U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
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Russian Thistle (Salsola tragus)
Goosefoot Family (Chenopodiaceae)
Description/Habitat: Russian thistle (Salsola tragus) is a large, bushy, noxious summer annual with rigid branches and reduced, stiff, prickly upper stem leaves (bracts) at maturity. Russian thistle grows best on loose, sandy soils, but will grow in any disturbed sites, waste places, roadsides, fields, cultivated fields, and disturbed natural and semi-natural plant communities. All species in the genus Salsola are native to Europe. The taxonomy of Salsola species in the western United States is complex and not fully understood. Russian thistles can impede traffic, create fire hazards, and are the alternate host for the beet leaf-hopper (Circulifer tenellus) that can carry the virus causing curly top of sugar-beets, tomatoes, melons, and other crops.
When this plant is consumed, toxins called oxalates can accumulate over time in livestock and become toxic, especially to sheep. Diarrhea and toxicity problems associated with high levels of oxalate ingestion most often occur when sheep have been foraging almost exclusively on Russian thistle or other Salsola species for several weeks. Symptoms of oxalate poisoning include abrupt onset of depression, weakness, labored breathing, prostration, seizures, and sometimes coma and death.
Leaves: Leaves are alternate, sessile, linear to needlelike, and gradate into rigid spine-tipped bracts in the inflorescences. The leaves are also fleshy to leathery, 8-52 mm long, mostly 0.5-1 mm wide, and the tips sharp-pointed to spine-tipped. Russian thistle grows approximately 1 m tall, usually height equal to the width, or taller than the plant is wide. Stems are rigid, typically curved upward, purple-striated or green. Foliage is bluish green and glabrous, but sometimes covered with short, stiff hairs. Bracts are usually awl-shaped, reflexed, and not overlapping at maturity; the margins are membranous and minutely barbed.
Flowers: Russian thistle flowers come in a two varieties, male and bisexual. The bisexual flowers are generally solitary on the branches, and positioned at the axil with no petals. The calyx has wing-like appendages that appear petal-like, but are paper thin in feel, and are usually 2.5-3.5mm long, 4-5 parted, persistent in fruit. The flowers have 5 stamens that protrude beyond the sepals. The sepal tips are acute, limp, and not spine-like. The sepal wings are 0.5-2.5 mm long, fan-shaped, and the margin is usually minutely toothed or scalloped, translucent, and often pink to deep red with conspicuous veins. These flowers are wind-pollinated, but can also pollinate themselves if needed.
Seeds: After drying the main stems of the Russian thistle can break off at the ground level under windy conditions, which allows the plants to disperse the seeds as they tumble with the wind. The skeletons will usually persist for at least one year and are typically found along fences and other structures. Russian thistle reproduces by seed, and germination can occur when night temperatures are below freezing and daytime temperatures reach 2º C, but optimal temperature for germination is generally 44º-95º F. Seeds require very little moisture (about 7.5mm of rainfall) to germinate, and after the moisture falls germination will occur within a few hours. To become established seedlings require loose soil. Seedlings that germinate on firm soil seldom survive because the young root is unable to penetrate the soil. Under field conditions, most seeds survive about 1 year, and a few seeds may survive up to 3 years. Plants about 0.5 m tall can produce about 1500-2000 seeds, and larger plants can produce up to 100,000.
Flowering Period: Russian thistle flowers from July-October. Male flowers often develop in early July, and bisexual flowers develop about mid-July to early October.
Management: At Fort Ord National Monument, the entire Russian thistle plant is pulled or dug out of the ground, bagged, and thrown away. Russian thistle has been present at Ft. Ord in small numbers for many years, but recently it has moved into new areas. This spread is likely related to gravel which was put on several roads throughout Fort Ord. Luckily, each new site contains only a few plants which were caught and treated early before they could become established.
In other places it has been found that seedlings cut just about the cotyledons seldom survive. Properly timed cultivation of seedlings prevents seed production and can control infestations, but cultivation must be repeated until the short-lived soil seedbank (≤ 2 years) becomes depleted.
DiTomaso, J. M. and E. A. Healy. 2007. Weeds of California and Other Western States. Volume 1: Aizoaceae to Fabaceae. University of California Department of Agriculture and Natural Resources: 622-632
|Last updated: 08-20-2012|
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