Description: Canada thistle is a perennial
reproducing by creeping, freely sprouting horizontal roots
and by seed. It grows in patches because of its root system
and perennial nature. The extensive roots are fleshy and send
up frequent new shoots. The stems are erect, hollow, smooth,
or slightly hairy, up to 4 feet high and branched at the top.
The plants are leafy but no wings or spiny appendages occur
on the stems. The waxy leaves are oblong to lance-shaped and
vary from very irregular and deeply cut to spiny-toothed on
the margins to almost smooth with few or no spines. The color
is usually bright green but the upper surface varies from
dark to light, and the leaves are sometimes very light green
and slightly hairy on the underside. The flower heads are
numerous, small, compact, and vary from light lavender to
rose-purple. The bracts on the heads are not spiny. The plant
is dioecious, so male and female flowers are produced on separate
plants. The seeds are oblong, flattened, curved, smooth, dark
brown, and approximately 1/8 inch long.
Distribution: Canada thistle is declared a noxious
weed throughout the United States and has long been recognized
as a major agricultural pest, costing tens of millions of
dollars in direct crop losses annually and additional millions
in cost for control. Canada thistle is found in Iowa in cultivated
fields, meadows, pastures, waste places, roadsides, and prairies.
This weeds spreads by producing enormous quantities of seed
that is carried by wind, mowers, and animals.
What’s The Problem?: Canada thistle is
one of Johnson County’s most problematic weeds. Canada
thistle establishes quickly in an area and crowds out and
displaces native plants and lowers plant diversity. The spreading
horizontal roots also allow this invasive weed to move into
uninfested areas to compete for light and moisture. Conservation
Reserve Areas (CRP) can become infested, which violates set
aside contracts. Row crops such as corn and soybeans can have
decreased yield and marketability if infested with Canada
thistle.
Management Methods:Since its introduction into
the United States in the 1600’s, Canada thistle has
been recognized as one of the worst weeds in respect to ability
to spread, ability to negatively impact agriculture, and its
tenaciousness. Due to its perennial nature and extensive roots
system, Canada thistle is very difficult to control, once
well established. The best control method is to identify the
weed early in its’ invasion, and to prevent infestation.
Management is typically through hand-cutting, mowing, controlled
burning, competitive seeding, and careful use of appropriate
herbicides. Generally, mowing alone will not eliminate Canada
thistle. Use of several of these methods will be necessary
to reduce infestations to manageable levels. Typically, several
or more years of management are required to deplete root reserves
and the seed bank. Seeds can remain viable in the soil for
50 or more years..
Above photos courtesy of King County, WA Noxious Weed Control Program
Photo couresy of Weeds of Nebraska and the Great Plains published by
Nebraska Department of Agriculture
Above photos courtesy of King County, WA Noxious Weed Control Program