Professor/Extension Weed Scientist
Iowa State University
Biennial thistles are commonly found in Iowa's pastures, roadsides, CRP and other un-tilled areas. Musk (Carduus nutans ) and bull (Cirsium vulgare) thistle are exotic species (originate from outside of North America) and are responsible for the majority of problems caused by this group of plants. Tall ( Cirsium altissimum), Flodmans ( Cirsium flodmani) and field thistle (Cirsium discolor) are native to Iowa and far less prevalent than musk or bull thistle. Although all species in the Carduus and Cirsium genera are classified as noxious weeds by Iowa's Noxious Weed Law, the less common, native thistle species rarely invade managed areas at densities that crowd out desirable plants or interfere with human activities.
Characteristics of Iowa's weedy biennial thistles are described in Table 1. Canada thistle is included in the table for comparison. Canada thistle, an exotic, is a perennial and thus management strategies are considerably different than for biennial thistles.
Musk thistle is the most prevalent and invasive of the biennial thistles. Rosette leaves of musk thistle usually have silvery margins that simplify identification. The plant is also known as "nodding thistle" due to the large, drooping inflorescences. Bull thistle has prominent, sharp spines on each of the leaves narrow, pointed lobes. The undersides of tall and field thistle leaves are hairy and white, with field thistle having leaves more deeply divided by numerous lobes.
Table 1. Identification Characteristics of Iowa's Weedy Thistle Species
|Tall and Field Thistle
|Heavily pubescent; leaves deeply lobed with narrow teeth, each tooth tipped with sharp spine.
|Leaves deeply lobed, typically with silvery or white margins; leaves lack hairs or have sparse hairs.
|Underside of leaves are white, heavily pubescent; hairless on top; basal leaves may be oval shaped or toothed.
Field thistle leaves more deeply divided by lobes.
|Highly variable; margins may be entire or deeply lobed; margins spiny or soft.
|2-6 ft; many spreading branches; hairy; wings on upper portion; dark purple veins.
|2- 8 ft; highly branched; spiny wings; leaves clasp stem.
|2-8 ft; lightly hairy; freely branching at top.
|1-4’; branched at top; ridged; hairy at base, smooth towards top; typically found in dense patches.
|2-3” tall, 1-2” wide; gumdrop shape; bracts tipped with long, stiff spine; purple flowers.
|1-2” tall, 2-3” wide; broad bracts with sharp tips; deep rose to violet flowers.
|1.5-2” long, 2” wide; bracts tipped by small spine; one or more long, undivided leaves present just below head; dark to light purple flowers.
|0.5-1” tall, 0.5” wide; present in clusters; bracts without sharp spines; plants are either male or female (dioecious), so some plants do not produce seed.
|Extensive creeping system
Biennials require two years to complete their life cycle (Figure 1). Seeds typically germinate in the spring or early summer. In the first year of growth, the plant forms a cluster of leaves at the soil surface (rosette). The following spring the stem elongates (bolts), producing an erect flower stalk. The plant dies in late summer or early fall after seed have matured. A small percentage of the population may act as annuals and produce flowers the same year they emerge. These plants typically are smaller and flower later in the year than plants that act as biennials.
Biennials only reproduce by seed, and individual plants die after seeds mature. Canada thistle is the only perennial thistle species prevalent in Iowa, and it reproduces both by seed and vegetative root stalks. Biennial thistles are much easier to control than Canada thistle due to the lack of perennial root systems.
All thistles produce an inflorescence composed of many small flowers in a compact head. Musk thistle averages approximately 50 heads per plant with each head producing several hundred seed. Each seed has a cluster of soft bristles (pappus) attached to the tip. The role of the pappus in many plants is to aid in the wind dispersal of seeds. However, thistle seeds are relatively large and thus poorly adapted for wind dispersal. For example, a musk thistle seed is approximately 6 times larger than a dandelion seed. In addition, the pappus on most thistle seeds detaches from the seed prior to the seed leaving the seed head. Thus, although a few seeds may be carried long distances by wind, the majority of thistle seeds fall within a few feet of the mother plant. Thistle seeds are consumed by birds which may transport the seeds long distances.
- Cultural control. Biennial thistles are most commonly found in pastures, roadsides and other areas that are not disturbed frequently. The most effective control strategy is to promote vigorous growth of the desired vegetation. Maintaining good fertility and avoiding overgrazing will reduce the establishment of biennial thistles in pastures. Over-seeding disturbed areas with adapted plant species to enhance competition will reduce biennial thistle infestations in other areas.
- Mechanical control. Mowing biennial thistles after they have bolted is an effective control tactic since this will prevent seed production. Biennial thistles have dormant buds in the crown that usually sprout when the main stalk is removed. Thus, at least two mowings during the summer are required to completely eliminate seed production. Isolated plants can be effectively removed by cutting the plant below the crown with a spade or hoe.
- Biological control. Musk thistle weevils have been released throughout the Midwest to manage musk thistle. The adults lay eggs on the underside of flower heads and the larvae burrow into the flower head. Larvae feed on the developing seed, therefore preventing seed production. Musk thistle is the preferred host since it flowers at the same time as weevil egg-laying, but the musk thistle weevil will lay eggs on other thistles that flower at the appropriate time. A second insect, the rosette weevil, feeds on the crown and leaves of musk and other thistles. The musk thistle weevil is well established in Iowa and can frequently be found in musk thistle infestations. These biological control organisms can provide economical, long-term control but typically do not impact thistle infestations until several years after release. They are best suited for large areas with heavy infestations that will not be disturbed for several years. Insects can be purchased from companies or individuals or can be collected from sites with established infestations.
- Chemical control. Several effective herbicides are available to control biennial thistles. Biennial thistles are much easier to kill when in the rosette stage, so applications should be made in the fall or early spring before bolting initiates (Figure 1). Effective herbicide options include: 1 to 2 qts of 2,4-D LVE (4 lb ae/gal); 1 qt 2,4-D LVE + 0.5 pt Banvel/Clarity; 0.2-0.3 oz Ally; 2 to 4 pt Grazon P&D; 2 to 3 qt Curtail and 3 to 5 oz Milestone. With the exception of Ally, all of the above products are plant growth regulator herbicides. Many of these herbicides are off-patent and sold under a variety of trade names and in different combinations. Grazon P&D contains picloram (Tordon), and is a classified as a Restricted Use Product due to its persistence and mobility.
Fall treatments are very effective against established rosettes, and can be made after several frosts have occurred. Spring treatments should be made prior to bolting since thistle tolerance increases greatly after bolting has initiated. Grazon P&D and Milestone provide more consistent control of thistles that have bolted than other treatments, but treatment costs increase since higher rates are required. A potential advantage of spring treatments is they can control both second year rosettes and first year seedlings, whereas fall treatments typically only control rosettes that established earlier in the spring.
Caution must be used when applying any herbicide to avoid off-target movement that may damage susceptible vegetation in adjacent areas. All listed herbicides will kill any legumes present in pastures.
Herbicides work best when used in combination with other control strategies. Due to persistent seed banks, biennial thistles are likely to reinfest treated areas. Enhancing the competitiveness of the desirable vegetation and implementing appropriate follow up tactics will provide economical long-term management of the biennial thistles.