bulbous oatgrass, button grass, knot oat-grass, pearl grass
Arrhenatherum elatius ssp. bulbosum, (A. tuberosum, A. avenaceum, A. elatior)
Onion couch is a tall perennial grass that occurs throughout the UK both as an arable weed and as a component of semi-natural grassland. The species exists in several sexually compatible forms differing mainly in the amount of swelling of one or more basal internodes. A survey of bulbous forms showed the arable weed form to occur mainly in the Midlands and central southern England. The grassland bulbous forms were found mainly in Wales and the South West. There was no obvious relationship with soil type but rainfall patterns may play a key role in distribution. The non-weedy form, without the swellings, is commonly known as tall oatgrass. In semi-natural grassland both bulbous and non-bulbous forms occur together but here the bulbous forms are not as extreme as the arable weed. Plants in these populations do not breed true and offspring are likely to be a mixture of weedy and non-weedy forms.
Onion couch cannot tolerate shade and is sensitive to low temperatures and exposure to wind. The bulbous form is more frost sensitive than the non-bulbous. It is not as drought tolerant as some grasses and is said not to withstand waterlogging. Onion couch does not tolerate trampling and is absent from gateways.
Onion couch flowers from June to July and beyond. The flowers are wind pollinated and self-incompatible. An abundance of seed is produced. Seeds are shed soon after maturing in July and are generally non-dormant. They germinate best at moderate temperatures, mostly in August and September but continuing into December. The seeds do not have a light requirement and many germinate following the autumn rain. The optimum depth for seedling emergence is 5 to 40 mm, the maximum around 130 mm.
Aerial shoots are formed from March onwards. Stem elongation begins in April and peaks in July.
Onion couch does not form a persistent seedbank. Viability in soil is thought to be relatively short and the seeds are often attacked by fungus. In cultivated soil most seeds germinated in the first year after shedding and few seedlings emerged in the following 2 years. Seeds held in dry storage at low temperatures may retain viability for a few years.
The abundant seeds are spread by the wind. The weed can become established in arable crops as seedlings from seed shed in the hedgerow or in the previous crop.
Onion couch may be a dominant grass that produces a high level of biomass in a productive pasture. It is less competitive than tall oatgrass under moist conditions and when nutrient levels are low, but under drier conditions onion couch is more competitive. In pasture it is favoured by undergrazing. There is some evidence that the bulbous form is at a disadvantage in grassland when regularly defoliated. It regrows readily after the first defoliation but after subsequent defoliations it rapidly declines. In roadside verges, increasing the cutting frequency reduced the frequency of onion couch.