Scotch Thistle Essays - Invasive Plant Species, Agricultural Pests

Forage Production Scotch thistle (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 sp read 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