Often the use of edging borders will give your garden a more formal appearance, however the choice of border edging plant you use will determine just how formal. If you use tidy, neatly shaped plants such as dianthus or tuft grass you will produce quite a straight formal look. On the other hand if you use plants with a loose habit such as nepeta, your garden will have a much more relaxed feel with plants intermingling with each other.
A well sheared low boxwood hedge is probably the most sophisticated of the garden borders. Boxwood can be very expensive and is not suitable for all gardening areas. To create a similar look you can use any small, woody perennial border plants, planted close enough together to fill a solid line. You will have to keep this regularly sheared to keep it looking good.
Larger, bushy perennial border flowers planted in a row edge can liven up a garden bed of evergreens. You might try daylilies, agapanthus, heuchera or bergenia. When edging a bed bordered by a gravel path use perennials that flop over the edges, or creep along such as the sedum family. Fleabane and hardy geraniums work wonders for softening paths. Some geraniums such as ‘Mavis Simpson’ will even flower for most of the summer. When you edge both sides of your path stay with the same plant for a more harmonious and natural appearance.