The recent discovery of spotted knapweed in Regina has prompted efforts to raise awareness of this noxious weed.
It is a scourge of forage lands in Montana, Alberta and British Columbia. Unless controlled early, it has the same potential in Saskatchewan.
“Spotted knapweed is not commonly known in Saskatchewan at present, but is considered to be well-adapted to the dark brown soil zone,” said Clark Brenzil, a weed control specialist with Saskatchewan Agriculture.
“By raising awareness of the weed and the potential for its spread, we hope to keep spotted knapweed from becoming a scentless chamomile of pasture land.”
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It has pink-to-purple, sometimes white, thistle-like flowers but has no thorns. Closer inspection will show deeply lobed leaves. It gets its name from black fringes on the tips of the flower bracts, which give the egg-shaped flower heads a distinct spotted appearance. It can grow almost a metre high, with bush-like proportions.
Brenzil said spotted knapweed is a biennial or a short-lived perennial. A true biennial will remain in a vegetative rosette stage in its first year and produce flowers and seeds in its second year. Then it dies, relying completely on seeds to reproduce.
Spotted knapweed, however, often remains as a rosette for up to four years and, although most biennials die after the second year, it has been reported to flower continuously for up to 10 years in Montana.
It is also prolific.
“One plant can produce up to 146,000 seeds, 30 percent of which may still be viable eight years after burial in the soil,” Brenzil said.
A unique characteristic of spotted and other knapweeds is the production of a substance that suppresses growth of the plants around them, which contributes to their spread.
Spotted knapweed does not tolerate tillage, tending to thrive in forage lands and waste or industrial areas. Rail and roadbeds are favourites. It establishes best in habitats that have been recently disturbed or where the existing plant cover has been compromised. Once established, it may rapidly expand into surrounding undisturbed areas.
“Spotted knapweed is also noted for catching a ride on vehicles travelling through infested areas, which spreads seeds over long distances.”
Small initial infestations of spotted knapweed can be managed by the same pick-bag-and-burn strategy used on scentless chamomile. Larger infestations can be managed, but not controlled, by mowing at the early flowering stage to prevent seed set and minimize seed rain, thus depleting the seed bank over a long time.
Because the tap root does not tolerate cultivation, tillage is effective in annual cropping systems.
Burning rangeland to control spotted knapweed is difficult because the airy architecture of knapweed does not support the intense, sustained burn necessary to reduce seed viability. Also, removal of cover by a burn may favour the weed’s establishment.
Herbicides containing picloram or clopyralid provide good control when applied at the bolt-to-early bloom stage.
Brenzil said picloram may also be applied to rosettes in the fall, but should not be applied to light soils, the preferred soils of spotted knapweed, due to its persistence and its mobility in soil with water movement.
He said herbicide applications should extend about three to 4.5 metres beyond the knapweed patch. Competitive grass cover should be reintroduced at the same time.
