The stems and leaves grow 1.2 to 1.5 m
tall and are ridged and branched.
will grow 1' to 4' tall and will branch in the upper part of the
The stems are a light green
and are ridged.
The leaves are
around 3 to 4'' long and 2 to 3'' wide, and have the capability of
a bit larger.
The leaves also have
yellowish thorns on them.
leaves may also have hair on them, which varies from plant to
The above ground portion of the shoot dies during the winter, but
ground part generally survives to produce new shoots the following
season. New shoots are also developed from lateral root
leaves are spiny, and the edges are serrated and ruffled. (2)
The flowers will grow from the upper stems,
and have one, to several flowers growing from different stems.
The flowers are unisexual, which means
contain male and female parts.
flower can grow to approximately 1/2'' to 1'' across. (3)(4)
head is urn-shaped (like a vase) and the bracts are
of the flowers may vary from plant to plant, being purple, pink or
Flowering occurs during July and August.
The plant can still bloom later if it was disrupted in any
Fruits and Seeds:
The seeds of the Canada thistle are
2.5 to 4 mm long and a straw to light brown color.
Each of the fruits is small and only contains
one seed. (1) Canada thistle will
seeds that can be spread many ways, the wind being the easiest
Most of the seeds will grow within a
being spread, but some seeds can stay dormant, and eventually
or more years. (
This weed reproduces by seed, but it
mostly spreads by lateral roots sending up new shoots each
the roots with cultivation implements only produces more plants,
fact that the weed can grow from small segments of cut roots. (5)
When using this method, cultivation it
repeated until the nutrients in the roots are depleted. (3)
A colony-forming, aggressive
perennial. (1) Canada thistle's habits are mainly that it
grows large underground root systems, and also grows rapidly which
hard weed to control.
It out competes
the natural plants in its area and can become a major problem.
It has been estimated that the plant will
for about 2 years, but it is continually growing new roots that
Environments Favorable to
grows in cultivated fields, pastures, range land, forests, and
ditches, and river banks. (2) This weed grows best in areas where the
and worked up. Canada thistle needs
sunshine, so it is not often found in shady areas. Also,
this weed doesn't do as well in wet
soils that are not well aerated. (4)
Methods of Control:
The larvae of the weevil Ceuthorhynchus
bore into the stems and weaken the plants, sometimes killing them.
Also, there is Larinus planus
which is a seed head
that lay its eggs in the seed head.
feed on the flower parts and developing seeds reducing the
production and stressing the plant. (7)
galling fly Urophora cardui
has been released in
area several times but has not established.
It may be too cold here for this fly. (1)
They remove or suppress the tops of
Canada thistle in crops which will grow rapidly enough to provide
canopy and shade the ground before the new thistle shoots recover.
many different types of herbicides that will work. The main result
from an herbicide is to destroy the top portion of the plant, so
Some people have
onto the foliage to kill the tops of the plants or injected it
with a syringe to kill that plant and some of its root system. (1)
Cultural: Burning does
not kill the roots and may even
stimulate the plant to send up many more stems.
Tilling if repeated until all the root fragments are