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Statewide IPM Program, University of California

Russian thistle  (Salsola ssp.)

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Life stages of Russian thistle mature plant seedling immature plant seeds flower in leaf axil flowering stem

Russian thistle is a large and bushy noxious annual broadleaf plant. It occurs throughout the western states, more often in drier areas. Recent taxonomic work has demonstrated that what has been named Salsola tragus likely consists of several morphologically similar species that differ in flower size and shape.  Besides S. tragus, these include S. australis, S. iberica, S. kali, S. pestifer, and S. ruthenica. Russian thistle is common throughout California, especially in the southern region to an elevation of 8900 feet (2700 m). It grows best on loose sandy soils and inhabits agricultural land, roadsides, and other disturbed places.

When consumed in moderate amounts, immature plants are nutritious for livestock. However, with maturity and under particular conditions, some Salsola species accumulate levels of oxalates toxic to livestock, especially sheep. Most often toxicity occurs when sheep feed almost exclusively on these species for many weeks. Russian thistle also can create a fire hazard or hinder traffic when it breaks off from its main stem and dries up. At this stage, it is commonly called tumbleweed. It is also an alternate host for the beet leafhopper, Ciculifer tenellus, which vectors the virus that causes curly top disease in melons, tomatoes, sugar beets, and other crops. Common soil-applied (preemergence) herbicides generally provide good control of this weed, but the seeds can germinate from up to 2.5 inches deep, so control may be poor where incorporation is shallow.

Habitat

Fields, unmanaged places, roadsides, cultivated and other disturbed sites.

Seedling

Stems are slender and flexible and often have reddish purple streaks. The cotyledons (seed leaves) and first true leaves are long and thin, like pine needles. The cotyledons are 2/5 to 1-2/5 inches (10–35 mm) long. Later leaves are soft and fleshy with a weak spine at the tip. Leaves are alternate to one another along the stem, but may appear opposite to one another because of the short length between stem joints. Later leaves are soft and fleshy, with small spines at the tips. Stems are thin, flexible, and often have reddish purple striations.

Young plant

Young plants are usually taller than wide. Their lateral branches are shorter than the main stem and point upward.

Mature plant

Mature plants are large and bushy with rigid, purple-streaked or green stems that typically curve upward giving the plant an overall round shape. They generally grow to about 3 feet (1 m) tall but can grow much larger usually with a similar height and width, or taller than wide. Leaves are somewhat bluish green, fleshy to leathery, hairless or covered with stiff short hairs, and 1/3 to 2 inches (8–52 mm) long and up to 1/25 of an inch (1 mm) wide. Leaf tips are sharply pointed to spine tipped. Upper stem leaves are reduced (bracts), stiff, and prickly. After they turn grayish brown, the plants break away from the roots at the soil line, becoming tumbleweeds that scatter their seeds as the plant skeletons are blown around.

Flowers

Flowers bloom from July through October. Flowers are produced in the junction between the leaf base and the stem (axils). Although they lack petals, they have an outer whorl of winglike sepals that are, translucent, petal-like, fan shaped and often pinkish to deep red with noticeable veins.

Fruits

Fruiting structures contain one seed, are somewhat round, and can grow to 3/10 of an inch (8 mm) in diameter—including the winglike sepals. Sepal wings are open and flat or folded over.

Seed

Seeds are compressed and round to somewhat conical, gray to brown, and have a thin, translucent seed coat, through which a dark, greenish-brown coiled embryo is visible.

Reproduction

Reproduces by seed.

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