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Pest Status
Chapter 31
Centaurea diffusa Lam. and C. maculosa
Lam. s. lat., Diffuse and Spotted
Knapweed (Compositae)
P. HARRIS and I.H. MYERS
Diffuse and spotted knapweed are herbaceous plants introduced from Europe to the dry
grasslands of western Canada and other parts of North America. The combination of
their allelopathic properties (Fletcher & Renney 1963), low forage value. and drought
adaptation have allowed these two knapweeds to displace most other herbaceous plants
over large areas.
Diffuse knapwced is typically a biennial. The species was recorded in Washington State
in 1907 (Howell 1959) and early Canadian herbarium specimens suggest a northward
spread into British Columbia: Oyama 1936, Penticton 1939, Grandforks 1940 (Groh
1943); however, it may also have been introduced directly into the province with Turkestan
alfalfa from the Caspian Sea region. Renney (1959) suggested that the weed occurred at
Pritchard and Lytton, British Columbia, prior to 1930 and certainly other weeds from
southern USSR were recorded at Kamloops (near Pritchard) as early as 1920 (Groh
1940). From these beginnings the weed spread by 1972 to infest 25 952 ha of dryland range
and roadside in British Columbia as well as small areas in Alberta (Watson & Renney
1974). Distribution of diffuse knapweed in 1977 is shown in Fig. 4. In 1981 about 25 ha of
diffuse knapweed, 8 heavily infested. were found on one farm near Morden, Manitoba
(B. Todd, 1982, personal communication). Harris & Cranston (1979) predicted that the
relatively high availability of summer moisture in Manitoba would put it beyond the range
of the weed. It is now not clear whether the potential of the weed was seriously
underestimated or the infestation merely represented a local site anomaly as it was on a
light soil with a southern exposure and dry. In 1982 several kilometers ofrailway track
were found to be infested at Walsh, Saskatchewan. This is only a slight extension of the
range from the stand at Irvine, Alberta (Fig. 4); but a small stand was also found on the
railway track at Colonsay, Saskatchewan, which is southeast of Saskatoon (S. McKell,
personal communication). Distance spread ofthe weed across the prairies often seems to
be associated with railway ties or treated bridge timbers from British Columbia.
It appears that diffuse knapweed will continue to spread until it dominates the herbaceous
community in open uncultivated sites in the brown chernozem and brunisol soils of
southern British Columbia and the brown soils of Alberta and Saskatchewan. This
involves about 7.5 million ha (Harris & Cranston 1979).
Spotted knapweed is a short lived perennial. The first Canadian specimen was collected
by McCoun from Victoria, British Columbia, in 1893 (Groh 1943). By 1972, it was
estimated to have infested 2410 ha in British Columbia as well as several small stands in
Alberta (Watson & Renney 1974); but these amounts are small compared to the estimated
800000 ha in Montana, United States (Maddox 1979). The western Canadian distribution
of the weed in 1977 is shown in Fig. 5.
It is predicted that spotted knapweed will spread to dominate the herbaceous community
in open uncultivated or lightly forested sites at the dry end of the Douglas fir zone
in British Columbia and in the dark brown soils of Alberta and Saskatchewan. About 3.2
million ha are vulnerable to invasion (Harris & Cranston 1979).
Assuming that the two knapweeds spread to their predicted limits of approximately 10
million ha. the loss in terms of dry native pasture species would be 2.5 million tonnes/year
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