Cirsium arvense

Thistles of the genus Cirsium are distinguished by their plumose pappus from members of the genus Carduus which are known as "plumeless thistles" because of their simple pappus. Within the genus Cirsium, C. arvense is the most often confused with the bull thistle, C. vulgare. The two species can be distinguished by the larger head and isolated distribution of the bull thistle (Hansen 1918). Gray's Manual of Botany (Fernald 1950) distinguishes C. arvense from all other thistles by its habit of arising from sprouts of a widely creeping root system and its almost universally dioecious plants.

The northern limit of the zone of highest density in Canada corresponds with the 18 degrees mean January isotherm, whereas the southern limit of the species is probably controlled by high summer temperatures and short-day length (Moore 1975).

Cirsium arvense is found in open, mesophytic areas, with optimal growth corresponding to between 50 and 75 cm annual rainfall (Hodgson 1968). It occurs on all but waterlogged, poorly-aerated soils (Rogers 1928, Bakker 1960, Hodgson 1968, Moore 1975) and in nearly every community within its range where sufficient light is available. Bakker (1960) found an average density of 39 shoots per meter with 41 flower heads per shoot in open sites, and a density of 11 shoots per meter with 18 flower heads per shoot in shaded areas. Patches are most prevalent in disturbed areas such as agricultural land, roadsides, ditch spoil banks, gopher mounds and overgrazed pastures (Moore 1975).

C. arvense apparently has difficulty becoming established from seed in undisturbed areas. Amor and Harris (1974) report no seedling establishment from seed artificially sown in pastures, whereas 7 to 13% of seeds sown on bare dirt emerged and 78 to 93% of these became established. In spring wheat fields, Canada thistle increased in abundance by 192% over a four-year period, whereas over the same interval it declined to 1% of its previous value in alfalfa fields mowed twice yearly for hay (Hodgson 1968).

The most outstanding biological characteristic of Cirsium arvense is its well developed lateral root system, which sends up new shoots at 6 to 12 cm intervals (Moore 1975). Morphological studies indicate that this is a true root system, developed at depths beyond those used by rhizomes (Friesen 1968). Lateral root growth can exceed 6 m in one growing season (Rogers 1928, Hayden 1934). The depth of vertical roots appears to be determined by the depth of the water table and has been reported to be as deep as 6.75 m (Rogers 1928). The roots are brittle and regeneration has been observed from cuttings as small as 10 mm long and 1 mm in diameter under laboratory conditions (Hamdoun 1972). Root segments 3 cm thick and 6 cm long can regenerate shoots in as short an interval as 5 days (Sagar and Rawson 1964).

Root bud elongation increases with increasing root temperature and photoperiod and is greatest at shoot temperatures of 25 degrees C daytime/15 degrees C night, with a 15-hour photoperiod and 30 degrees C root temperature (McAllister and Haderlie 1985). Under laboratory conditions root buds are inhibited by internal competition for a limiting N supply (McIntyre and Hunter 1975).

Phenology of Cirsium arvense varies with different ecotypes, but follows a general pattern. In Washington State, overwintering roots develop new underground roots and shoots in January and begin to elongate in February (Rogers 1928). Shoots emerge in May, when mean weekly temperatures reach 5 degrees C. Rosette formation follows, with a period of most active vertical growth (about 3cm/day) in mid-to-late June. Flowering in Montana and South Dakota is from early June to August and in Canada from June to September (Hodgson 1968, Van Bruggan 1976, Moore 1975).

Root carbohydrate reserves exhibit an annual cycle. Minimum reserves occur in early June, just before flowering. As growth declines in July, these reserves begin to rebuild and increase in the early fall (Hodgson 1968, Bakker 1960, Arny 1932, Welton et al. 1929).

Cirsium arvense is a long-day plant. Flowering of different ecotypes has been observed under daylengths of 14 to 18 hours, but not under light of 8 to 12 hours (Linck and Kommedal 1958, Hunter and Smith 1972).

Because of the species' dioecious plants and its prolific vegetative reproduction, whole patches of C. arvense are usually one sex or the other. Cross-pollination success is a function of distance. Hodgson (1964) reports that a high proportion of seeds are produced when parent plants are 17 m apart. Seed production decreases with distance between 33 m and 200 m (Hayden 1934) and only a small percentage of seeds are produced from plants 390 m apart (Amor and Harris 1974).

One plant can produce up to 5,200 seeds with an average annual production of 1,530 seeds per plant (Hay 1937) with between 83 and 90 seeds per head (Derschied and Schultz 1960, Hayden 1934). Seed size varies among different ecotypes, ranging from 298,000 to 677,000 seeds per pound (Hodgson 1968). Seeds are known to be dispersed by wind (Bakker 1960) and by run-off in drainage or irrigation ditches (Wilson 1979, Hope 1927).

A succession of seedlings is produced from a single crop of seeds. In a Montana study that compared several western ecotypes Hodgson (1964) found that germination of fresh seed varies with ecotype. Rate of germination ranges between 50 and 95% (Hayden 1934, Hodgson 1964). Under natural conditions, seeds of some Canadian plants germinate immediately, produce rosettes before winter, and emerge to flower the next spring, whereas germination of others is delayed until the following year (Moore 1975). About 90% of all seeds germinate within one year of dispersal (Roberts and Chancellor 1979). Seeds buried as part of Duval's long-term buried seed experiment germinated after 21 years, but peak germination occurred during the third year (Toole and Brown 1946).

Seedling establishment requires high light intensity. Growth is reduced in light of 60-70% full daylight and death ensues when light intensity falls to below 20% of full daylight (Bakker 1960). Under optimum conditions seedlings begin to develop roots capable of vegetative reproduction at 8 weeks (Bakker 1960). Detmers (1927) observed a 101 cm root with 10 shoot buds on a 4-month seedling. Rogers (1928) states that a root fragment more than 6 weeks old can regenerate an entire plant.