For more images, click here.
Canada thistle (Cirsium arvense) is extremely difficult to kill. However,
September is one of the best times to start your assault. This article
will describe Canada thistle and how to eradicate it.
Canada thistle (note that it's NOT 'Canadian' thistle) is in the family Asteraceae. It
is an invasive weed native to Europe and Asia. The
generic name Cirsium is derived from the Greek word kirsos which means
'swollen vein' (Clark, 1998). Plants of this genus were used as an herbal
remedy to relax swollen veins. Arvense means 'of cultivated fields',
a word you will notice that is used in the name of many of our most problematic
weeds (Cerastium arvense, Anagallis
arvensis, Convolvulus arvensis, etc.). The specific name is
appropriate since Canada thistle is so common and problematic in cultivated
fields. Canada thistle is not native to Canada, I don't know how the common
name came about.
Canada thistle is a perennial that spreads by seed and an underground
of vertical and horizontal roots. Canada thistle is diecious, which means
male and female flowers occur on separate plants. Flowers are pink, bristly,
1/2 inch long and wide. Characteristics of Canada thistle are extremely
variable when examining populations from different regions. Although, my
observation within the northern Willamette Valley is that
most plants are similar. They are 3 to 5 feet tall, with glossy foliage
on the upper surface and woolly on the lower leaf surface (this is reportedly
one of the more variable characteristics). Leaves are alternately arranged,
lobed, and armed with stiff spines.
Seed are attached to a cotton-like pappus that aids in wind dispersal. Seed
can survive in soil for up to 20 years (Ross and Lembi, 1999). A
seedling can reproduce vegetatively in as little as 6 weeks after germination,
and a single plant can develop a lateral root system with a 20 foot spread
in a single season. Severed roots can produce new plants, thus tillage
and/or cultivation spread the weed throughout the field. Vegetative
reproduction, through a spreading root system and/or dissemination via
equipment, are the primary methods of Canada thistle infestation.
Canada thistle control
Control can be accomplished mechanically by tilling every 3 weeks for
an entire growing season. If this option is not feasible, herbicides are effective
when used properly. Roundup and Basagran
are effective when applied at bud to early bloom stage, but more than one application
will be necessary for controlling established colonies. Herbicides containing
the active ingredients clopyralid (Stinger or Lontrel) are reported to be the
most effective, and should be applied as soon as plants have emerged from the
soil. Arnold Appleby, a retired OSU weed scientist, reports that the most
effective control is achieved by applications of Lontrel in late September with
2/3 pint/acre followed by application in spring with 1/3 pint/acre (Appleby,
1999). So mark your calendars, and as soon as the Farwest Show is over,
get after that thistle!
Clopyralid can cause severe injury to some crops. Always use directed applications
to avoid injury on nursery crops. When making applications in nursery or
landscape sites, Lontrel is the labeled product (Stinger is
labeled for ag crops).
Note: Due to issues surrounding
residual clopyralid in compost, the ODA developed new restrictions for using
the herbicide in turf areas. Generally this does not apply to agricultural
sites, but first read the ODA rules or
check with your local Dow Agrosciences rep to be sure you are permitted to
use these products.
- Appleby, A. 1999. Canada
thistle (Cirsium arvense). http://www.css.orst.edu/newsnotes/9903/weed.html#Canada
- Clark, L. 1998. Wild Flowers of the Pacific Northwest. Harbour
Publishing, Madeira Park, BC.
- Ross, M.A. and C.A. Lembi. 1999. Applied
Weed Science. Prentice
Hall Inc., Upper Saddle River, NJ.
For more images, click here.
|Canada thistle is a serious weed in nursery crops. Not only does
it compete with crops, but it will infest fields and landscapes of
customers who purchase your plants (very bad news).
|Flowers are pink and bristly.
|Flowers fade in late summer to pale brown seed heads. The brown
color is the color of the pappus (hairs attached to seed) that aids
in wind dispersal.
|Small plants that have just emerged from the soil are connected
by underground system of horizontal
|Foliage is lobed, dark glossy green on the top,
and woolly on the underside. Leaves are
clearly painful to touch.
|Seed plumes develop
in large numbers in a local field. Canada thistle will continue to send
up new plants from roots after setting seed.