Scotch Thistle

Composite or aster (Asteraceae family):

Onopordum acanthium L.

EPPO code:

ONRAC

Other names:

White thistle, cotton thistle

Species information

Lifecycle:

Biennial, sometimes annual.

Propagation:

Reproduces by wind-dispersed seed.

Emergence:

Scotch thistle seed germinates in the spring and late summer.

Habitat:

The weed is found primarily in pastures, roadsides and waste places. It is rarely found in cultivated crops.

Competitiveness:

Scotch thistle reduces productivity in pastures as livestock avoid it.

Identification clues

Seedling

Cotyledons:

Orbicular (round) to oval with whitish veins, tapering into a stalk, approximately 30 mm long.

Young leaves:

Its first leaves are lanceolate, white and woolly, lobed or coarsely toothed, and have spiny margins.

Mature leaves:

The mature leaves are similar to the rosette (first) leaves; however, they are smaller and alternately oriented on the stem.

Mature plant

Stems:

Erect, tall (1–2 m), white and woolly with broad and spiny wings.

Flowers:

The inflorescences are similar to those of bull thistle, but the spiny region below the purple florets is white and wooly.

Seeds:

The seed, which is light brownish-gray, is 5 mm long and attached to a white feathery pappus, often referred to as “fluff.”

Roots:

Thick, long and fleshy taproot.

Note of interest

Humans have utilized the plant in many ways, both as food and for medicinal purposes.

Often mistaken for

I know it's not Bull thistle because the leaves and stem of Scotch thistle have a white and wooly appearance, while bull thistle has dark green leaves and a dark green stem.