Plumeless or Bristly Thistle (Carduus acanthoides)
DESCRIPTION: Several native thistles exist in prairies that may be confused with exotic species. Verify identification before initiating control work.
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Plumeless Thistle |
Plumeless thistle is similar, but with many leaf-like spines on the stem, and hair on the underside of the leaf. In addition, the red to purple flowers of plumeless thistle are much smaller--usually about one third the size of musk thistle flowers. Plumeless thistle flowers are single or clustered, erect on the stems, and usually do not droop or nod. These two thistles can hybridize, and both are very similar in the rosette stage.
Bull thistle is also a biennial with prickly, winged stems. The leaves are coarse and spiny above with wooly white hairs below. A purple "brush" of flowers emerges from a spiny green ovoid base.
DISTRIBUTION AND HABITAT: All of these thistles are exotics introduced from southern Europe and western Asia. Musk thistle is a particularly pervasive weed--it has been reported in 42 states and labeled a noxious weed in 16. It is especially problematic on the Great Plains and in mesic pastures of the Intermountain West.
These thistles are most commonly found in disturbed areas such as pastures, roadsides, waste areas, and ditch banks. They are also a problem in prairies, old fields, and hay fields.
LIFE HISTORY AND EFFECTS OF INVASION: The life cycles of these species are similar. They are primarily biennial or winter annuals, but may act as annuals in areas of sparse vegetation and fertile soil. Seedlings emerge from early spring to late fall, and the length of time to flowering can vary from 4 months to 22 months. A single taproot is formed. Reproduction is by seed.
Stem elongation takes place in early May. Blooming starts with the terminal head in early June and continues until mid-August with the lower branches. Seeds mature and may disperse within 7 to 10 days of flowering. The bulk of seed is produced on the upper branches, and germination may run as high as 95%. Wind dispersal allows for movement over long distances. Seeds can remain viable in the soil for over 10 years. Each plant is capable of producing up to 10,000 seeds, which are dispersed by wind.These thistles typically do not pose a great threat to high quality natural areas, although they have been known to invade native and restored grasslands despite the presence of dense, native prairie vegetation. Glade communities are also likely areas for thistle establishment. These species are very aggressive in disturbed areas, and can pose a major problem in buffer and restoration areas. Control of these thistles is important before beginning a prairie restoration.
CONTROLLING EXOTIC THISTLES
Mechanical Control: Eliminating seed production is the most effective mechanical control technique. Thistles mowed in bud or early bloom stage will produce new branches from buds in the axils of the basal leaves. However, close mowing or cutting twice per season will usually prevent seed production. This can be done at any time during the growing season, although cutting is easier when the thistles are smaller. Mowing once flowering has begun may result in the spread of viable seeds with the mower. For effective selective control, plants should be cut with a sharp shovel at 1" to 2" below the soil surface before flowering. Competition with native vegetation decreases seedling establishment. A buffer strip between the preserved area and external sources of exotic thistles should be maintained to prevent invasion.
Biological Control: Two exotic weevils, the flower head weevil (Rhinocyllus conicus) and the rosette weevil (Trichosirocalus horridus) have been introduced in several states, and appear to be effective biological control agents that limit populations of musk thistle. However, these insects are not in use in Wisconsin due to the risks presented to rare native thistles.
Chemical Control: Herbicides are not recommended for use on exotic thistles in high quality natural areas. Chemical control is most effective when plants are in the rosette stage and least effective when thistles are flowering.
On severely disturbed sites, 2,4-D ester at 2-4 lb/acre can be applied using a backpack or tractor-mounted sprayer or in granular form. It is most effective when applied 10-14 days before bolting of the flowering stems. Dicamba can be used earlier in the spring than 2,4-D. One lb/acre of dicamba provides the same control as 2 lb/acre of 2,4-D; spring applications of 0.25 lb/acre in combination with 2,4-D ester at 0.5 lb/acre have resulted in 97% control by the fall of the same year.
Picloram alone or in combination with the other herbicides mentioned gives the best late-season control, but is more expensive, cannot be used near groundwater or during certain seasons of the year, and presents a greater risk of damaging nontarget species. However, nontarget plants are less susceptible during the cool, dry autumn season, and picloram has been effective when applied during fall at a rate between 0.27 lb/acre and 0.5 lb/acre.
More Information on the Web (links exit DNR)
Plumeless Thistle - Wisconsin State Herbarium
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