Ryegrass (Lolium perenne), a cool-season grass hardy in U.S. Department of Agriculture plant hardiness zones 2 through 8, is often used to overseed Bermudagrass (Cynodon spp.) in the fall so the lawn remains green during winter when Bermudagrass is dormant. Normally, the process works well, because ryegrass dies out as the temperatures rise, but on occasions when cool weather lingers in spring -- or in grass transition zones -- ryegrass can persist, intermingle with the Bermudagrass and weaken it. In these cases, you'll need to take steps to remove the ryegrass to keep your lawn healthy.
Wear gloves and long sleeves, and apply a herbicide containing glyphosate during wintertime when your Bermudagrass is dormant -- normally in December or January, because it is hardy in USDA zones 9 to 12. Follow the manufacturer's instructions for rate of application. Within 24 hours of application, the lawn needs at least 1/2 inch of water to activate the herbicide. Apply a second application a few weeks later if ryegrass is still present.
Reduce fertilizing and watering your lawn in late spring for overseeded lawns -- or if you prefer not to use a herbicide. Mow the ryegrass progressively shorter each week, until you are cutting it to a height of about 1 inch. This approach weakens the ryegrass while rejuvenating your Bermudagrass. Once the Bermudagrass has started to green up, resume your warm-season grass schedule of mowing, watering and fertilizing.
Call a professional lawn service to remove ryegrass from your lawn while your Bermudagrass is actively growing. Sulfonylurea-type herbicides can remove ryegrass without damaging or killing your Bermudagrass, but these herbicides are not available to consumers so they must be applied by a licensed professional. The most commonly used herbicides on ryegrass include formasulfuron, metsulfuron or trifloxysulfuron, instead of sulfosulfuron. Herbicides with sulfosulfuron tend to be the least effective on ryegrass, often requiring two treatments to eradicate it.