Salsola paulsenii
(barbwire Russian thistle)

Russian thistles or tumbleweed of the genus Salsola are annual weeds, mostly native to Europe and Asia, but introduced globally. Salsola infests many tens of millions of hectares, especially in North America. S. paulsenii is a coarse, spiny annual weed native to Central Asia and introduced to North America. It tends to be found in drier and more lowland desert areas in the western USA as compared to other weedy Salsola species, with which it is often confused. S. paulsenii can be controlled by cultivation and chemicals, and a number of biological control agents have been tested. However, it is likely to remain a troublesome weed unless biocontrol eventually offers a viable option.

S. paulsenii is native to a large part of Central Asia, from southern European Russia (Dagestan, Kalmykia, Orenburg, Rostov and Volgograd) to Mongolia and western China (Xinjiang), and from Kazakhstan in the north to Pakistan in the south (USDA-ARS, 2015).

It has been widely introduced, and is especially prevalent in the western USA, particularly California, Oregon, Nevada, Utah, Arizona, Colorado, New Mexico and Texas (USDA-NRCS, 2015). It is likely to be present in many more countries than those listed in the Distribution table, such as Mexico; its distribution is likely to become clearer following taxonomical clarification of the genus. However, it is not reported in Australia (CHAH, 2015), Pacific islands (PIER, 2015) or the Caribbean (Kairo et al., 2003).

S. paulsenii is a declared noxious weed in California (USDA-NRCS, 2015), and closely related Salsola species are declared noxious in many other US states and Canadian provinces. S. paulsenii is adapted to range of dry, cold and temperature climates, and has the potential to invade large areas around the world if it was introduced. No Salsola species should be intentionally introduced to any new areas for any purposes, and care should be taken to reduce any risks of accidental introduction, such as with the movement of hay made from S. paulsenii or hay containing its seeds.

In its native range, S. paulsenii is a widespread rangeland and agricultural weed. Where introduced, it is also a rangeland and agricultural weed, especially in sandy soils, disturbed natural and semi-natural plant communities, semi-deserts, deserts, eroded slopes, sand dunes and sandy waste places. Beatley (1973) observed S. paulsenii to be common in western USA on disturbed land below 1200 m, especially on limestone-derived soils. It dominates severely degraded native plant communities in the most arid portions of the Great Basin (western USA) and southwest USA.

Genetics

S. paulsenii is a tetraploid species with a chromosome number reported as 2n = 36 (Zakharyeva, 1985; Wang et al., 2008). Ploidy levels may be a useful means for separating it from some other species of Salsola that are diploid or have higher ploidy levels. Reports of a chromosome number of 2n = 18 for S. paulsenii are probably erroneous and based on misinterpretation of data (Flora of North America Editorial Committee, 2014). Other forms conventionally named as ‘Salsola X’ and ‘S. paulsenii lax form’ with 2n = 54 (e.g. Arnold, 1972; Ryan and Ayres, 2000) are probably of hybrid origin.

Reproductive biology

S. paulsenii reproduces by seed, which can only survive for up to two years in the soil. S. paulsenii seed has temperature related after-ripening requirements, but these are less stringent and break down sooner after maturity than S. kali seed. The shorter after-ripening requirements allow for germination during the winter months, giving S. paulsenii a distinct competitive advantage under arid conditions with winter precipitation and summer drought (Young and Evans, 1979b). S. paulsenii is more suited than S. iberica to arid environments because S. paulsenii can germination at a broader temperature range and sooner after maturity, is more rapid at low temperatures and during the first 10 days of incubation, and because seed dehiscence occurs without the uprooting and tumbling of the plants (Young and Evans, 1979b).

Seed production of S. paulsenii was correlated strongly with herbage yield and weakly with plant density, and was markedly lower than S. iberica (Evans and Young, 1980). Later studies by Evans and Young (1982) on seed germination and seedbed ecology of S. paulsenii and S. iberica confirmed that both exhibited temperature related after-ripening requirements after maturity in autumn and winter, and once requirements has been met both species germinated well at a wide range of temperatures. However, S. paulsenii seeds did not germinate well under very low osmotic potentials. Seedling establishment was increased by soil and litter coverage and by favourable microtopography of the soil surface.

Specific adaptations of S. paulsenii include earlier maturation and seed dispersal, seed dispersal under parent plants, and less specific temperature related after-ripening requirements as compared to S. iberica; but even with these characteristics, S. paulsenii establishment was lower in arid environments compared with S. iberica (Evans and Young, 1982).

Physiology and phenology

Most Salsola species exhibit C4 photosynthesis. Four species from Central Asia were investigated by Pyankov et al. (2000) to determine the structural and functional relationships in photosynthesis of cotyledons compared to leaves, using anatomical and biochemical criteria. These included S. paulsenii from section Salsola, S. richteri from section Coccosalsola, S. laricina from section Caroxylon, and S. gemmascens from section Malpigipila. Results show that all species have a C4 type of photosynthesis in leaves with a Salsoloid type Kranz anatomy, whereas both C3 and C4 types of photosynthesis were found in cotyledons. S. paulsenii (and S. richteri) have NADP- (NADP-ME) C4 type biochemistry with Salsoloid Kranz anatomy in both leaves and cotyledons (Pyankov et al., 2000).

S. paulsenii flowers in spring and summer.

Environmental requirements

S. paulsenii is adapted to range of dry, cold and temperature climates. Native to Central Asia, it can tolerate great extremes in both diurnal and annual temperature, tolerating cold and frost as well as daily highs above 40ºC. The native range is often characterized by low rainfall areas, and it can also tolerate long dry seasons.

Where introduced, it is commonly found in sandy soils, eroded slopes and sand dunes, in dry areas, up to 1900 m altitude. It has been observed that S. paulsenii is similar in ecological requirements to S. kali (Young and Evans, 1979b).

The transport of aqueous-soluble solutes in dust was investigated in Nevada, USA, and it was concluded that increases in potassium, nitrate, ammonium, sulphur and salt content of aeolian dust were likely to be responsible for the invasion of S. paulsenii (Blank et al., 1999).

Animal feed, fodder, forage

• Forage

S. paulsenii is similar to S. tragus sensu stricto. Intermediate forms are known, and S. paulsenii × S. tragus hybrids have been identified, identical with S. × gobicola. Forms conventionally named as ‘Salsola X’ and ‘S. paulsenii lax form,’ with 2n = 54 (e.g. Arnold, 1972; Ryan and Ayres, 2000), are probably also of hybrid origin.

S. paulsenii is also noted to be similar in flowers and fruits to S. tragus and S. vermiculata, but it is easily distinguished from them by its shrubby perennial habit and oblong to ovate leaves with round tips (Flora of North America Editorial Committee, 2014). Crompton and Bassett (1985) provided a key that separates the closely related species S. paulsenii, S. kali, S. collina and S. pestifer.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/mechanical control

Cultivation can effectively control S. paulsenii, but must be repeated for at least two years until all seeds in the seedbank loose viability, as well as ensuring that no more seeds are re-introduced.

Biological control

The eriophyid mite, Aceria salsolae (Acari: Eriophyidae), was collected in Greece and evaluated for host plant specificity as a prospective biological control agent of invasive alien tumbleweeds in the USA, including S. paulsenii, S. tragus, S. collina and S. australis. The mite does not form galls, but is a vagrant that inhabits leaf and flower bud crevices, and feeding damage stunts the plant. The mite was able to multiply only on species in the Salsola section Kali subsection Kali, which includes the alien weeds Salsola collina, Salsola kali and Salsola paulsenii. It did not damage or multiply on Salsola soda, which is in a different section, nor on Halogeton glomeratus, which is in the same tribe. The mite reduced plant size by 66% at 25 weeks post-infestation under artificial conditions (Smith, 2005). Further studies concluded that there would be no significant risk to non-target plants as a result of using A. salsolae as a biological agent to control Salsola species in North America (Smith et al., 2009).

The search for biological control agents on related species continues, and a recent study found that a gall forming midge, Desertovelum stackelbergi Mamaev from Uzbekistan and a fungal pathogen, Colletotrichum gloeosporoides (Penz) from Hungary had much higher rates of attack and damage to S. tragus than S. australis (Smith et al., 2013). It is also clear that different agents have different impacts on closely related species, and so further work on taxonomic elucidation is required.

Chemical control

Herbicides found to be effective with other Salsola species, especially S. kali and close relatives, are likely to be effective for the control of S. paulsenii.

30/04/15 Original text by:

Nick Pasiecznik, consultant, France