(Onopordum acanthium) also known as cotton thistle or woolly thistle is native of Europe and eastern Asia and is probably an escaped ornamental plant. Scotch thistle stands are dense and practically impenetrable due to the weed’s spiny nature and large size. It spreads by seed and generally inhabits moist sites or drainages in dry locations. It was introduced in the 1800s to the United States and it crowds out native species and crops.
Scotch thistle is a branched, robust biennial (or sometimes annual) although it can behave as a winter or summer annual or a short-lived perennial under certain situations. Often grows 8 feet or more in height and 6 feet in width. Main stems may be up to 4 inches wide at the base. Stems have vertical rows of prominent, spiny, ribbon-like leaf material or “wings” that extend to the base of the flower heads. Leaves, which are armed with sharp, yellow spines, are up to 2 feet long and 1 foot wide. Upper and lower leaf surfaces are covered with a thick mat of cotton-like or woolly hairs, which give the foliage a gray-green appearance. Plants flower in mid-summer from July to September. The globe-shaped flower heads are borne in groups of 2 or 3 on branch tips. Flower heads are up to 2 inches in diameter, with long, stiff, needle-like bracts at the base. Flowers range in color from dark pink to lavender.
Seeds are smooth, slender, and plumed. As a biennial, Scotch thistle typically lives for two growing seasons. Seeds usually germinate in the late fall, but germination can occur at other times, as well. Seedlings that appear in late autumn behave as true biennials, but seedlings produced during late summer or early autumn behave as annuals. During its first year, Scotch thistle produces a rosette with a taproot that may extend down 1 foot or more. Early in the second year, the plant bolts. Plants produce 8,400 to 40,000 seeds. Reports on seed longevity in the soil vary from unknown to up to 20+ years. Seeds are dispersed locally by wind; humans, water, livestock, and wildlife are involved in longer-distance dispersal. Seeds are sensitive to light. While some seeds will germinate in the dark, studies indicate that most germination occurs with alternating light/dark cycles, with 8 hours being the optimal day length
Scotch thistle is a wasteland weed that generally inhabits moist sites or drainages in dry locations. If not controlled, it presses into farmland or forms dense canopies in any area overgrazed or not under intense cultivation. It is a major agricultural weed in the western United States. If the soil is moist enough, it has the ability to re-sprout when its roots are cut up during cultivation. In the western U.S., Scotch thistle infests wet meadows and pastures, as well as more arid big sagebrush sites. Scotch thistle is often associated with waste places, as well as rivers, streams, canals, or other waterways. It can also be abundant in dry pastures, fields, and rangeland. In particular, the plant thrives in light, well-drained, and sandy or stony soils. Temperature and moisture, rather than soil nutrient concentrations determine the ecological performance of Onopordum species. It is a major issue in rangeland management in northeastern Oregon, Idaho, and Utah. Scotch thistle can spread rapidly. For example, it was first found in Utah in 1963. By 1981, it covered approximately 6070 hectares in 17 counties. Eight years later, it had spread to cover more than 22,540 hectares in 22 counties.
Small areas can be eradicated by digging. Plants must be cut off below the soil, leaving no leaves attached. Mowing has limited effectiveness for controlling Scotch thistle. It usually only prevents seed production if done either immediately prior to flowering or when plants are just starting to flower. When mowing is conducted too early, it may only delay flowering. However, when plants are cut too late in the flowering process, viable seed may still develop in the capitula following cutting. Because there can be a wide variety in the maturity of plants, a single mowing is unlikely to provide satisfactory control.
Establishing and maintaining dense, vigorous, competitive pasture can effectively prevent Scotch thistle establishment. Healthy pasture is particularly important in the autumn, when most Scotch thistle seeds germinate. Thistle invasion in unlikely to occur in un-grazed pasture. Goats will graze Scotch thistle, reducing plant numbers and preventing seed production.
Herbicides, Picloram, dicamba, 2,4-D, dicamba + 2,4,-D, and metsulfuron are effective for controlling Scotch thistle. Application rates vary depending on stand density and environmental conditions. Herbicides should be applied in the spring before Scotch thistle bolts, or in the fall to rosettes (Beck 1991). . For chemical control recommendations, refer to the Pacific Northwest Weed Control Handbook, an annually revised publication available from Washington State University Cooperative Extension. Here are a few guidelines to help lessen the spread of noxious weeds in Idaho.
?Avoid driving in noxious weed infested areas. Seeds can become stuck in tire treads or mud on the vehicle and be carried to unaffected areas.
?Don’t transport flowering plants that you cannot identify.
?If you find a small number of isolated noxious weeds that have no flowers or seeds, pull the weeds and leave them where you found them to dry out.
?If you find noxious weeds and they have flowers or seeds, pull them, place them in a plastic bag or container to avoid spreading seeds, and either burn them or dispose of them in a sanitary landfill.
?Report newly-found noxious weeds to the county weed superintendent or county extension office.
Here are some interesting facts about the Scotch thistle. It has been credited with helping Scotland fend off Viking invasion. As the Vikings moved into Scotland for a sneak attack, they yelled out in pain when they stumbled through thistle plants. Their cries alerted the Scots and allowed them to push out the Vikings. Since then, Scotch thistle has been the national emblem of Scotland. Scotch thistle was probably introduced to North America as an ornamental plant in the late nineteenth century Scotch thistle is sometimes sold as an ornamental plant. It has reportedly been used to treat cancers and ulcers and to diminish discharges of mucous membranes. The receptacle was eaten in earlier times like an artichoke. The cottony hairs on the stem have been occasionally collected to stuff pillows. Oil for Scotch thistle seeds has been used in Europe for burning and cooking.