A wide variety of thistles can be seen everywhere this summer and appear to be in abundant supply. Musk, purple, bull and spiny thistles are all eyesores but they also hinder grazing, compete for available moisture and will continue to multiply if left alone.
"Thistles are found primarily in pastures, hay meadows, roadsides and waste areas. Musk thistle generally is not a problem in cultivated cropland, but may germinate and develop in fallow fields or after wheat is planted in the fall," said Tim Schnakenberg, agriculture and rural development specialist with University of Missouri Extension.
Thistles can be either annuals or biennials, but most are biennials. These germinate and grow into the rosette stage in the first year. They remain in the rosette stage over the winter and then bloom and produce seeds the next growing season.
Musk thistles reduce forage production and use. Cattle will not graze forage plants in heavy infestation of this thistle. Dense infestations of musk thistle compete strongly with grass or other desirable plants for water, light and nutrients.
"Research in Kansas indicates one plant per square foot nearly decreases forage production in half. This results in lower income from grazing areas. Loss in income also results from lower quality hay due to contamination by this noxious weed," said Schnakenberg.
Musk thistles usually start flowering in spring and can bloom from seven to nine weeks. Musk thistle flower heads are "powder puff" in shape, in contrast to "saving brush" flower heads of some other thistles. The colorful flowers are usually deep rose to violet or purple. The leaves are coarsely lobed and dark green with a light green midrib. Leaves have an alternate arrangement on the stem and are smooth and hairless on both sides.
A musk thistle plant may produce from one to over 100 heads depending on soil and growing conditions. University of Arkansas research shows, 80 percent of seeds from a thistle are deposited within 55 yards of the original plant. A single plant can produce up to 10,000 good quality seeds.
Mechanical methods include mowing and hand cutting. Schnakenberg says mowing needs to be done with a rotary mower before the first appearance of pink on the flowers. Mowing at full bloom will prevent seed production.
"Producers should mow cleanly and closely and repeat as needed for control and cutting should take place between the first appearance of pink and the first appearance of brown on the pappus of the earliest head," said Schnakenberg. "You can also dig the root at least two inches below ground level and remove all soil from the roots. If you pick heads that are beyond the bud stage you should burn them in a controlled fashion."
Spraying is an effective way to control thistle outbreaks. Musk thistle plants with flower stalks are more difficult to kill than the rosettes. Rosettes need to be treated when they are actively growing and not under drought stress. This probably means in the fall (late October) or spring (mid April). The younger the rosette, the more susceptible it is to the herbicide.
Schnakenberg says you can use 2,4-D but you will get better thistle control by using a combination of 2,4-D and dicamba (Banvel). Other herbicides labeled for controlling musk thistles in pastures include Grazon Next HL, Grazon P+D, Chapparal, Surmount and Cimarron Max. Always be sure to read and follow label instructions, and treat at the proper time.
Another control is biological. Specific natural enemies of musk thistle can aid in regulating the spread of this weed. Such natural enemies include the musk thistle rosette weevil, the musk thistle flower head weevil and a musk thistle rust fungus.
The larvae of the rosette weevil feed on the underside of the leaves of the rosette and on the leaves and stems of bolted plants; larvae of the flower head weevil feed in the base of the developing flower of the thistle. The two weevils are compatible with each other as biological control agents because they do not compete with each.