Arrhenatherum elatius ssp. bulbosus (Willd.) Hyl.

Common Name: False oat grass, Onion couch grass

Family: Poaceae

Plant description

Arrhenatherum elatius is a tall perennial tussock forming grass that in one subspecies bulbosus has a swollen lower internode on the stem. The plant can grow to 180cm tall, and has long and broad bright green leaves. It is common today in the UK, temperate Europe, North Africa and West Asia as a coloniser on roadsides, abandoned fields, meadows, wastelands, gardens and grasslands. Gazing is the key limiting factor for the spread of A. elatius ssp. bulbosus. When under grazing pressure A. elatius ssp. bulbosus declines and later returns when grazing pressure is reduced (Robinson 1988). As the plants begin to die off, the swollen nodes detach from their stems and can be dispersed by wind or water to propagate in new areas.

Tuber description

Storage organs are formed by a swollen internode at the base of the stem. These swollen internodes are up to 7mm across and can be as long as 4cm (Hather 1993). The swollen internodes form one on top of the other and there are generally 3-5 swollen internodes per stem (Bond and Turner 2007). They can be a variety of shapes ranging from squat and to round to elongated and teardrop shaped. At the top of the swollen internode there is a constriction on the stem which forms a ridge, which is the point at which the tuber breaks from the stem leaving a sharp abscission scar. The surface has longitudinal striations still visible when charred. At the base of the internode the tuber constricts where the root attaches to the stem. The root is covered with root attachment scars in alternating rings of circular scars. The root can break off from the swollen internode and leave a circular scar on the base of the tuber.

When charred from a fresh state, cavities form in the ground tissue and create a large central void that may make the internodes fragile and easily crushed. Charred when dried, Arrhenatherum internodes can preserve well. The epidermis can fuse with the outer parenchyma cells to form a solid carbon ring. Vascular bundles are located in a ring beneath the epidermis up to two to three bundles deep. Internal to this, the ground tissue is composed of polygonal parenchyma cells. In the rootstock beneath the internode the vascular bundles are concentrated in the centre of the root with parenchyma beneath the epidermis. See Hather (1993: 112-113) for more information.

Archaeobotanical information

Arrhenatherum elatius tubers have been found in archaeological assemblages across the UK from the Neolithic to Medieval periods and in graves in France and Sweden (for more details click here). Although suggested as a plant food (Preiss et al. 2005) and described as tasting like raw potato (Englemark 1984) recent research into potential ancient plant foods deemed Arrhenatherum tubers to be quite inedible (Mears and Hillman 2007). Instead, it is possible that Arrhenatherum plants were used as tinder for starting fires and the swollen internodes were charred and preserved in this process (Robinson 1988). Couch grass tubers are frequently found in cremation graves, particularly in Neolithic and Bronze Age UK () Iron Age Scandinavia (Englemark 1984, Robinson 1994) and Roman France (Preiss et al. 2005). Although Preiss et al. (2005) and Englemark (1984) interpret A. elatius tubers as food remains, Englemark admits there is no evidence from historical or ethnobotanical literature that the internodes were a food source. Robinson (1988) suggests that the dried stems of A. elatius would have been useful tinder for lighting fires and that the swollen internodes could easily have been uprooted with the plant and burnt.

A. elatius ssp. bulbosus is an effective coloniser of arable land but is susceptible to grazing pressure (Bond and Turner 2007). Archaeologically, the presence of onion couch grass tubers indicates that there were cleared areas near to the site that were not being agriculturally utilised, either abandoned arable fields or grassland not being grazed by animals, and it was from these lands that tinder was collected (Robinson 1988). Being an invasive weed of arable fields A. elatius tubers could have been collected with crops if they were being harvested by hand and uprooted as was suggested for their presence in samples from Nettlebank Corpse (Campbell 2000). Charred A. elatius ssp. bulbosus tubers may also be the product of intensive land management, weeding arable fields. Fields infested with onion couch grass can have a yeild loss of over 60% (Miller, Cussans, Froud-Williams 1996). Because A. elatius ssp. bulbosus propagates vegetatively through tuber dispersal, agricultural practices like tillage that disturbs the soil may promote the spread of onion couch grass. To eliminate onion couch grass modern agricultural literature recommends either burning the surface soil or harrowing the field and burning the weeds that are uprooted and spreading the ashes over the land (Bond and Turner 2007).

Arrhenatherum elatius ssp. bulbosus (Willd.) Hyl. modern material collected by Dr J. Hodgson in Autumn 2007 on the Coast near Liverpool. Scale 2cm. Photograph C. Longford. SLR Digital.
Arrhenatherum elatius ssp. bulbosus modern dried material charred at 230oC for 3 hours. Scale 2mm. Collected by J. Hodgson Autumn 2007, Liverpool, UK. Photo C. Longford.