curly dock, dockum
Rumex crispus L.
Perennial Broad-leaved Weeds
The curled dock occurs in arable and meadowland, waste places, on sand dunes, and shingle. It is the more common dock in arable land especially on drier soils. Curled dock is universally distributed in Britain but is more a plant of clay, chalk or gravel than light sandy soils. Docks are common on soils deficient in potassium, and on soils rich in nitrogen. However, research results are contradictory on whether there is a link between dock populations and the level of soil potassium. Fewer docks may occur on fields subject to flooding, cutting for hay or grazing by sheep. However, studies have shown that curled dock is resistant to flooding and can survive 8 weeks of submergence.
Curled dock is capable of behaving as an annual, biennial or perennial but plants only persist for several years when regularly cut down and prevented from setting seed. Variants have developed that are adapted to specific conditions. Four varieties are recognised. Curled dock is able to survive in a changing environment due plasticity and genetic heterogeneity. Hybrids between curled dock and broad-leaved dock (R. obtusifolius) are common. They exhibit a range of intermediate characters and may be more vigorous than the parents. The primary hybrids are thought to produce little viable seed although the infertile panicles may still develop and turn red in autumn. Hybrids that result from backcrosses with a parent are more fertile.
There are some who would argue that docks in grassland are not weeds because they contribute to the herbage and hence do not need to be controlled. They may also contribute trace elements to a grazing animals diet. The leaves of curled dock, for example contain unusually high amounts of zinc. However, it also has a relatively high content of oxalic acid that may affect dietary calcium bioavailability.
Inflorescences are initiated in April or early May. Curled dock generally flowers earlier than the broad-leaved dock which flowers from June to October. Flowering is delayed by cutting. The flowers are wind pollinated but are also visited by bees. Between 25 and 100% of plants are self-fertile and both outbreeding and inbreeding may occur.
Plants may flower and set seed in the first year. Curled docks that flower in their seedling year do so from July onwards. Some plants die after flowering while others overwinter as rosettes. The upper part of the flower panicle may be in bud while the lower is forming fruit. Once seeds have begun to form, around 14 days after flowering, they will ripen even if the plant is cut down. Seed numbers per plant range from 100 to 40,000. Dock plants can shed seed from late summer through to winter but the seeds may require a short after-ripening period before being ready to germinate.
There is considerable variation in germination characteristics between seeds from different populations, different plants, different panicles on the same plant and seed from different positions on the same panicle. Some of this is due to seed size and seedcoat thickness, some to the time of ripening and some is due to maternal factors. Defoliation can also affect the development and germination characteristics of curled dock seeds.
Seeds near the soil surface are subjected to periods of wetting and drying, alternating temperatures and light that prime them to germinate rapidly after periods of heavy rainfall. Seeds buried deeper in the soil are not subjected to this and may remain dormant. Seeds germinate mainly from March to September but odd seedlings can emerge at anytime. Dock seedlings have a low competitive ability and find it difficult to become established in closed vegetation. Seedlings can emerge in dense patches but mortality is high and fewer than 10% may survive.
The underground parts of a dock consist of a vertical stem and a branched taproot with a transition zone between them. The underground stem may reach 5 cm in length and is kept below ground by root contraction. Regrowth from overwintered rosettes begins in early spring as temperatures rise. New lateral shoots develop from axillary buds in the crown.
Persistence and Spread
Curled dock reproduces from seed and by vegetative regeneration of the underground organs. The seeds are capable of surviving in undisturbed soil for more than 70 years and seed numbers in soil have been estimated at 5 million per acre. The seeds contain a chemical that inhibits microbial decay.
The seeds are shed continuously from late summer through to winter. There is no obvious natural seed dispersal mechanism but some seeds are light enough to be blown by the wind, as well as distribution by clothing and animal fur. Curled dock seed can be carried on farm machinery and in the straw as well as in the harvested grain. Seed is shed and spread during crop harvests. Seed that had been combine-harvested germinated more readily than hand-harvested seed. This was probably due to scarification of the seed coat during mechanical harvesting.
The main method of long distance dispersal is as a contaminant in crop seed, animal feed, straw and manure. The seeds can pass through cattle unharmed and will survive for several weeks in manure. The seeds can also survive long periods of immersion in slurry that is not aerated. Seed viability is reduced in silage particularly when additives are used to aid fermentation. The normal treatment temperatures may not kill dock seeds that occur in sewage sludge. Dock seeds are destroyed when fed to chickens but dropped seeds may contaminate the poultry manure. Dock seedlings have been raised from the droppings of birds and viable seeds have been found in worm cast soil. Curled dock seeds can float for up to 2 days and have been recovered from irrigation water.
There is considerable confusion about the ability of docks to regenerate from their underground organs. Some authors maintain that true roots do not regenerate and only the stem and transition zone can regenerate. Others insist that all parts will form new shoots if detached from the parent. Curled dock does not regenerate vegetatively as extensively as broad-leaved dock.
Once out of the seedling stage, docks are resistant to grazing, mowing and competition from grass. Infestations may be contained but not eliminated. It is prudent to avoid sward damage from trampling and poaching that will allow seedlings to become established. In grassland, frequent cutting encourages regeneration of the taproots and branching of the shoots, increasing the potential for future growth. It was reported that when the sward was cut frequently (5-7 cuts per year) the presence of docks had little effect on yield. When the sward was cut less frequently (3-4 cuts per year) total yields were reduced and the herbage contained a high proportion of dock foliage.
Docks are grazed off by cattle, sheep, goats and deer but not by horses. It has been suggested that sheep are used to graze off seedling docks in the autumn and mature docks in March.
Updated November 2007