Rumex acetosella
(sheep's sorrel)

Holm et al. (1997) listed Rumex acetosella as one of the world’s worst weeds, infesting 45 different crops in 70 countries. In 1891, the government of New South Wales pronounced R. acetosella to be one of the worst weeds introduced into Australia (Holm et al., 1997). Although R. acetosella is not shade tolerant, it still may be competitive in forage situations where grazing opens up the canopy (Leege et al., 1981). Its ability to recover quickly from grazing or clipping impacts also aids in its persistence in grassland and pasture habitats (Val and Crawley, 2004). Another aspect increasing the invasiveness of R. acetosella is its relatively large seedbank (Frankton and Mulligan, 1987).

R. acetosella is native to Europe and southwestern Asia but has been introduced and spread throughout many regions of the world. R. acetosella is reported from 70 countries, including most agricultural areas except for equatorial regions of South America and Africa (Holm et al., 1997).

Due to its broad range of environmental tolerances and large seedbank, there is a risk that R. acetosella could become established in new areas following introduction as a seed contaminant.

R. acetosella is common in grasslands, pastures, rangelands, waste areas, forest clear cuts and along roadsides (Fowler, 1981; Leege et al., 1981; Alex, 1992; Anon., 2002), where it successfully competes under poor soil conditions, but its impact is reduced on good soils (Fitzsimmons and Burrill, 1993) and in cultivated fields (Alex, 1992). Stopps et al. (2011) list a number of plant communities that play host to R. acetosella in Canada including Douglas-fir forests, cedar-hemlock forests, Garry oak ecosystems, white spruce communities, jack pine communities, hardwood forests, cereal crops and lowbush blueberry fields. R. acetosella colonizes acidic tussock grasslands in New Zealand (Harris, 1970b). In the Pacific Islands, R. acetosella is primarily seen as an invasion risk at high elevations (PIER, 2013).

R. acetosella has been listed among the world’s worst weeds, infesting 45 different crops in 70 countries (Holm et al., 1997). It is a serious pest of lowbush blueberry (Vaccinium angustifolium) in Eastern Canada  (McCully et al., 1991; Stopps et al., 2011). R. acetosella impacts blueberry yield via reduced floral bud numbers that result in considerably lower yields (Kennedy et al., 2010).

Genetics

R. acetosellacan be diploid, tetraploid, hexaploid or octoploid with chromosome numbers of 2n=14, 28, 42, 56 (Löve, 1944; Mulligan, 1959; Harris, 1969; Gleason and Cronquist, 1991). R. acetosella populations in North America are hexaploid with 2n=42 (Farris and Schaal, 1983), while European and Asian populations are primarily hexaploid, with some being diploid (2n=14), tetraploid (2n=28), or octoploid (2n=56) (Löve, 1944; Mulligan, 1959; Harris,1969).

R. acetosella is highly variable, encompassing a large number of genotypes with relatively specific ecological tolerances (Korpelainen, 1992a) likely determined by geographic origin (Korpelainen, 1993). High levels of phenotypic plasticity have also been observed in R. acetosella (Farris and Schaal, 1983; Houssard and Escarré, 1995), which may decline in long-term stable populations (Escarré et al., 1985).

Reproductive Biology

R. acetosella is primarily dioecious (Proctor et al., 1996) with andromonoecious intersexes occurring occasionally (Singh and Smith, 1971). Sex determination in R. acetosella and its subspecies is characterized by an XY sex mechanism that utilizes a strong male determinant in the Y-chromosome (Löve, 1983). Löve (1983) reported that the strong X-suppressant features of the Y-chromosome allow these polyploids to remain dioecious up to at least the dodecaploid level in experimental material. Sexual dimorphism in R. acetosella with respect to various morphological features is likely due to the higher reproductive effort in females (Fujitaka and Sakai, 2007).

R. acetosella is wind pollinated (Houssard and Escarré, 1991). Friedman and Barrett (2009) reported that pollen limitation was rare. Seeds are usually produced by allogamy but Löve (1944) reported that pseudogamy may occur in some hexaploids, and agamospermy may occur in some diploids. 

Female plants produce one three-sided seed (achene) per flower with seed production ranging from 124 to 247 seeds per flowering shoot in lowbush blueberry fields in Nova Scotia, Canada (Kennedy, 2009). Stevens (1932) observed 250 seeds per stem in Pennsylvania, USA; and as many as 1622 seeds per ramet were observed in France (Escarré and Thompson, 1991). Seed weight per 1000 seeds varies from 0.450 g to 0.525 g depending on location (Stevens, 1932). Escarré and Thompson (1991) observed a decrease in mean weight of filled seeds as ecological succession progressed at a site in France, involving genetically-based differences in resource allocation among different populations of R. acetosella.

Pakeman and Marshall (1997) reported that seeds of R. acetosella persisted in the seed bank of British heathlands; Granström (1987) observed viable seed buried for at least 5 years under coniferous forests in Sweden. Livingston and Allessio (1968) observed germination by R. acetosella seeds taken from soil of 80-year-old stands of eastern white pine (Pinus strobus) even though R. acetosella had never been observed in the ground cover.

Optimal germination of R. acetosella occurred between 17.5 and 30ºC in Belgium (Assche et al., 2002). Seed germination appears to be inhibited by plant canopies (Harris, 1972). Putwain and Harper (1970) reported increased seedling populations upon removal of associated grass and broadleaf vegetation. Seedling emergence tends to occur throughout the spring, summer, and autumn in lowbush blueberry fields in Nova Scotia, with high seedling mortality (SN White, Dalhousie University, Halifax, Canada, unpublished results).

In investigations into the effects of fire, exposure to temperatures over 70ºC greatly reduced seed viability, with pre-wetted seeds showing greater sensitivity (Granström and Shimmel, 1993). Immersion in concentrated sulfuric acid for 2-10 min stimulates germination (Andersen, 1968).

R. acetosella reproduces vegetatively via adventitious root buds arising from creeping roots (Kiltz, 1930; Escarré et al., 1994; Klimeš et al., 1997). R. acetosella is classified as “root sprouting” and not stoloniferous or rhizomatous because the buds arise directly from horizontal roots (Klimeš and Klimešová, 1999; Klimešová and Martínková, 2004). These buds develop into aerial shoots, i.e., true stems with scale-like leaves below the surface of the soil (Kiltz, 1930). Colonization by seed produces populations of individuals that may persist via vegetative propagation for at least 15-20 years (Escarré et al., 1994).

Physiology and Phenology

Production of root buds from horizontal creeping roots in autumn allows R. acetosella to overwinter under harsh conditions (Hoeg and Burgess, 2000). The base temperature for ramet emergence from creeping roots in Nova Scotia, Canada is approximately 5ºC, and optimal temperatures for ramet emergence range from 20 to 25ºC (SN White, unpublished results). Ramets fail to emerge at 40ºC or higher (SN White, unpublished results). In spring, root buds sprout to produce new basal rosette growth. In areas with mild winters such as southern British Columbia, Canada, the species is known to grow continuously through the winter (Anon., 2009).

Leaf size and shape remain constant under drought stress (Farris, 1983). Under dry conditions (c. 10% soil moisture), the photosynthetic rate declined to 10 mg CO₂/g dry weight and leaf conductance declined to 0.05 cm/s; optimal rates at 40% soil moisture were 30 mg CO₂/g dry weight and 0.15 cm/s, respectively (Zimmerman and Lechowicz, 1982). Supraoptimal soil moisture levels (e.g., 60%) decrease photosynthetic rate and conductance (Zimmerman and Lechowicz, 1982). Houssard et al. (1992) reported a differential response to moisture stress in male and female plants collected from 2- and 15-year-old successional sites. Males from both sites had reduced transpiration and water consumption rates under drought conditions. Females from the 2-year-old site responded similarly to water stress, but females from the 15-year-old site maintained high rates of transpiration and water consumption.

In greenhouse studies, Harris (1972) found photosynthates were transported among ramets of R. acetosella but not mineral nutrients. Similarly, Klimeš and Klimešová (1999) observed that ramets grown in a nutrient-rich soil failed to transport nutrients to connected ramets grown in nutrient deficient soil. Leaf tissues of R. acetosella contain large amounts of oxalic acid, but produce exudates with only moderate levels of oxalates (Tyler and Ström, 1995).

In terms of phenology, germination occurs through a broad span of time, from February to October in British Columbia, Canada (Anon., 2009). In the same region, bolting, flowering, and seed production occur from May to September (Anon., 2009) and from May to July in Ontario (OMAFRA, 2009). Very few ramets of R. acetosella flower in the first year in cooler regions such as eastern Canada; flowering in these populations appears to be induced by vernalization (SN White, unpublished results). Plants produced from seed either bolt and produce flowers or remain vegetative in the first year (Harris, 1970a). Bolting and flowering of first-year as well as mature plants is thought to be induced by long photoperiods (Carlson, 1965; Harris, 1970a). R. acetosella ramets flower in early June in lowbush blueberry fields in Nova Scotia (Kennedy, 2009). Esser (1995) provided a comprehensive listing of the flowering times of R. acetosella in various US states.

Nutrition

Increased herbage yield of R. acetosella was obtained when nitrogen and potassium were applied separately, but not together (Harris, 1971) but other tests have recorded increased density (Kennedy et al., 2003) or increased panicles per plant or biomass (Fan and Harris, 1996) with higher NPK fertilizer concentrations (Kennedy et al., 2010). The species appears to exhibit a high demand for phosphorous (Fransson et al., 2003). With fertilizer application, Kennedy et al. (2011) observed a shift from away from allocation to vegetative growth towards more allocation to reproduction.

Population Size and Structure

R. acetosella tends to colonize disturbed sites as an early successional species, quickly colonizing disturbed sites and waste places. As succession progresses R. acetosella declines rapidly in abundance, particularly on good-quality soils (Fitzsimmons and Burrill, 1993). As populations age R. acetosella allocates relatively more resources to vegetative propagation; younger populations that invest a greater proportion of resources to aerial biomass production and flowering (Escarré et al., 1994). 

In competitive situations, such as grass swards, R. acetosella populations may be maintained primarily by vegetative reproduction (Putwain et al., 1968). Putwain et al. (1968) observed that the number of vegetative shoots increased in the spring until flowering, and then declined through the remainder of the season. Removal of associated species within the sward resulted in a rapid increase of the mature R. acetosella population through vegetative reproduction (Putwain and Harper, 1970). In contrast, R. acetosella populations were not found to increase significantly following grass removal in lowbush blueberry fields in Nova Scotia (White, 2007). Ramet populations in lowbush blueberry seem to be regulated by a constant cycle of ramet emergence and mortality, but mortality of ramets tends to occur primarily in lateautumn (SN White, unpublished results).

Female plants tend to produce more vegetative shoots than male plants, but mortality of female shoots is generally higher (Lovett Doust and Lovett Doust, 1987). This higher mortality can be compensated for somewhat by renewed vegetative reproduction after flowering (Lovett Doust and Lovett Doust, 1987).

Female plants often suffer from reduced clonal growth following flowering, which can also result in male-biased sex ratios (Houssard et al., 1994). Korpelainen (1991) observed high variability in sex ratios of spatially segregated populations of R. acetosella, with some populations composed entirely of male or female plants. A male-biased sex ratio may be promoted under drought conditions because of greater tolerance of males to water stress (Houssard et al., 1992). Males also devote more resources to vegetative reproduction than females which by contrast allocate more energy into sexual reproduction growth in height (Fujitaka and Sakai, 2007). Males senesce earlier in the season than females (Putwain and Harper, 1972; Lovett Doust and Lovett Doust, 1987; Korpelainen, 1992b).

Associations

R. acetosella is reported to be a non-mycorrhizal species (Wang and Qiu, 2006) in North America (Medve, 1984; Dhillion and Friese, 1994), South America (Fontenia et al., 1998), and Europe (Harley and Harley, 1987; Pawlowska et al., 1996). Eriksen et al.(2002) observed the presence of internal fungal hyphae without arbuscules or vesicles in R. acetosella roots from Norway.

Environmental Requirements

R. acetosella is a cosmopolitan species well adapted to a broad range of climate conditions including temperate, subtropical and polar regions. Altitude ranges from  sea level to 1800 m in Sri Lanka (Harris, 1969), with extensive stands occurring as high as 1100 m in New Zealand (Moore, 1953). In interior regions of Canada, R. acetosella is capable of surviving both the harsh cold winters and the relatively hot dry summers, although it tends to be more abundant in temperate coastal regions.  

R. acetosella grows on a variety of soil types, thriving on silty loam (Zimmerman and Neuenschwander, 1984), sandy loam (Biswell, 1956; Wilson and Tilman, 1991), heavy clay soils (Moore, 1953) or gravelly soils (DeFerrari and Naiman, 1994) including acidic soils (Harris, 1969; Esser, 1995) but rarely on calcareous soils (IPANE, 2009) and is considered a calcifuge (Tyler and Ström, 1995). In acidic soils in Lithuania, 300-500 R. acetosella seeds per square metre were observed, whereas no seeds were found in limed soil (Ciuberkis et al., 2006). R. acetosella is more often associated with light soil texture and low soil fertility than low pH (Archer and Auld, 1982). R. acetosella grows on serpentine soils and on mine tailings and can persist with high nickel concentrations (Bagatto and Shorthouse, 1999; Wenzel et al., 2003).

Sheep, cattle and deer (Esser, 1995) graze leaves and shoots, but R. acetosella plants can recover fully, with the number of shoots potentially increasing following grazing (Leege et al., 1981). The plant is grazed by various grouse species in North America including the sharp-tailed grouse (Tympanuchus phasianellus campestris) the greater prairie chicken or pinnated grouse (Tympanuchus cupido cupido) and the ruffed grouse (Bonasa umbellus) (Johnson, 1928; Schmidt, 1936; Brown, 1946; Treichler et al., 1946; Stafford and Dimmick, 1979). The red grouse (Lagopus lagopus scoticus) feeds on R. acetosella seeds in the UK (Schmidt, 1936).

R. acetosella is host to over 30 lepidopteran species (Robinson et al., 2007) including the American copper (Lycaena phlaeas), one of the world’s most widespread temperate zone butterflies (León-Cortés et al., 2000). It is also host to at least 16 species of aphid, including the black bean aphid (Aphis fabae) and the potato aphid (Macrosiphum euphorbiae; Holman, 2009).

Nematode species associated with R. acetosella include Xiphinema americanum and X. rivesi, in apple and peach orchards of Indiana, New York and Pennsylvania (Powell et al., 1984), and Meloidogyne arenaria and M. incognita, in tobacco crops of South Carolina (Tedford and Fortnum, 1988).

R. acetosella hosts more than 40 fungal species (Farr et al., 2010). Many of these such as Cercospora spp., the cause of leaf spot (Farr et al., 2010), are pathogenic to agricultural crops. Tomato spotted wilt virus (TSWV) has been detected on R. acetosella plants collected from commercial farms in southwestern British Columbia (Bitterlich and MacDonald, 1993). The Tomato ring spot virus (TmRSV), transmitted by nematodes, is associated with R. acetosella in apple and peach orchards of Indiana, New York, and Pennsylvania (Powell et al., 1984). Hughes (2012) found that the incidence of botrytis blight, a major disease in wild blueberry (caused by Botrytis cinerea) was increased in the presence of R. acetosella.

General

• Research model

Human food and beverage

• Leaves (for beverage)
• salad

Materials

• Chemicals

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

Rumex acetosella may commonly be mistaken for R. acetosa L. (common or garden sorrel). R. acetosella and R. acetosa may have been used interchangeably or together in herbal medicines and foods (Pieroni, 2000), as both contain oxalic acid, conferring a sour taste and poisonous properties if consumed in large quantities (Cooper and Johnson, 1984). Rumex acetosa is distinguished by its larger size (up to 90 cm in height with leaves up to 10 cm in length) (Cooper and Johnston, 1984; Frankton and Mulligan, 1987), the presence of a distinct joint at the midpoint of the pedicel (Frankton and Mulligan, 1987), and the presence of valves that have expanded into broad reticulate wings surrounding the achenes (Gleason and Cronquist, 1991). 

R. acetosella may also be mistaken for R. rugosus Campd. (syn: R. ambiguous Gren.) and R. thyrsiflorus Fingerh. (Frankton and Mulligan, 1987). Both R. rugosus and R. thyrsiflorus are similar to R. acetosa and can likewise be distinguished from R. acetosella on the basis of their large size and the presence of a distinct joint near the middle of the flower stalk (Frankton and Mulligan, 1987).

Another similar species is Rumex hastatulus, which can be distinguished by the presence of broad expanded valves that form reticulate wings around the achenes (Gleason and Cronquist,1991). Leaves of R. hastatulus are often arrow-shaped, but they also appear entire (Gleason and Cronquist,1991), unlike those of R. acetosella. Rumex acetosa and R. hastatulus also differ from R. acetosella in chromosome number, with 2n=14 (females) or 15 (males) in R. acetosa, and 2n=26 (female) or 27 (male) in R. hastatulus (Gleason and Cronquist, 1991).

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.

Control

Cultural control and sanitary measures

As R. acetosella prefers poor soils with low pH, liming to raise soil pH may inhibit its growth relative to other species (Clark and Fletcher, 1923; Juska, 1960; Fitzsimmons and Burrill, 1993; Anon., 2006). The application of lime and 2,4-D improved control of R. acetosella from 40.9% (2,4-D only) to 73.1% (2,4-D + lime) (Juska, 1960). Combining lime with nitrogen fertilizer is also recommended to shift competition in favou rof other species over R. acetosella (Clark and Fletcher, 1923; Fitzsimmons and Burrill, 1993). 

Prescribed burning has been suggested for R. acetosella control, but it is unlikely to provide satisfactory control unless fires are extremely severe (Esser, 1995). Following burning of wild blueberry fields, a large increase of R. acetosella was observed (Penney et al., 2008), though subsequent burnings did reduce the occurrence of R. acetosella, likely due to destruction of seed in the seed bank. 

Physical/mechanical control

Attempts to eradicate R. acetosella through cultivation may be possible but creeping rootstalks and long-lived seeds may hinder such efforts. A 3- to 4-year rotation of crops with clean cultivation, followed by a grain and/or a cover crop, and finally a return to pasture or perennial crop is effective at reducing R. acetosella infestations (Clark and Fletcher, 1923; Fitzsimmons and Burrill ,1993). Infested areas should be cultivated at regular intervals allowing for some regrowth before re-cultivation to exhaust food reserves in root fragments (Fitzsimmons and Burrill, 1993). 

Movement control

As R. acetosella can be a contaminant in forage seed (Mudie and Byrne, 1980; McAndrews, 1988; Fitzsimmons and Burrill, 1993), it is imperative that measures are in place to prevent long-distance dispersal via that pathway. Boyd and White (2009) recommended reducing the movement of R. acetosella seeds in blueberry crops through harvesting by avoiding high density patches or adjusting harvest times, as well as cleaning of harvest equipment and other agricultural implements.

Biological control

Biological control has not been attempted for R. acetosella; neither have potential biological control agents been clearly identified (Stopps et al., 2011). 

Chemical control

R. acetosella is seldom completely controlled by 2,4-D (Juska, 1960; Harper, 1977; Burrill et al., 1990; Smith, 1995), but mixtures of 2,4-D with dicamba, dichlorprop, triclopyr or glyphosate can provide a high level of control (Lorenzi and Jeffery, 1987; Smith, 1995). Dicamba alone was shown to provide 85-100% control (Burrill et al., 1990; Smith, 1995). Other effective chemicals include paraquat (Burrill et al., 1990) or picloram (Harper, 1977). Hexazinone has been used in wild blueberry fields in eastern North America for more than 30 years (Li, 2013) and hexazinone-tolerant R. acetosella may be present in some areas (McCully et al., 2005). Li (2013) showed that hexazinone in combination with either rimsulfuron or nicosulfuron provided a good alternative to hexazinone alone. Other suggested candidates for chemical control of R. acetosella include mesotrione, sulfentrazone and atrazine (Hoeg and Burgess, 2000; Graham and Melanson, 2007).

Much is known about the biology of R. acetosella, although some notable gaps still exist. For example, more could be done to understand the apparent lack of mycorrhizal associations. Much of its basic phenology or physiology in certain environments, such as blueberry fields is unknown (Kennedy et al., 2011). A lengthy list of herbivores and pathogens found on R. acetosella is available but the actual impacts of most of these species on growth and reproduction are largely unknown. A better understanding of its life history, especially with respect to seed banks could be helpful in developing management strategies. In areas where it is a serious pest, such as in blueberry crops in eastern North America (McCully et al., 1991; Boyd and White, 2009; Stopps et al., 2011), there is a need for extensive research into management. Considerable research is in fact being pursued in this region (SN White, personal communication), which may prove useful for management of R. acetosella in other world regions.

11/10/13 Original text by:

David R Clements, Trinity Western University, 7600 Glover Road, Langley, British Columbia V2Y 1Y1, Canada