Common couch

Common couch
Other names: 

couch, couchgrass, quackgrass, quicks, scutch, squitch, twitch, wickens, wicks

Latin names: 

Elytrigia repens (L.) Desv. ex Nevski. (Agropyron repens , Elymus repens, Triticum repens)

Occurrence: 

Common couch is a native perennial grass found throughout the British Isles. It grows on most soil types except those with a low pH. It prefers heavier land but is able to spread more readily in lighter soils. The UK distribution of common couch closely follows that of cultivated land. On arable land it is concentrated in the hedge bottoms and field margins from where it can spread out into the field. It is a frequent weed of cereals and other arable and horticultural crops. Couch growth is especially vigorous on fallow land and in the first year after tillage ceases in abandoned fields. However, it is sensitive to shading and gradually dies out as scrub takes over. Couch is less frequent in old grassland and permanent pasture.

Common couch can form dense stands that exclude other vegetation. If left undisturbed a mat of young rhizomes forms in the upper 10 cm of soil. Common couch thrives in cooler climates. At higher temperatures rhizome production is reduced and the plant is less invasive. The aerial shoots are not harmed by freezing but the rhizomes are said to suffer damage if exposed to frost.

There is considerable variation in the growth and morphology of plants from different clones. Seedlings from seeds collected in different areas continue to exhibit differences when grown under uniform conditions. Varietal names have been given to several recognised forms.

Common couch may be infected with fungal cereal diseases including ergot (Claviceps purpurea). Like many other grasses it is an alternative host for the frit and gout flies.

Biology: 

Common couch flowers from May to September. Couch is self-sterile and a large patch may consist of a single clone but as the flowers are wind pollinated, fertilisation need not be a problem. The seed heads mature during August and September, at around the time of cereal harvest. Some seeds become viable 10-18 days after flowering despite appearing green and immature but the level of germination increases with time. There are usually 25-40 seeds per stem but on average only 13 are viable.

Shed seed may germinate at any time that conditions are favourable. Light is not important but alternating temperatures promote germination. Seeds do not germinate at a constant temperature. In the UK, germination occurs mainly during the autumn but seedlings also emerge in spring especially when autumn germination is delayed by cold temperatures, inadequate moisture or deep burial. Seeds readily emerge from 0-5 cm deep but few from 10 cm and none from 12.5 cm or deeper.

Seedlings begin to develop rhizomes at the 4- to 6-leaf stage, around the time of first tillering. In most situations, vegetative reproduction is more important than seed. The aerial shoots of the parent plant die down in the autumn and new primary shoots start to develop. These grow slowly until temperatures rise in spring when active growth begins. New leaves are produced and dormant buds at the base of each shoot grow out to form upright tillers or horizontal rhizomes. The rhizomes themselves form numerous lateral rhizomes in July. More form in an open or disturbed habitat than in a closed community. In compacted soil, the rhizomes grow more or less horizontally. Rhizome growth increases with nitrogen level. Rhizomes grow horizontally in summer before turning erect in autumn ready to form a new aerial shoot.

Cultivation disrupts the seasonal growth cycle. If a rhizome is separated from the parent plant, the axillary buds develop into aerial shoots that grow vertically upward. After extensive rhizome fragmentation at least one bud per fragment develops a new shoot. New rhizomes begin to form when the shoot has 3-4 leaves. Renewed tillering and rhizome production will follow soil disturbance at any time except mid-winter but regeneration is greatest between October and April and least in May-June. Regeneration increases with the level of soil fertility. The longer the length of a rhizome fragment the more likely it is to regenerate and the greater the depth it will emerge from. In a given area, more shoots are produced when rhizomes are cut into smaller pieces but these are less vigorous and mortality is greater.

Persistence and Spread: 

Common couch seed is not innately dormant and most will germinate within 12 months of shedding. Seed shallowly incorporated in soil germinates more readily than seed left on the surface. Seed buried more deeply in soil can lie dormant for 2-3 years and may remain viable for about 5 years. However, studies have shown that although it is well represented in the vegetation common couch may be absent from the seedbank.

Nevertheless, seed could be a more important source of new infestations than is realised. Common couch allowed to flower in the field margins may set seed that could be dispersed within the field or beyond. Contamination of cereal, grass and other crop seeds with common couch seed was and remains a common source of spread. The seed retains viability after passing through horses, cows and sheep but not pigs. Common couch seed may also survive in manure. The seed has been recovered from irrigation water.

Field margins infested with common couch also act as a reservoir for repeated vegetative spread into arable fields. The rhizomes extend readily into the cultivated soil and, once there, tillage fragments the rhizomes and scatters the pieces further into the field. The survival and regeneration of rhizome pieces increases with the length of the fragments. Rotting from the cut ends kills shorter fragments faster. Longer fragments can emerge from a greater burial depth. The stem bases of aerial shoots are also able to regenerate after fragmentation. The parent rhizome can survive for 2 or more seasons but active growth gradually ceases unless the rhizome is disturbed or fragmented. In a 3-year old grassland, only 33% of the rhizomes were viable.

Management: 

Although vegetative spread is considered to be the main problem it is important to ensure that common couch is not introduced as a seed in contaminant in crop seed. Any new clones introduced in this way will increase the likelihood for cross-pollination and enhanced seed production in future crops.

Once couch is established repeated cultivations must be practiced to reduce it. The land should be ploughed shallowly and as much weed as possible collected by grubbing and harrowing when the soil is dry. It can be almost completely killed in one season by repeated cultivations that begin in spring. The optimum time for tillage to be repeated is when regrowth has reached the 3- to 4-leaf stage. In a fallow period, progressively deeper spring-tine cultivations aim to bring rhizomes to the soil surface to desiccate. Machinery has been developed with two banks of rigid soil-loosening tines fitted with 30 cm wide wing- or duck-foot shares that tear up the stubble ahead of a pto-driven horizontal rotating shaft fitted with long curved tines. These flick the rhizomes out onto the soil surface where they can be left to desiccate or can be collected up for burning.

Actively growing rhizomes are readily killed when exposed to dry air for a few days at moderate temperatures. However, if covered even with a shallow layer of dry soil the rhizomes may survive. The best time to work the land is when the soil is beginning to dry and falls readily from the rhizomes. In drought conditions the rhizomes are less susceptible to desiccation because growth is restricted and the rhizome buds become dormant. Pigs in a moveable pen will root out and consume the rhizomes. Cattle and horses are also said to relish the rhizomes. Geese will eat common couch and may be selective in certain crops.

Repeated cultivations are not good for a poorly structured soil but a full fallow should not be needed on light land. A bastard or half fallow can precede fodder or vegetable crops in spring or ploughing can be delayed following a forage crop or early cereal harvest. In cropping systems without a fallow period, apart from repeated inter-row cultivations, the main period for couch control is after harvest. In cereals it is critical that rhizome fragmentation begins straight after harvest. The first cultivation with a rotovator working to 15 cm should aim to cut the rhizomes into short lengths. Each fragment will develop a new root and shoot at one node and a further rotovation after 2-3 weeks will kill many of these. Rotovations need to be repeated when survivors have shoots 5-10 cm tall (2-leaf) and before they reach 15 cm tall (3-4-leaf). Two cultivations may be needed on light soil but up to six on heavy land. Ploughing to 30 cm will bury short rhizome fragments down to a depth from which some will not be able to emerge. Cutting the aerial shoots of regenerating rhizome pieces at weekly intervals inhibits further rhizome production but less frequent cutting does not.

Competition from the crop can enhance the control of couch weakened by burial or fragmentation but in general smother crops alone have less effect on couch growth than cultivations. Couch is sensitive to shading, however, and when continually shaded the grass gradually dies out. Seedlings of couch are more sensitive to crop competition than regenerating rhizome fragments. It has been said that if land is laid down to grass, common couch will be eradicated within 3 years. If a suitable mixture of grasses and white clover is sown and efficiently managed for a few years the weed will be gradually suppressed. Couch will not persist under a system of close grazing.

Common couch is not controlled by flame weeding and regenerates rapidly after treatment. An old method of dealing with the weed was to light a series of small fires over an infested area of field.

Updated November 2007.

Fully referenced review: