Perennial Canada Thistle
Perennial thistle plant
Perennial thistle rosette
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BOTANICAL NAME:Cirsium arvense
Asteraceae (daisy family)
Perennial, Canadian or Californian thistle is declared noxious in the Southern Tablelands and South East Region only in Cooma-Monaro and Snowy River LCAs, in class 4.
Most thistles are erect single-stemmed or branching biennial herbs ranging from 30cm to 2m high depending on species and growing conditions. They are characterised by having long spines on the leaf margins and stems. The plant begins life as a rosette, from which an elongated flowering stem arises. Flower heads consist of numerous small flowers clustered into cylindrical or hemispherical heads at the branch tips, and usually surrounded by spiny bracts. Flower colour is pink to purple for most of the species found in the region, except for a few with yellow flowers.
Perennial thistle is distinctive in having the purple flower heads enclosed by several layers of non-spiny bracts, which are green at the base, grading to purple towards the top. The rosette leaves have wavy margins and are sparsely spiny on the margins, at the tips of the lobes. Perennial thistles differs from all other thistles found in the region in spreading by creeping rhizomes (underground runners).
preferred habitat and impacts
Perennial thistle is highly invasive because of its habit of spreading vegetatively. It is strongly competitive in crops and pastures.
Thistles in general are invasive weeds of pasture, reducing carrying capacity. The broad flat rosette habit in the early stages of growth smothers surrounding grass plants, and the density of stands which can occur after disturbance such as over-grazing or cultivation can choke out all other vegetation. Unpalatable to stock because of the spines, it is favoured by heavy grazing. The spiny nature of thistle plants restricts stock and human movement in infested pasture.
Thistles are a more troublesome weed in the drier tablelands and slopes of southern NSW than on the coast, and more species are present in these areas. Black or spear thistle is the most common thistle found on both the coast and the tablelands, but other thistle species are quite uncommon on the coast.
Thistles are also environmental weeds, invading grasslands and grassy woodlands and occasionally moving into forest where there is sufficient soil moisture for them to overcome the competitive effect of the trees, particularly along road edges.
Lateral growth from the root system of perennial thistle can increase the size of patches by up to several metres per year, and pieces of root may be moved around during cultivation and give rise to new plants. Seed is generally wind-blown, and moved around in soil and on animals, vehicles and machinery. Perennial thistle has been dispersed around the world as a contaminant of agricultural seeds. In Australia fertile seed was seldom produced because the two sexes of flower occur on separate plants and both need to be present within 100m of each other to set seed. As the weed becomes more common, this requirement is more often met. The degree of wind movement of thistle seed varies with the species. Although a cluster of fine hairs is present to assist with wind dispersal this often breaks off, and distances over which seed will travel has varied in studies between 10 metres and 1km. The lighter seed which travels the furthest may be sterile.
All thistles are broadly similar in appearance and perennial thistle is distinguished by its lack of spines on the bracts enclosing the purple or pink flower heads. In this character it is most similar to knapweeds (Centaurera nigra and C. maculosa), but these have non-spiny leaves.
Young plants are similar in appearance to prickly sowthistle (Sonchus asper), but this species does not spread by underground runners and has small yellow flowers in a tight cluster at the tip of the plant. It is a common and widespead weed.
Cultivation is not very effective with this species because of the spreading root system, which is hard to exhaust and may be spread to uninfested areas during cultivation. However, repeated cultivation during hot, dry conditions, preventing the plants from developing above ground between cultivations may exhaust the root system in 3 years. Spot spraying or boom spraying with suitable herbicides is likely to be more effective and selective herbicides can be used to remove them from pasture.