Thistle weed seeds

Scotch thistle

Regionally prohibited in the North Central, Port Phillip and Western Port catchments.

Regionally controlled in the Glenelg Hopkins, Corangamite, Goulburn Broken, North East, West Gippsland and East Gippsland catchments.

Restricted in the Mallee and Wimmera catchments.

Plant biology

Appearance

Herbaceous plant — Forb (flowering herbaceous plant — not a grass)

Description

Scotch thistle is an erect annual or biennial herb growing to 2m high and reproducing by seed and root pieces.

Stems

Scotch thistle generally has 1 main stem with numerous branches and broad spiny wings covered with dense woolly hairs, giving a whitish appearance.

Leaves

Leaf margins of scotch thistle are cut or toothed, spiny and undulating. Dense, white woolly hairs grow on the undersides of leaves and are sometimes sparser on upper sides.

Rosette leaves are stalked and grow up to 40cm long. Stem leaves are smaller and without stalks, extending into wings along the stems.

Flowers

Scotch thistle florets are purple or mauve in heads surrounded by numerous spiny bracts (modified leaves at the base of flower). Heads are 2 to 6cm in diameter, solitary or in groups towards the ends of the branches.

Bracts are woolly at the base and end in orange spines. Flowers are produced in late spring and summer.

Fruit

Seeds

Seeds of scotch thistle are 4 to 5mm long, grey with dark mottling and are attached to a pappus (parachute) of toothed hairs or bristles up to twice as long as the seed.

The pappus is often detached from the seed in the head.

Growth and lifecycle

Method of reproduction and dispersal

The major means of dispersal for the scotch thistle is by seed, but it can also be spread from severed root pieces.

Rate of growth and spread

Scotch thistle seeds can germinate at any time of the year, hence infestations consist of plants of various ages and sizes. There are 2 main periods of germination — late summer-autumn and late winter-spring.

Seedbank propagule persistence

Scotch thistle is a prolific seeder and a single plant can produce more than 20,000 seeds.

Preferred habitat

Scotch thistle prefers sub-humid temperate regions and grows well in soils of moderate to high fertility. The weed is competitive in annual rainfall areas of 500 to 850mm. It does not grow well on waterlogged soils.

Growth calendar

The icons on the following table represent the times of year for flowering, seeding, germination, the dormancy period of scotch thistle grass and also the optimum time for treatment.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Flowering
Seeding
Germination
Dormancy
Treatment

Impact

Agricultural and economic impacts

Scotch thistle competes well with pasture resulting in them being overrun by this weed. It is not grazed by stock due to its dense spines. Animals rarely eat the plant.

Management

Prescribed measures for the control of noxious weeds:

  • application of a registered herbicide
  • physical removal.

Other management techniques

Changes in land use practices and spread prevention may also support Scotch thistle management after implementing the prescribed measures.

Canada Thistle

Efforts must be made to prevent seed maturation and dispersal of plants into new areas. Additionally, no transportation, propagation, or sale of these plants is allowed. Failure to comply may result in enforcement action by the county or local municipality. Minnesota Noxious Weed Law

Background

Canada thistle is native to Europe. It was introduced to North America in the 1600s, probably in agricultural seed shipments and is now widespread throughout the United States and Canada.

Description

  • An aggressive perennial with a vigorous root system that continually produces new shoots, invading new areas and outcompeting other vegetation types.
  • Grows 2 – 5 feet tall.
  • Leaves are alternate, lance shaped, irregularly lobed, and have wavy spiny/toothed margins.
  • Stems are usually smooth, but sometimes have short hairs and are slightly grooved.
  • Flowers are purple and pink, occasionally white, and are borne at the end of the stems in clusters. Buds are 1/2 inch wide by 3/4 -1 inch long, have a tear-dropped shape, and lack spines.
  • This plant is a prolific seed producer and also spreads by roots.
  • Seedlings emerge as small rosettes in the fall or early spring, eventually bolting into erect branched flowering stems. Flowers begin to develop in late June, blooming between July and August.
  • This plant is most recognizable in mid-July when flowers change to seedheads with obvious white fluffy tops. Seeds are attached to the “fluff” and can become airborne and spread to new areas.

Habitat

Found growing in a wide range of habitats. Typically infests a variety of disturbed landscapes and is commonly found along roadsides, trails, natural areas, pastures, forest and field margins, mining locations, waste areas and unmaintained gravel pits. This plant establishes quickly after new road construction, housing and development projects, overgrazing of pastures, forestry clear-cuts, and destructive flooding events.

Means of spread and distribution

Spreads primarily by rhizomes and seeds. Found commonly throughout Minnesota.

Impact

This plant is highly invasive, severely reduces pasture capacity and desirable forages, degrades wildlife habitat, and can hinder reforestation and landscape restoration efforts. Once a population gets established, it begins to quickly displace native vegetation, including desirable pollinator habitat, creating large stands with little biological diversity and low habitat value.