Scotch thistle
Onopordum acanthium

Common Names: Scotch thistle, heraldic thistle, and cotton thistle. (2)
Scientific Name:
Onopordum acanthium (1)
picture from source (4)

History: This noxious weed was first found in spots of Europe, as well as parts of Asia.  It developed in the Mediterranean regions, but now has begun to show up in places all across North America, including Montana, where it has begun to sprout only recently.  Some believe Scotch thistle to have first been introduced in the Americas around the 19th century as a decorative plant. (4)

: Taproots. (2)

Stems: Scotch thistle has one large stem that connects to its roots, as well as its flowers and leaves.  Its stem is gray-green in color, due to the number of small, thick, white hairs that run along the length of it. (2)  The stem may contain branches and spiny wings, and can be around four inches thick, and may reach a height of up to eight feet. (3) 

The leaves of the Scotch thistle plant can grow up to being two feet long and one foot wide, but these are just the leaves of the rosette. (4)  The leaves of the stem are much smaller and are not connected to stalks, but rather just continue on to become wings connected to the stem.  Both types of leaves contain the same type of white hairs found on the stem, just on the bottom of them.  These hairs, sometimes found on the tops of leaves, give the plant its gray-greenish color. (2)  The leaves may also contain sharp, yellow spines. (4) 

The flowers of the Scotch thistle plant usually bloom around the middle of summertime, but can bloom in late spring.  The flowers range in color from dark pink to violet and have a round, spherical shape about them.  This shape allows the flowers to, at most, group in twos or threes on the tips of branches. (4)  The flowers can range from two to six centimeters in diameter, and are accompanied by small leaves at their bases which contain pointy, yellow spines. (2) 

The seeds of the Scotch thistle vary in length of around 4mm to 5mm, and are produced in great numbers (anywhere from 8,400 to 40,000). (4)  The seeds are dark gray in color, and are sometimes connected to many toothed hairs, that of which are in the shape of a parachute. (2)  

Scotch thistle reproduces by use of seeds, which are spread by wind, humans, animals, and waterways that come into contact with the seeds, and (purposefully or accidentally) take them off of the plant and move them to a new location where they fall into the soil and stay there, sprouting into a new plant. (4)  These seeds can easily catch on to passing objects or creatures by using their hairs to grip onto them. (2)  Scotch thistle seeds can take 0-20 years to sprout, a factor that may be affected by amounts of light (it grows best when it spends about 1/3 of its time in light and the rest in the dark), type of soil (the plant prefers numerous amounts of nitrogen) and type of environment (the plant prefers a moist one). (4)  Other than the seeds, though, this noxious weed can be spread if root pieces are cut off and distributed to other places in the soil. (2) 

Life Style/Habits/Life Duration:
Scotch thistle is a biennial mainly, but there have been cases where it has also been both an annual and perennial.  The seeds sprout usually from late spring to late autumn, and during the first year, this weed will grow a rosette accompanied by a tap root.  In the 2nd year, the plant will bolt (grow long flowering stems), and it can flower any time between mid-summer and early autumn. (4)  Scotch thistle can infect a variety of places, and it usually does so in large groups. (2) 

Environments Favorable to Infestation:
Scotch thistle can be found in varied environments, but the weed does prefer to grow in places that are generally spaced out, and very wet.  Therefore, it tends to grow in places with a high annual rainfall, anywhere from 500 mm to 900 mm.  Also, Scotch thistle must grow in light soil, preferably containing high amounts of nitrogen. (2)  It can be found in places such as: roadsides, pastures, waste sites, waterways, and fields. (3) 

Methods of Control:

There are no biological control agents present in America at this time, but there have been trials in both Europe and Australia in which insects were used to cut down on Scotch thistle by destroying a large number of the weed's seeds.  Some of these insects have been studied in America, but they failed their tests.  However, the USDA is currently checking up on more types of insects to see if they are fit to be used in our country as biological control agents. (4) 

Cultivation:  One way to stop Scotch thistle is to plough the plants when they are young, which will work only if the roots are completely pulled from the ground and cannot grow back in. (2)  Also, you can help prevent the spread of this weed if you make sure that any pastures or fields that you may own are healthy, dense, and competitive to the Scotch thistle plant.  A very healthy native plant community can usually withstand thistle invasion. (4)  

Grazing is preferred to actually cutting down the weeds, for when you cut them, they can re-grow very easily, and sometimes the cut parts can still create seeds.  With grazing, when the plants are eaten, the seeds are eaten as well, so they are taken care of.  Goats are the preferred animal to be used for grazing Scotch thistle. (2)

The only manual way is to remove with your hands, thus pulling up the roots so it cannot grow back.  If the weed has already flowered or is seeding, then you must put the plant in a plastic bag when you are done, so that it does not still spread seeds.  Otherwise, just leave it where you pulled it. (1)

Herbicides should be used in the spring when the plant begins to bolt, or in the autumn, on the rosettes.  Dicamba, metsulfuron, picloram are some of the only chemicals proven to destroy Scotch thistle. (4) 

(1) Pierson, Kim. Scotch thistle Onopordum acanthium. USFS. February 13, 2006.

(2) Turnbull, Keith. Scotch thistle. Frankston Research Institute. February, 1998. February 13, 2006.

(3) Scotch thistle. Colorado Weed Control District. 1995-2000. February 13, 2006.

(4) Scotch thistle. State Noxious Weed Control Board. 3/12/05. February 13, 2006.











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By: Jonathan Bateman, WHS Student 2006