Canada thistle is an herbaceous perennial with erect stems 1½" — 4' tall, prickly leaves and an extensive creeping rootstock. Stems are branched, often slightly hairy, and ridged. Leaves are lance-shaped, irregularly lobed with spiny, toothed margins and are borne singly and alternately along the stem. Rose-purple, lavender, or sometimes white flower heads appear from June through October, generally, and occur in rounded, umbrella-shaped clusters.
The small, dry, single-seeded fruits of Canada thistle, called achenes, are 1" — 1½" long and have a feathery structure attached to the seed base. Many native species of thistle occur in the U.S., some of which are rare. Because of the possibility of confusion with native species, Canada thistle should be accurately identified before any control is attempted.
Ecological threat
Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. As it establishes itself in an area, Canada thistle crowds out and replaces native plants, changes the structure and species composition of natural plant communities and reduces plant and animal diversity. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and possibly through the release of chemical toxins poisonous to other plants.
Canada thistle is declared a "noxious weed" throughout the U.S. and has long been recognized as a major agricultural pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control. Only recently have the harmful impacts of Canada thistle to native species and natural ecosystems received notable attention.
Habitat
Canada thistle grows in barrens, glades, meadows, prairies, fields, pastures, and waste places. It does best in disturbed upland areas but also invades wet areas with fluctuating water levels such as streambank sedge meadows and wet prairies.
History
Canada thistle was introduced to the United States, probably by accident, in the early 1600s and, by 1954, had been declared a noxious weed in forty three states. In Canada and the U.S., it is considered one of the most tenacious and economically important agricultural weeds, but only in recent years has it been recognized as a problem in natural areas.
Biology
Canada thistle produces an abundance of bristly-plumed seeds which are easily dispersed by the wind. Most of the seeds germinate within a year, but some may remain viable in the soil for up to twenty years or more. Vegetative reproduction in Canada thistle is aided by a fibrous taproot capable of sending out lateral roots as deep as 3' below ground, and from which shoots sprout up at frequent intervals. It also readily regenerates from root fragments less than an 1" in length.
Management options
Management of Canada thistle can be achieved through hand-cutting, mowing, controlled burning, and chemical means, depending on the level of infestation and the type of area being managed. Due to its perennial nature, entire plants must be killed in order to prevent regrowth from rootstock. Hand-cutting of individual plants or mowing of larger infestations should be conducted prior to seed set and must be repeated until the starch reserves in the roots are exhausted. Because early season burning of Canada thistle can stimulate its growth and flowering, controlled burns should be carried out late in the growing season for best effect.
In natural areas where Canada thistle is interspersed with desirable native plants, targeted application of a systemic herbicide such as glyphosate (e.g., Roundup or Rodeo), which carries plant toxins to the roots, may be effective. For extensive infestations in disturbed areas with little desirable vegetation, broad application of this type herbicide may be the most effective method. Repeated applications are usually necessary due to the long life of seeds stored in the soil.





Leafy Spurge

Other common name: Faitour's grass

Native range: Eurasia
Entry into the United States: The plant was first reported in the United States in 1827.

BIOLOGY

Life duration/habit: Leafy spurge is an aggressive, persistent, deep-rooted perennial, growing to a height of 1 meter (3 ft) or taller. Vegetative stems manufacture sugars for root reserves while other stems produce flowers.
Reproduction: Leafy spurge reproduces by vegetative re-growth from spreading roots and by the production of large quantities of seeds that are often dispersed by birds, wildlife, humans, and in rivers and streams.
Roots: Leafy spurge roots are brown with pinkish buds. Plants are able to maintain high root reserves through an extensive root system, ranging from a massive network of small lateral roots near the soil surface [within 30.5 cm (12 in)] to deep, penetrating taproots that may extend to depths of 3 to 7 meters (9 to 21 ft). This ability to maintain high root reserves permits the plant to recover quickly from physical and most chemical damage.
Stems and leaves: The stems are thickly clustered and bear narrow, 2.5 to 10 cm (1 to 4 in) long leaves that are alternately arranged along the stems. When damaged, leaves and stems produce a milky latex.
Flowers: The small flowers are yellowish-green, arranged in clusters, and enclosed in yellow-green bracts.
Fruits and seeds: Seeds are oblong, gray to purple, and occur in clusters of three. When dry, the seed capsules shatter, scattering seeds away from the plant.
text authors: N.E. Rees, N.R. Spencer, L.V. Knutson, L. Fornasari, P.C. Quimby, Jr., R.W. Pemberton, and R.M. Nowierski
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INFESTATIONS

Worst infested states: Leafy spurge now extends from southern Canada through the northern United States, and is approaching areas as far south as Texas. (see Leafy Spurge Distribution)
Habitat: It has become dominant on rangelands and pastures in a wide range of environments throughout much of the United States.
Impact: Leafy spurge produces a milky latex that is poisonous to some animals and can cause blistering and irritation on skin. The digestive tract is similarly affected when this plant is eaten by humans and some animals. In cattle it causes scours and weakness. When ingested in larger amounts it can cause death. Cattle usually refuse to eat leafy spurge unless it is given to them in dry, weedy hay or when better forage is not available.
A conservative 1979 estimated loss in the United States of $10.5 million annually was based on expenditures for controlling leafy spurge and loss of productivity. Although leafy spurge infestations are most severe on undisturbed lands, on cultivated cropland the weed can reduce crop yields by 10 to 100%. A 1990 study conducted by North Dakota State University estimated the direct annual financial impact in Montana, North Dakota, South Dakota, and Wyoming to be $40.2 million with secondary impacts at $89 million and the potential loss of 1,433 jobs annually.
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COMMENTS

Leafy spurge is extremely difficult to control by chemical means and almost impossible to control by cultural or physical methods in rangelands. It apparently has the ability to purge undesirable chemicals from the root system in approximately the top 45 cm (18 in) of the soil, allowing the remaining portion of the root system to regenerate as soon as the effect of the chemical in the soil has dissipated. Although leafy spurge causes problems with cattle that consume it, sheep generally can be taught to feed on it and goats will seek it out. Both sheep and goats are utilized in weed control programs to "keep the yellow out" and to retard the spread of leafy spurge. People should handle the plant with caution because the latex can cause irritation, blotching, blisters, and swelling in sensitive individuals. The eyes should never be rubbed until after the hands are thoroughly washed. The dried latex is often very difficult to wash off, consider wearing lightweight latex gloves when handling the plant.

Canadian Thistle