division , Colorado Plateau Field
Station-Flagstaff, Arizona. Internet 02/16/98. Available:
http://www.nbs.nau.edu/FNF/Vegetation/Exotics/diffusa/
diffusa.html
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