One plant that we are constantly fighting here at the weed board is the dreaded Canada thistle. This plant was actually the main reason that the weed board was formed.
Although this plant is called Canada thistle, it is not of Canadian origin. It is actually native to Europe. Canada thistle is a perennial plant, and as such will form monocultures when able.
Canada thistle will readily invade disturbed areas, and can take over very quickly if not controlled soon after invasion. Once established in an area, it can be quite a chore to get rid of.
The roots of this plant can spread 10-12 feet in a single season! Canada thistle can spread by seeds or by rhizomes which makes this plant even more difficult to control. While the rhizomes are slowly taking over an area, seeds blow around to sites further away to take those over.
Because Canada thistle is so aggressive in its growing habit, it is even considered a weed in its native range. A single plant can colonize an area 3 to 6 feet in diameter in a year!
So, imagine more than a few of these plants and you can quickly see what the big deal is. The seeds of Canada thistle are an important food source for goldfinch, and many species of butterfly and moth.
Canada thistle produces purple flowers and spiny leaves. Canada thistle is very competitive, generally growing in disturbed sites first and spreading outwards.
It can reduce the amount of forage for cattle, as they generally will not graze near an infestation. Canada thistle seeds can survive about 22 years in the soil, and deep burial improves survivability.
Now let’s talk about how to control this plant. If the timing of the application is correct, the plant will take in the herbicide. It will move through the plant, and you will have an effective application.
Just like with early spring treatments, these lush young plants are relatively easy to control, which means the lower labeled rate of herbicide will get the job done on most species, which translates to more money in your wallet.
Residual herbicides control through the fall and into the following spring. Herbicides such as Milestone, Telar or Perspective, are slower acting, therefore will get to the roots and control the whole plant, and will provide pre-emergence control of germinating seeds, giving your pastures a head start come spring.
Being that Canada thistle is a perennial, mowing or hand pulling/tilling are ineffective for the most part. This is because these methods leave the underground root system intact which can generate new vegetation. Herbicides are generally the best bet because the chemicals can get down into the roots to spread to other plants.