An easy way to add definition between your lawn and landscape beds is to plant border grasses.
These clump-forming perennials can range in size from 2 inches to 2 feet tall.
One of the most widely planted border grasses is Liriope muscari. Commonly called liriope or lilyturf, this grass has solid green or variegated foliage, depending on the variety. Some varieties bloom in warm months with small purple flowers.
Mondo grass (Ophiopogon japonicus) is another popular border grass. It has deep green, variegated, or black foliage and ranges from 2 to 24 inches in height. It's sometimes planted as a groundcover, creating a shade-tolerant lawn alternative.
Mondo grass is quite often confused with liriope. Common names such as Aztec grass and monkey grass are used for both. However, the leaves of mondo grass are more narrow than those of liriope and have smaller fruit.
Border grasses are relatively low maintenance once established, though they'll look better if you mow or cut them back in late winter or early spring.