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Rumex crispus L., Curled Dock

Account Summary

Native, common, widespread and locally abundant. Eurosiberian southern-temperate, but very widely introduced worldwide and now circumpolar.

1881; Stewart, S.A.; Co Fermanagh.

Throughout the year.

Growth form and preferred habitats

R. crispus is an extremely variable short-lived perennial, or rarely an annual, which flowers in its first year after germination. Perennial forms develop a thick, fleshy, yellow taproot that can reach to a soil depth of 150 cm. Autumn germinated seedlings overwinter as small rosettes which grow rapidly in the spring to produce tall flowering stems up to 2 m in height, but often reach only half this height. In comparison with R. obtusifolius (Broad-leaved Dock), the other most competitive Dock species, the basal leaves are narrower, more parallel-sided and their margins are more wavy (undulate), crenate (with small, blunt, rounded teeth) and crisped (tightly undulate). The leaf base is wedge-shaped or only slightly indented at the stalk, which is long (sometimes almost as long as the blade). Another distinguishing feature observed in the ripe fruit is that the three inner perianth segments (tepals), which become enlarged to form valves surrounding the fruit (a solitary nutlet in each flower), have margins almost entire, and each may or may not bear one smooth, oblong tubercle (Lousley & Kent 1981).

Like the other very common weedy dock, Broad-leaved Dock (R. obtusifolius), R. crispus is a very troublesome plant for farmers, being a particularly serious weed in arable fields, a habitat nowadays almost completely absent in Fermanagh (Foster 1989). Unfortunately, Curled Dock is also all too abundant in pastures. It is believed to be one of the twelve most widespread and successful non-cultivated, colonising species in the world (Allard 1965; Holm et al. 1977). Typical Fermanagh habitats are damp to wet lowland lakeshore pastures, marshy grasslands in general, plus garden lawns and beds, in parkland grassed areas, roadsides, old quarries and waste ground. In other counties with coastal habitats, subspecies of it are also common on shoreline shingle, dune and estuarine salt marsh habitats.

Variation

Five subspecies and a "numerous" range of minor forms, all coming true from seed, are recognised in Britain & Ireland (Sell & Murrell 2018). Probably the only native forms of R. crispus occur on coastal shingle, dunes, salt-marshes and muddy river estuaries (Sell & Murrell 2018). The other more inland forms commonly found in cultivated, rough and waste ground are regarded as weeds introduced at an early date by man and his domestic animals, namely, subsp. crispus, subsp. robustus (Rech.) Pestova and subsp. strictissimus (Rech.) Pestova. The most likely native form is subsp. littoralis (Hardy) Akeroyd, followed subsp. uliginosus (Le Gall) Akeroyd, which appears to be restricted to a number of muddy estuaries in the S of England and Wales, and is probably also mostly native (Sell & Murrell 2018).

Fermanagh occurrence

While Curled Dock is generally regarded as the commonest native Dock in Britain & Ireland as a whole (Lousley & Kent 1981), it is less common than R. obtusifolius in Fermanagh. Nevertheless, it is common and widespread, being recorded in 315 of the tetrads, 59.7% of those in the VC. It is particularly widely scattered across lowland areas of the county. In comparison, R. obtusifolius is represented in the Fermanagh Flora Database by records in 456 tetrads, 86.4% of the total area.

Reproduction

R. crispus reproduces mainly by seed (nutlets). The inflorescence is a lax to dense panicle 30-60 cm long, ± simple, or with branches held upright at an angle of no more than 30° to the main stem. Flowers are borne in whorls of 10-30. The species has a very long flowering period extending from June to October. One plant can produce from 100 up to 60,000 seeds per year. Buried seed longevity is impressive, some remaining viable for up to 80 years. The plant can also regenerate vegetatively, but only the uppermost 4 cm of rootstock is involved. The deep taproot enables the species to survive both very dry seasons and severe freezing. Vegetative regeneration is most successful when it occurs in the early part of the growing season (Hudson 1955). Regeneration from root fragments, together with prolonged seed survival, makes eradication of the plant infested ground almost, or practically, impossible (Holm et al. 1977).

Seed dispersal

Seed is lightweight, has an attached wing and can readily be dispersed by wind. Seed also floats and can be transmitted in flowing water or transported in mud attached to feet and coats of animals, including man. It may also be transported in mud on machinery. Viable seed can also pass through the digestive tracts of birds and cattle. Despite these dispersal possibilities, in pastures and in open, neglected ground R. crispus often forms patches. This reflects the fact that in very many species, including Docks, the majority of seeds produced fall near the parent plant and germinate there (Holm et al. 1977). R. crispus can establish quickly from seed, colonising pasture openings caused by trampling, gouging, fire or dung patches.

Germination

Seeds can only germinate on open ground as the presence of a leaf canopy reduces red light wavelengths (in absolute terms), and increases far-red radiation (relatively), which 'switches off' germination (Foster 1989). Temperature fluctuations reaching 15°C at the upper limit, promotes germination of buried seed, which helps explain the observed flush of spring germination (Roberts & Totterdell 1981). Until a tap root develops, seedlings are vulnerable to competition from other more established plants, and to disturbance, such as further cultivation.

Polymorphic variation exists in seed germination between plants in different habitats, and even within different parts of the same plant (Cavers & Harper 1966). Differences in germination between seeds of separate plants was greater than between habitats. Interplant differences were greater than differences between the germination of seeds borne on different parts of the same plant, plants of different age, or different degrees of ripeness (Cavers & Harper 1966). Once well established, plants of both R. crispus and R. obtusifolius can easily withstand even quite severe trampling, although flowering may be inhibited. In growth experiments in pots, R. crispus had much higher rates of germination and growth rate in freely drained and partly waterlogged conditions than under fully waterlogged conditions (Cavers & Harper 1964). The species appears to be unaffected by severe drought or frost, and while occasionally it can survive periods of very waterlogged conditions, it is not very successful there (Cavers & Harper 1964).

Weed ecology and control measures

Curled Dock is a primary coloniser of disturbed ground and is regarded as the most 'weedy' species in the genus Rumex due to its ability to establish quickly from seed, flower in the first year of growth, and because its dormant seed can persist for many years buried in soil until another favourable environment arises (Cavers & Harper 1964). Weed infestation of pasture tends to be encouraged by: 1. spreading of slurry and farmyard manure, 2. cutting for silage, 3. high nitrogen levels, 4. soil disturbances, 5. previous damage to the sward (eg direct drilling), and 6. selective grazing by horses (Haggar 1980). The same author also found that Docks are discouraged by flooding, sheep grazing and cutting for hay, but poaching of soil and grazing by cattle have apparently no effect on the abundance of Docks.

Mowing of grassland containing Docks is often advantageous to the establishment of both R. crispus and R. obtusifolius, since after mowing new shoots are quickly sent up from ground level and these two species can very quickly flower and set seed before winter. Both these Dock species produce large numbers of persistent seed and they also show considerable powers of regrowth from vegetative fragments left in the soil (Foster 1989). Apparent elimination of established Dock plants only gives short-term control due to their very large and long persistent seed bank.

Practical methods for the non-chemical control of Docks include pulling by hand (ie 'docking' them) in growing crops or on waste ground. Pulled Docks should be burned in case they should still go to seed. Hoeing of ploughed or dug ground, especially in the autumn, is effective in removing seedlings. French et al. (1986) worked on natural flavour-related chemicals which might be used to stimulate the germination of R. crispus seed at a time unfavourable to successful growth of the weed. They also suggested the premature stimulation of seed germination, before shedding from the mother plant, could be developed as a control method against the species.

European and world occurrence

R. crispus is a very obvious follower of man (ie an apophyte), having spread with agriculture and become a serious weed in every continent and across a wide range of crops. It is considered one of the five most widely distributed plant species in the world, its spread completely masking its area of origin (Hughes 1938). On the other hand, Hultén & Fries (1986, Map 667) consider it originated in Eurasia. R. crispus occurs throughout Europe up to a latitude of 69°N in both Norway and Russia. It is present in Iceland and the Faeroes, but interestingly not in Greenland (Cavers & Harper 1964). Despite existing discontinuities, mainly in C Asia, Hultén & Fries (1986) regard it as circumpolar, and it is widely introduced in the southern hemisphere, including remote areas such as the Falkland Isles, Réunion and Tasmania.

Toxicity

R. crispus plants have a pleasantly sharp taste and are apparently palatable to farm animals, which often eat them without showing any obvious ill effects (Cooper & Johnson 1998). However, Curled Dock is also known to have caused fatal poisoning in both sheep (Panciera et al. 1990) and humans (Reig et al. 1990). The active toxic substances include oxalic acid, oxalates and anthraquinone glycosides. Leaf and 'tops' samples of R. crispus measured by Panciera et al. (1990) contained from 6.6 to 11.1% oxalic acid calculated on a dry-weight basis.

The quantity of oxalate required to induce acute poisoning in ruminants is found to be quite variable (0.1-0.5% of body weight), and the active dose depends on several factors. Ruminal gut microflora can readily detoxify a proportion of ingested oxalate, so that mature animals that slowly ingest potentially toxic concentrations do not become affected. In addition, because the population of oxalate-degrading microflora in ruminants increases with gradual exposure to higher concentrations of oxalate for a few days, such animals are able to consume considerably greater quantities of toxic plant material than are animals that rapidly consume a single comparable dose (James & Butcher 1972).

Most reported cases of Dock poisoning in Britain and in the USA have involved sheep. It has also been noted that well-fed sheep can tolerate considerably greater doses of ingested oxalate than can animals suffering from restricted feed and water intake (Cooper & Johnson 1998).

Uses

R. crispus has a long history of use in herbal medicine. It produces an acid called rumicin, and the root contains chrysarobin (chrysophanolanthranol) and other substances of similar composition. The large leaves of both common 'Dock' or 'Dockens' species, ie R. crispus or R. obtusifolius, are very generally reached for in many communities when anyone suffers a nettle sting, since the leaf or its sap, provides a welcome cooling effect when rubbed on the affected skin (see the Urtica dioica (Stinging Nettle) species account on this website). Root extracts of R. crispus have been used as laxatives, astringents in piles, rheumatic tonics and for blood diseases, chronic skin diseases and jaundice. A homeopathic tincture made from unopened inflorescences has also been used to treat sore throats and coughs (Grieve 1931).

Names

The genus name 'Rumex' is an old Latin name for Sorrel from Pliny derived from the Latin 'rumo' to suck, from the Roman habit of sucking Sorrel leaves to allay thirst (Johnson & Smith 1946). The Latin specific epithet 'crispus' means 'curly', a reference to the undulate, crisped leaf margin of the plant (Gilbert-Carter 1964). The English common name 'Curled Dock' is a typical book name based on the scientific name and the general appearance of the leaf. As such, it has no folklore significance, and Britten & Holland (1886) do not list any other English names specifically attached to R. crispus. However, the commonly used names 'Dock' and 'Docken' apply equally to this species and to its very common relative, R. obtusifolius (Grigson 1987; Allen & Hatfield 2004), their most frequent use being as a coolant for nettle stings, burns and scalds.

Threats

None.