What is the best way to clear a weed- and grass-infested established flowerbed with plants that you want to protect. How do you eliminate ryegrass from a bed?
No eager gardener wants to hear this piece of advice, but you must move those plants and kill the weeds and grass, once and for all. Western weeds are tough. When we moved to our current house -- before I knew that we had bindweed and bermuda grass in the backyard -- I raced out and planted a huge border garden. All of the plants in it eventually had to be moved to new locations (at least for the season) while I dug and poisoned my way through the former border. Many experts say don't dig because it can spread weed seeds; I find that at least cutting off the heads of the culprits with a hoe cuts off their food supply and weakens the weeds until they die. This process can take all season.
Try putting layers of black plastic over the soil to cut off light, air, and water and smother the weeds. This process should take one growing season. Then clean all weed debris from the bed, and spray any remaining culprits with glyphosate, the active ingredient in Roundup. In fall, dig in compost and pray for spring. You might want to cover the bed until then, too, with a permeable landscape fabric, to keep topical weeds from spreading to the patch. To keep ryegrass from creeping back in, line your new bed with metal edging that goes down into the ground several inches.