Scotch thistle (Onopordum acanthium)
Keys to ID:
- Purple flowers in clusters of 2 to 5 flowers.
- Leaves are alternate, stalkless and hairy underneath.
- Thistle-like appearance
Identification:
- Lifecycle: Biennial
- Growth Form: Forb
- Flower: Purple to rosy violet flowers with spine-tipped bracts in clusters, with dozens of flowers per plant. Blooms mid-summer.
- Seeds/Fruit: One-seeded fruit is wrinkled, brown to grayish black, and tipped with a plume of slender bristles.
- Leaves: Leaves are alternate, large, irregularly lobed, and have sharp yellow spikes. Rosette leaves may be up to 2 feet long and 1 foot wide. Upper and lower surfaces are covered with a thick mat of cotton or woolly-like hairs, giving the plant a gray-green color.
- Stems: Mature plant can grow up to 12 feet tall. Stems are numerous, branched, and have spiny wings.
- Roots: Thick, fleshy taproot.
- Propagation: Seeds. From 110 to 140 seeds per flower; seeds can lay dormant up to 5 years, and are dispersed by water, wind, animals, and human activity. (Belliston et al. 2004).
Control:
Mech: Sever the root below the surface. Removal by pulling or digging the rosettes before they have begun to flower is the simplest and most effective method of control. Plant does not reproduce vegetatively so severing the roots will kill the plant. Tilling is another effective method in a garden area, while mowing is not as effective because the flower can still produce seed. Plant pieces must be carefully bagged and disposed of to prevent spread. Control of these plants must include preventing new seed dispersal for up to 6 years.
Cautions:
Sharp spines have a tendency to punch through even the thickest leather gloves. Caution is advised when pulling and long-handled cutting tools are often helpful.