Two types of ryegrass typically grow in North America: perennial and annual. Annual ryegrass grows wild in several areas, whereas perennial ryegrass often starts its life as a turf grass. But both of these weeds are aggressive spreaders, and they each offer their own obstacles to control. Annual ryegrass will respond to only a limited number of pesticides, and perennial ryegrass's root system must be completely eradicated or it will crop up again.
Mow the ryegrass as low as possible. Wait for it to grow back 3 to 4 inches before treating it. Weeds must be actively growing for most herbicides to work effectively.
Spray annual ryegrass with herbicide. According to specialists at Virginia Tech, annual ryegrass responds to applications of 3 pints of Roundup per acre (mixed according to the manufacturer's instructions for the type of sprayer you are using), spaced six weeks apart.
Spray perennial ryegrass with a nonselective, post-emergent herbicide that contains glufosinate (Finale) or glyphosate (Roundup). Re-spray at intervals prescribed by the manufacturer. Use a different herbicide in your second application to prevent the ryegrass from developing resistance.