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1) upper part of plant; 2) seed; 3) down with seed; 4) new shoot; 5) base of stem; 6) root system.
Canada Thistle (Cirsium arvense (L.) Scop.)
Family: Compositae
Native of Eurasia and North Africa, it was introduced to North America as a crop contaminant during the eighteenth century. It is distributed throughout the northern half of the United States and southern Canada and infests cropland, pastures, meadows, road sides, and waste places. It is considered a noxious weed throughout the United States. Other common names include field thistle, creeping thistle, and California thistle.
Seedling: Cotyledons are rounded to oblong, dull green, 16 mm long and 8 mm wide, and somewhat thick.Seedling leaves are alternate,club-shaped, covered with short hairs, and have a serrate margin with spines on the tips of the serration. Initially forms a rosette.
Root: Fleshy, creeping rhizomes extending several feet deep and spreading horizontally.
Stem: Erect, rigid, grooved, and hollow. One to four feet tall, glabrous on the upper portion with some pubescence below. Pubescence increases with maturity. Branched on upper portion. Green in color.
Leaves: Simple, alternate, oblong to oblanceolate,sessile to petiolate, clasping or not, margins shallowly lobed with crinkled edges and short spines, upper surface generally dark green and glabrous, lower surface white and hairy, 5-18 cm long and 1.5 -6 cm wide. Upper leaves are smaller, less lobed, and generally sessile.
Inflorescence: Male and female flowers are on separate plants. Numerous corymb-like clusters of heads form on the upper portion of the plant. Involucre is 1-2 cm tall by 5-10 mm wide and is composed of 5 to 6 rows of bracts, each gradually pointed and spineless. There are no ray flowers. Disk flowers are purple to pink and rarely white. Male corolla are 1.2 to 1.5 mm long while female corolla are 1.8 to 2.5 mm long. Fruit that forms is a single seeded achene.
Seed: A single seed is enclosed in an achene that is 2.5 to 4 mm long by 1 to 1.5 mm wide, dark brown to tan, oblong, flattened, curved or straight, and with a pappas of tannish-white bristles that is easily detached. The blossom end of the achene has a ridge and is blunt with a rounded bump in the center.
A cool season perennial, herbaceousforb. Reproduces by windborne seed and from shoots from creeping rhizomes. Plants can also form from one inch or longer severed root sections. Plants are dioecious. Flowers June to August. Seed production can be as high as 4000 to 5000seeds per plant. Seeds may become viable 8 to 10 days after flowers open. Seeds generally have a low level of dormancy but can remain viable many years if buried.Germination occurs at shallow depths. Seedlings do not survive well under competition for light. Rapidly forms colonies due to the rhizomatous nature.
Causes reduced crop yields when infestations are high. Canada thistle is a particular concern in canning peas as bud formation occurs around the time of pea harvest, resulting in product contamination by the pea-size thistle buds.
Canada thistle control is difficult due to the extensive root system. Prevent the spread of seed by mowing or chemical treatment prior to flowering. Seedling establishment can be prevented by competition from desirable plants. In pastures for instance, proper grazing management will provide poor light conditions preventing Canada thistle from establishing from seed.
The goal of any control program is to deplete the root reserves. Cultivation will deplete root reserves over time but must be implemented every 2 to 3 weeks throughout the growing season. Mowing will also help reduce root reserves but will be less effective than cultivation. Chemical control options exist in crops ranging from burndown products to those that translocate deep into the roots.
Several biological control agents are being tested for control options. These include the stem gall fly (Urophora cardui), seed head weevil (Larinus planus), stem mining weevil (Ceutorhynchus litura) and a foliage feeder (Cassida rubiginosa). Research has also been conducted with the phytopathogenic bacterium Pseudomonas syringae pv. tagetis .