Thistle Biology & Identification
South Dakota is home to many thistle species, with the most prevalent being Canada thistle followed by bull and musk thistle (Figure 1). Other species include plumeless, tall and Flodman’s thistle.
Understanding thistle biology is necessary to know how to control them. Canada and Flodman’s thistle are perennials, meaning one plant can exist for more than two years. Bull, plumeless and tall thistle biennial, in which they finish their life cycle in two years. Musk thistle is typically biennial but also could be a winter annual. This depends on environmental factors to when it germinates and emerges.
In addition to knowing how each species grows, we need to properly identify the problem thistle to know the best way to manage it. There are resources on iGrow and your local SDSU Extension office for thistle identification. There is also a free app for smart phones that gives an in-hand on-the-go identification tool for everyone.
Figure 1. Top left: Canada Thistle. Top right: Bull Thistle. Bottom: Musk Thistle.
Integrated Pest Management
There are chemical options to control thistle, but we should consider integrated pest management (IPM) before spraying. IPM is an integrated approach to controlling pest (weed) issues. This approach includes chemical, mechanical, biological and cultural control measures.
- Chemical Control
At this time of year (July) the majority of thistles are already in reproductive stages of growth. Mowing (mechanical control) now may be the best option to keep the thistle under control. Returning this fall before a killing frost, or early next spring when these thistles are in the rosette stage would be the time to best use chemical control. Chemical options in non-crop land areas include: Tordon+ 2,4D Ester, Milestone, Curtail, Perspective and others. A fall application or early spring burndown could be used in crop land with tank mixes that include: 2,4D, dicamba, glyphosate, liberty, etc. depending upon cash crop rotation.
- Biological Control
Biological control is the use of a biological agent (in this case, insects) to manage a pest. Examples include the seedhead gall fly for bull thistle and Canada thistle. Canada thistle can be feed on by the stem weevil and bud weevil. For musk thistle the flower head weevil is quite effective in reducing seed output.
- Cultural Control & Other Methods
An unthought-of, but effective biological control could be through mob grazing livestock which has been shown to decrease some thistle species. Cultural control in range areas can be done through burning, increased grass establishment, etc. In crop land areas crop rotation, cash crop row-width and seeding rate can help thistle control.