Onion Couch [Arrhenatherum elatius]

Pest Profile

About the pest

About the weed

Onion couch is a very tall, loosely tufted perennial grass, growing to 150cm. A series of bulbous swellings at the base of stem gives the grass its common name of onion couch. Leaf blades are flat and finely pointed. The flowerhead is compact and narrow.

Key features

Plant: Bulbous swellings at the base of the stem and yellowish roots.

Flowers/fruit: The spikelets have a single long awn.

Biology

Onion couch is a troublesome weed and difficult to control on cultivated fields. The plants can overwinter and new shoots are produced from March. The non-bulbous form can grow from stem bases detached during ploughing, but the bulbous form grows only from seed. It is encouraged by direct drilling of arable crops.

Symptoms & Diagnosis

Life cycle

Seed longevity: 1 - 5 years

Seed weight: 3.33 mg

Seeds/head: 100

Treatment

Management

Mouldboard ploughing can bury the stem bases too deep to emerge. Best control will be achieved by herbicides such as glyphosate applied when the grass is actively growing. This can be difficult near and around crops post-emergence and is best done in uncropped land such as summer fallows.