Today’s entry was written by Alexis:
Anne Elliott (annkelliott@Flickr) shares this photo via the Botany Photo of the Day Flickr Pool and writes, “The nodding/musk thistle is my favourite kind of thistle, especially at this stage, when the pinky purple flower has died and the beautiful pattern of the spiny bracts can be seen more clearly. Flower head is 4-6 cm in diameter. Photographed at the Erlton/Roxboro Natural Area on July 27th, when I called in for a short walk after my afternoon volunteer shift. There were a lot more of these plants this year, so I guess this invasive weed really does spread”. Thank you, Anne!
The name nodding thistle refers to the plant’s large flower head, which is often seen bobbing up and down. Wikimedia Commons has a picture of the flower in full bloom.
Carduus nutans occurs naturally in western Asia and across Europe. In North America, however, the species is a major weed, especially in farmlands. It out-competes native plant species, creating dense colonies that livestock refuse to walk through and cannot eat because of the spines. As an invasive species, the musk thistle has an advantage since it lacks natural predators and can spread its seed quite quickly.
In the United States, biological methods of pest control have been implemented in addition to chemical and mechanical approaches. Two species of weevils from Europe, Rhinocyllus conicus and Trichosirocalus horridus, have been introduced to some areas affected by Carduus thistle invasions; the former target the plant’s developing seeds, while the latter eat the root crowns. Yet, as with many pest control methods, this one is not without risk, as the introduced insects may also affect non-target species. In Wisconsin, for example, the weevils are not used due to the associated threat to rare thistles native to the state.