After the recent years of drought, thistles have taken advantage of weakened grass stands and full-bloomed plants are visible in many fields through the area.
"Many tracts of land in Southwest Missouri are inundated with heavy populations of musk and bull thistles," said Tim Schnakenberg, an agronomy specialist with University of Missouri Extension. "Some landowners have taken extra steps this year to keep the problem in check, while others have done nothing."
As a reminder for all Missouri landowners, section 263.190 of the Revised Statutes of Missouri reads: "It shall be the duty of every owner of lands in this state to control all Canada, musk or Scotch thistles growing thereon so often in each and every year as shall be sufficient to prevent said thistles from going to seed."
"Thistle control can be very difficult but it is not impossible," said Schnakenberg.
Think before you mow
What is to be done at this point in time? Since the majority of the seed for the growing season is already produced, control measures at this time are after-the-fact. Most thistles are biennials, meaning they germinate in the fall, bolt with a seed head in the spring, produce seed and die by mid-summer.
"Since the plants that have seeded out are almost dead because of the proximity to the end of their lifespan, spraying is almost fruitless now," said Schnakenberg.
Mowing is the first impulse of many to control it now, but one risk of mowing is the spreading of the seed to other areas on the mower deck, making matters worse for the fall germination period. Sometimes this is what it takes however to clean up a mess.
Using biological controls
More than 30 years ago, University of Missouri Extension and USDA introduced the flower head weevil and rosette weevil to Missouri fields. These weevils specifically target thistles.
"There is lots of evidence that these weevils are doing a massive job of consuming many of the seed in the flower heads scattered across our county. There is no way that they can keep up with all the seed produced, but if they are getting perhaps 30% to 40% of the billions of seeds that are produced each year, they are having an impact," said Schnakenberg.
Schnakenberg says to watch for dried up seed heads and cut them open for evidence of weevil damage. Many times, a person will find two to four flower head weevils in the heads.
"At this time of year, the weevil is probably the best control," said Schnakenberg. "Biological control does not take us, as landowners, off the hook for keeping thistles from going to seed. Obviously the weevils need our help."
Using chemical control
Other control options include spraying at appropriate times of the year with products such as 2,4-D, dicamba, Grazon P+D, GrazonNext, Chaparral or other registered products.
Schnakenberg says the best times to spray are when the plants are still in the rosette stage which is the stage these plants are in for 70% to 80% of their lifespan. This corresponds with an ideal time of the year to spray being in the fall (October) or early spring (March-April). Sometimes widespread broadcast spraying is necessary for control over spot spraying.
Mowing multiple times is also an option in the spring or early summer. A Kansas study found that only 11% of the musk thistles mowed at the early bud stage were killed. When mowed a second time four weeks later, 79% of the thistles were controlled.
The best time to start mowing is within two days after the terminal flower head blooms in order to inhibit seed production and prevent rebolting. Remember however, that viable seed can start to develop within seven days of the first pink coloring in the heads.
Source: University of Missouri Extension