Family Asteraceae

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Introduction

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.

Physical description

Canada thistle is a perennial that spreads by seed and an underground system 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 tillage 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.

Literature cited

• Appleby, A.  1999.  Canada thistle (Cirsium arvense).  http://www.css.orst.edu/newsnotes/9903/weed.html#Canada thistle.
• 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.
  
  

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