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Thistles compete for water, light and nutrients in pastures

published: April 17th 2009
by: Martha Hollida Garrett

Thistles are everywhere, and they appear to be in an abundant supply this year. There are different varieties of thistles and they have many nicknames. Thistles are eyesores for starters, but they also hinder grazing, compete for available moisture and left on their own, will continue to multiply.
    The musk thistle appears to be very prevalent this year. These may also be called by other names like purple thistle, bull thistle, or spiny thistle.
    They are found primarily in pastures, rangeland, 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.
    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.
    According to work by Walter Fick and Dallas Peterson with K State University, Musk thistles reduce forage production and utilization. Cattle will not graze forage plants in heavy infestation of this spin 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.
    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. White-flowered plants may occur, but they are not common.
    The leaves are coarsely lobed and dark green with a light green midrib. They may have a silver-gray leaf margin. Leaves have an alternate arrangement on the stem and are smooth and hairless on both sides. Each lobe has three to five points that end in a white or yellowish spine. The spiny musk thistle leaves extend onto a hairless stem, resulting in a winged appearance.
    A musk thistle plant may produce from one to over 100 heads depending upon soil and growing conditions. The upper one or two heads develop individually on long stems while lower branches may have from two to nine heads per branch. The large terminal head is one and one-half inches or more in diameter, solitary and is usually bent over slightly. Prebud, flower and muster seed stages of development often occur simultaneously on the same plants.   
    According to John Boyd an extension weed scientist with the University of Arkansas, 80% of seeds from a thistle are deposited within 55 yards of the original plant. Less than 1% blows more than 110 yards from the point of origin.
 Thistles are prolific, as Boyd says a single plant can produce up to 3,500 good quality seeds. The seeds are 90% viable up to two months after they are produced and may retain the ability to germinate for more than a year.
    Brian Cummins, Texas AgriLife county extension agent, recommends two methods of control - mechanical or use of herbicides
    “Mechanical methods include mowing and hand cutting. 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. Mow cleanly and closely and repeat as needed for control,” says Cummins adding that cutting should take place between the first appearance of pink and the first appearance of brown on the pappus of the earliest head. This presents a problem there may be plants of varying ages in an area.
    He recommends digging the root at least two inches below ground level and removing all soil from the roots. Pick heads that are beyond the bud stage and place in a tight container. Bury the container at a landfill or other site that will not be unearthed.
Cummins also says that 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 in Texas.  The younger the rosette, the more susceptible it is to the herbicide,” says Cummins.
    Individual plant spot spraying can be done with a 2% glyphosate solution at any time. 2,4-D and other commonly used herbicides registered for use in pastures are also effective when the plant is in the growing stage.
    Controlling the thistle will increase the quality of your pastures and hay.