||Perennial (up to 9 yrs)
||Fall or early Spring
knapweed was first introduced into America from Eurasia (central Europe
and east to central Russia) as a contaminant in alfalfa and clover seeds
in the late 1800's. Until 1920, it was limited to San Juan Island, Washington.
By 1997, it could be found in 14 western states and in every county in
Washington, Idaho, Montana, and Wyoming.
aggressive plant is very competitive and can form a monoculture.
It releases allelopathic
chemicals from its roots that stunt the growth of many of the surrounding
native plants. Early spring growth and lateral shoot development allow
it to effectively compete for water and nutrients with desirable native
knapweed is capable of invading both well-managed pastures and disturbed
areas, although a higher soil disturbance creates a greater knapweed density.
Each plant starts out as a rosette
the first year and produces one to six flowering stems the following
years. Spotted knapweed can send lateral shoots and form new rosettes
less than a inch from the parent plant. It is not uncommon to see multiple
rosettes on a single root crown.
production is extensive, ranging from 5,000-40,000 seeds/m2/year
(greater number during wet years). Germination
occurs under a wide variety of conditions. Seeds are generally dispersed
by water and animals, and with crop-seed, or hay.
knapweed suppresses the growth of other plants by releasing inhibiting
chemicals from the roots and by aggressive competition. Loss of native
plants has reduced available forage for wildlife and livestock. In areas
of Montana, elk use on winter range has been reduced up to 98% due to
spotted knapweed invasions. Loss of vegetation has also produced an increase
in surface water runoff and stream sediment yields, and a decrease in
produces a great nectar for domestic bees.
knapweed's natural location is the forest grassland interface in Europe,
but it can easily invade and dominate rangelands that receive less than
8 inches of annual precipitation. Areas without disturbance are not safe
from invasion by this plant. It occupies a wide range of sites, including
elevations from 1,900 to more than 10,000 feet, and precipitation zones
of 8-79 inches.
a large scale, simply going out and killing the existing plants is not
effective. Small patches may be eradicated, but with existing technology
and money, larger populations can only be slowed and hopefully contained.
Once again, a combination of the following tools provides the best control:
are the most effective (example
herbicides: Tordon 22K, 2-4,D, Clarity, Curtail [look
at herbicide label: free search]).
A combination of plowing or herbicide and seeding has been effective.
control agents have not been able to completely control knapweed,
but they have reduced the competitive edge and reduced seed production
in some areas.
• Persistent and thorough hand-pulling can be effective.
• Don't burn. Burning has been known to increase knapweed densities by