Couch grass originated in Europe and Western Asia but has spread around the world. Couch grass is extremely invasive and can quickly choke out and destroy other grasses in a lawn or completely dominate a flower bed or vegetable garden. Couch grass should be dealt with as soon as it appears. Couch grass spreads by both seeds and rhizomes (roots), and is very difficult to eradicate.
Keep couch grass mowed so it cannot seed. However, frequent mowing will not kill couch grass, as it can still spread by underground rhizomes.
Talk to your local nursery or home improvement center about a weed-specific herbicide designed to kill couch grass but not other plants you have. Spray the herbicide on a windless day and do not spray if rain is forecast within 24 hours.
Use a broad-spectrum herbicide such as Roundup if the selective herbicide fails to kill the couch grass. Be as selective as possible when spraying any herbicide containing glyphosate, as the chemical will kill virtually all plants it touches, not just couch grass. Spray when there is no wind and do not spray if rain is predicted within 12 hours.