Thistle (Cirsium spp.) Family Asteraceae
The generic name Cirsium is derived from the Greek word kirsos which means 'swollen vein'.
Photo by Bob Webster
Habit: Thistle is a perennial that spreads by seed and an underground system of vertical and horizontal roots. It is diecious, meaning male and female flowers occur on separate plants. Flowers are pink, bristly, 1/2 inch long and wide. Characteristics are extremely variable when examining populations from different regions. They are 3 to 5 feet tall, with glossy foliage on the upper surface and woolly on the lower leaf surface although this is one of the more variable characteristics. Leaves are alternately arranged, lobed and armed with stiff spines.
Culture: Seed are attached to a cotton-like pappus that aids in wind dispersal. Seed can survive in soil for up to 20 years. A seedling can reproduce 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 can spread the weed. Vegetative reproduction via tillage equipment is a primary method of Canada thistle infestation.
Photo by Bob Webster
Control: Control can be accomplished mechanically by tilling often and encouraging soil health and competition. Organic herbicides like vinegar and fatty acid products are effective when used properly. Roundup, Basagran , clopyralid (Stinger or Lontrel) are recommended by the organiphobes. 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).
Notes: The chemical pushers say this about the product they recommend: 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. Of course, I didn’t recommend that toxic crap in the first place.
Here’s some good information from Barney Lipscomb of BRIT in Fort Worth
Probably most common three of the seven native thistles:
1) Cirsium undulatum (wavy-leaf thistle or pasture thistle) (native)
2) Cirsium texanum (Texas thistle, southern thistle) (native)
3) Cirsium altissimum (Iowa thistle, tall thistle) (native)
Nonnatives include Bull thistle or yellow thistle (C. horridulum)
And totally different “thistles”:
Milk-thistle (Silybum marianum) introduced.
Scotch-thistle (Onopordum acanthium)
Musk-thistle (Carduus nutans) introduced
Slender bristle-thistle (Carduus tenuiflorus) introduced
Star-thistle (Centaurea melitensis) introduced
Yellow star-thistle (Centaurea solstitialis) introduced