Thistle Type Weeds
- Thistle weed control depends on identifying the physical characteristics separating thistle weeds from less harmful related species. An untrained observer may overlook differences among teasels, sowthistles and true thistles. Teasels (Dipsacus spp.) have serrated, wrinkled leaves. Their egg-shaped, white or lavender blooms measure as long as 4 inches. European and Eurasian sowthistles (Sonchus spp.) pair tiny, yellow flowers with bluntly spined foliage. Their cut stems exude white sap. True thistles have sharply spined leaves. Colors of their 3/4- to 2-inch blooms include lavender, purple, red, pink or -- infrequently -- white.
- Thistle weeds grow as biennials or perennials. Biennial plants, including bull (Cirsium vulgare), musk (Carduus nutans) and plumeless (Carduus acanthoides) thistles, germinate and produce a clump of leaves in their first year. They flower, usually between May and October, set seed and die in their second year. Their vigorous spread results from large numbers of seeds; a single plumeless thistle plant may yield 120,000 of them. Perennial thistles, on the other hand, survive for years. They spread by roots and seeds.
- Canada thistle arrived in the United States in early Colonial times. The Vermont Legislature labeled the Eurasian native a weed in 1795. Canada thistle's 2- to 3-foot stems have oblong, spiny-edge green leaves, spiny roots and small, lavender, white or pink flowers. Plumeless thistle, from Europe and Asia, has 1- to 4-foot stems with spiny branches of wavy-edged green leaves. Their rounded, pink or lavender booms usually open from May until July. Eurasian musk thistle weeds reach an imposing 6 feet or higher. Their large, rounded rose-purple blooms open on nodding stems from early to late summer. The stems rise from 2-foot clumps of smooth green leaves. Bull thistle, although less invasive than musk and plumeless plants, now grows across the United States. The European, Asian and African native's bushy, 2- to 5-foot frame has deeply serrated, prickly-surfaced spiny leaves. Yellow-tipped spines also enclose its conical, purple mid- to late-summer blooms.
- The 3- to 4-foot Flodman's thistle weed grows across the northern central United States to Colorado. Drought tolerance permits it rangeland spread. Its leaves have glossy green surfaces, downy white undersides and deeply lobed margins. Its red or violet, oval flowerheads trap insects in a sticky substance. They typically bloom from mid- to late summer. Closely resembling Flodman's thistle, wavyleaf thistle weed blooms up to two weeks earlier with pink or purple, spherical spiny-tipped flowerheads. Its foliage has heavier down, shallower lobes and denser spines. Wavyleaf plants are frequent sagebrush companions.