In the UK, there are five main species of perennial grassweeds that are commonly known as couch or twitch. These are common couch, black bent, creeping bent, onion couch and creeping soft-grass.
Common couch is characterised by having creeping, white underground stems (rhizomes), shoots with small auricles and a short ligule at the base of the leaf blade.
The heads (spikes) superficial resemble perennial rye-grass, but the spikelets sit flat against the stem rather than with the narrow, rounded side adjacent to the stem as in rye-grass.
Black bent is characterised by having white underground stems (rhizomes), with a long ligule, no auricles and the heads are large, dark read panicles.
Creeping bent is somewhat similar to Black bent, although mature plants are shorter and produces creeping stems (stolons) on the soil surface, not underground rhizomes.
Onion couch looks like flase-oat grass with large loose panicles producing relatively large seeds with awns. However, the stem bases are swollen to form several round, onion-like corns, up to 1cm in diameter, hence the common name.
Creeping soft-grass produces rhizomes, has conspicuously hairy nodes on the stem and a compact panicle.
The threat that common couch, black bent, creeping bent, onion couch and creeping soft-grass present makes it vital to take a zero-tolerance approach to these perennial grass weeds.
Not least because they remain so widespread in rough grassland, hedge bottoms and field margins, and can thrive in conservation headlands, buffer strips and other environmental areas.
Couch grasses are also very easily spread through cultivations and, being particularly difficult to control in the spring and especially competitive in more open crops, can be favoured by spring cropping.
The fact that current couch grass problems are highly localised within farms and fields means control measures are best focussed on the particular areas infested.
Given the differences in their biology and behaviour, it is vital to tell perennial couch grasses apart from annual grass weeds so the most appropriate control measures can be adopted. It’s also useful to know which species of couch grasses you are dealing with as they also vary in their biology.
Correct identification and mapping of problem areas is best undertaken when grasses are flowering above the cereal canopy in the late summer.
WARNING: The most effective stubble control programmes for annual weeds may be the least effective for perennials, and different couch grasses may require different treatment strategies for the greatest success.
Glyphosate application is the best method of controlling couch grasses. It is more effective and economic than the repeated cultivations that would otherwise be necessary.
Pre-harvest glyphosate in cereals can be particularly effective because it targets couch grasses at the time their translocation is directed downwards to store nutrients ahead of the winter. This ensures the most effective kill of the entire root system.
Unlike annual grass-weed control, the most effective post-harvest approach involves leaving stubbles uncultivated for 6-8 weeks, allowing couch grasses to grow to 4-5 leaves before spraying them off with glyphosate.
WARNING: Cultivating stubbles prior to spraying in a typical annual grass-weed control strategy will only spread damaged fragments of couch grass roots and shoots in the soil to emerge later and escape pre-planting spraying.
Cultivations can be an effective non-chemical way of controlling couch grasses, but only if they are of the right type and, crucially, repeated several times.
A single cultivation can be worse than none as it chops and spreads the rhizomes, stimulating new growth and increasing plant numbers. Ideally three successive cultivations are needed to achieve a reasonable degree of control, together with sunny weather to complete the desiccation of rhizomes on the surface.
Rotary cultivators have been found to be much more effective than tines in fragmenting common couch rhizomes, a sequence of three rotary cultivations achieving a 71% reduction in their weight against just 44-56% with a similar rigid tine cultivation sequence.
WARNING: Cultivations alone are unlikely to be cost-effective in controlling couch grasses. Repeated passes mean a high energy as well as labour requirement, together with significant potential for damaging the soil structure.
Like black-grass and Italian ryegrass control, ploughing can reduce couch grass infestations by burying the weed reservoir to a depth from which it struggles to emerge.
However, all parts of the weed must be completely buried to at least 15 cm – preferably 20cm – which means quality ploughing to at least 30 cm with the use of efficient skimmers to dispose of all trash.
Cultivations for the next few years will also need to be shallow to avoid bringing viable couch grass roots and stems back to the surface where they can regrow.
WARNING: Poor or shallow ploughing will achieve little and may make future control more difficult by distributing Couch grass roots and stems throughout the plough depth.
Fallowing can provide valuable extra opportunities for controlling couch grasses as well as annual grass weeds with Roundup in the rotation, providing the spray programme is carefully timed and effectively structured.
Couch grasses need to be sprayed once all the ears are fully emerged but while the seed heads and leaves are still green to maximise the downward flow of Roundup to the roots. This is significantly later than the ideal annual grass weed timing, so two sprays are likely to be needed with mixed weed populations.
Grass margins and buffer strips can easily become infested with couch grasses unless any weeds present are effectively controlled ahead of their establishment.
