Most thistles go unnoticed until they bolt (put up a flowering stalk). When thistles bolt and begin to flower each spring, folks who want to control them call the Noble Research Institute. By the time they call, however, it is usually too late in the season. Once thistles become reproductive, they are much harder to control and may have already produced viable seed.
Thistles can be easily controlled with herbicides when they are young and in the rosette, or low growing, vegetative stage. Plant identification can be a little more difficult when the weeds are immature, but there are many good resources available on the Internet. An excellent resource for Oklahoma is Oklahoma Cooperative Extension Service Fact Sheet PSS-2776. You can also use a search engine such as Google to find images of "musk thistle" or any other thistle.
Once you determine if and what kind of thistles you have, consider how many and where they are in your fields or pastures. Are there so many that you want to treat them with herbicides or are there few enough that you can hoe them out by hand? Are they isolated so you can use a hand sprayer or so widespread that you will need a broadcast application? Are temperatures favorable enough that herbicides are an option?
If herbicide is the method chosen and the thistles are in grass pastures or crops, then there are many herbicide products to choose from. If thistles are in broadleaf crops or pastures, then herbicide options are more limited. For the sake of brevity in this article, we will focus on thistle control in grasses.
The most common, most available and least expensive herbicide for thistle control in grasses is 2,4-D applied in February. There are many other products that will also work in late winter and early spring, but they typically will cost more. When choosing a 2,4-D formulation and rate, always read and follow label directions. A 2,4-D application in February will also control many other broadleaf weeds that may be present, such as mustards and chickweed. However, do not expect much activity from 2,4-D on henbit. Be mindful of nearby sensitive crops, just as you would at any other time of year.
If the grass pasture you are treating is bermudagrass and it is completely dormant, you can also use glyphosate products like Roundup®. I prefer a tank mix of glyphosate and 2,4-D to get maximum effectiveness and broad spectrum weed control. However, glyphosate alone will also work. For formulation and rate selection, be sure to read and follow label directions.
Whichever herbicide you use, read and follow label directions. In addition to all the other information it contains, it will recommend the appropriate adjuvants or surfactants. Most thistle leaves have either a waxy coating or a covering of fine hairs. Both of these will inhibit the herbicide spray from reaching the leaf surface and being taken up by the plant. Surfactants reduce the surface tension of the spray solution and help it to spread over the leaf surface so that the chemical can be taken into the plant. Some herbicides already have the surfactant premixed into the product and no additional adjuvant is needed.
Thistles and most other weeds are most susceptible to control with herbicides when they are young and actively growing. If temperatures are cold enough that plants are not actively growing, then herbicide action will be limited. Try to plan your spraying when several days in a row are forecast to be above 60 degrees Fahrenheit. Under conditions of warm sunny days, cool-season weeds like thistles do have some active growth and should be acceptably controlled with herbicides. Read and follow label directions.
Once you are finished spraying, you will want to drain and re-winterize your sprayer, unless it is stored in a heated shop. Even though the weather may be nice on a particular day in February, late cold snaps in March can freeze the water in sprayer components and crack or rupture the plumbing.