There are a number of plants that are routinely recommended as edging plants, and few unusual ones. Here are some suggestions.
Check for potential slipping hazards when planting something near a walkway
When choosing an edging plant, it is important to consider what your goals are and what needs you may have. As an example, if a stone walkway is present, you must take care that the plants do not emerge onto the stone, because that can create a slipping hazard for you and your guests. I remove the hazard at some gardens I tend by cutting back the sections that emerge from the ground onto the stone, but you can save yourself many hours over the years by avoiding plants that trail sideways from ever emerging onto the potentially slippery surface. Ivy is a perfect example of a potential slipping hazard. The plants you choose should be more upright, or should have flowers and foliage that are closer to the ground, or more compact.
Choose plants that thrive in a wide range of conditions
Next, is the site more sunny or shady? I have tried to choose plants that will thrive in either set of conditions. I should add that I have grown every one of these plants, some for many years, and have grown them many of them in full sun to full shade, and in a pH of 7 to7.9, and with amended clay and loam.
When actually picking the plants you will install, think about appearance throughout the season. If a plant goes completely dormant, like Mertensia virginica (Virginia bluebells) lovely as they are, you will have a big hole in a very prominent place. I have actually seen bluebells promoted as an edging plant, which bluntly mystifies me. Hostas are a poor choice if you have a slug problem. I have seen them used, to the same sad effect by summer. Edging plants have to be tough, but that doesn't mean they cannot be beautiful. The same with daylilies in my zone, which is 5b/6a. I have spent the last few weeks removing what I call daylily "smutch" for clients. Perhaps this works well in warmer climates, but certainly not in cooler ones like mine. On the other hand, trilliums may bloom in spring, but also present substantial and attractive foliage for most of the rest of the year, so they can be considered.
Parsley is a great border choice
Parsley (Petroselinum crispum) is my personal choice. It is quite easy to grow. It is also outstandingly attractive. I have read stories of its difficulty and I find them untrue. However, parsley is hard to transplant, which is another thing altogether. You can purchase it, but you can also start it by laying it in cells, lifting the entire cell, and putting it in the ground. It is hardy in zones 5-8 and is a biennial. The first year you get the leaves. The second-year it blooms, and then drops seeds. The result? I have only had to plant it once in any given location. If you are growing it for consumption, the first year flavor is superior, but if you are growing it as an edging plant, I suggest just leaving it and letting it seed itself. And given that I have seen it sold for $8.50 a plant, it seems a good policy to grow your own. It takes a couple of weeks to germinate, but it's simple because you can actually start it directly in the ground. It is hardy in zones 3 to 9.
Eragrostis spectabilis is a native that withstands harsh conditions
Also known as purple love grass, I consider it to be a real find. Its native range is North America, in zones 5 to 9, and it grows and blooms in the most inhospitable environments you can imagine. It's black walnut tolerant, drought-tolerant, and it withstands air pollution. It can be grown in any average well-drained soil. I grow it along my cement driveway, right by the edge! Its hight and spread are 1 to 2 feet. It supposedly blooms from July into August, but here in my zone, which is 5b, it is in full bloom at the end of September. The soft reddish-purple flowers are very decorative and can be cut and dried for the house. It has no insect or disease problems.
The flowers are a soft purple. It does require full sun, and tends to naturalize but not spread excessively. It has minimum watering requirements, although I do water mine a bit while establishing it, and is handsome even in the winter. It is readily available online. It is not widely grown, and mine appears to be the only one in my city. I think that the word has simply failed to be spread.
It looks really cool in the winter, too!
Fragaria vesca 'Reugen' are woodland strawberries
These are also known as woodland strawberries. They come in a variety of colors, but the most important thing to remember is that they do not spread by runners, instead of clumping, and the berries are edible. They are upright, zone 4 hardy, have excellent flavor, and can grow in part shade to sun.
The plants themselves are quite compact. They are everbearing, and if you start them early in the season, you can actually have fruit the first year. They are really easy to grow from seed. They should be planted six to 12 inches apart. and will grow together. They are also amazingly easy to transplant.
Nepeta 'Dawn to Dusk' is great for edging
Nepeta grandiflora 'Dawn to Dusk' is a plant I covered when I wrote about nepetas on this forum. I never see it in gardens, but it is in fact available, and beautiful enough to be a great edging plant. It has a strong constitution; mine actually seeded in a previous garden under hot, dry conditions, but to my regret pretty much stays in place now. It is about 2 feet tall and wide, and blooms repeatedly if you deadhead it. And it's a nice change from blue Nepeta.
Alchemilla mollis (Lady's Mantle) works as an edging or a stand alone specimen
This is a plant typically associated with cottage gardens, but it actually originates in the Caucasus. This plant has a horticultural range of zones 3 to 8, grows one to one and a half feet high, and one and a half to two and a half feet wide. It can handle anything from full sun to part shade, (some state full shade) and is a gorgeous chartreuse color, producing sprays of flowers that make good cut or dried flowers. It blooms in early to mid-summer. Supposedly it can be somewhat invasive but I have grown it for years and it has never spread, only becoming larger. If invasiveness is an issue, it can be controlled by deadheading the lovely flowers when they are spent. In my experience in zone 5a it can become a bit brown in full, hot sun with southern exposure, but that can be countered by watering.
This is a plant that looks great as a specimen, great grouped, and great as an accompaniment, particularly to traditional plants like peonies and roses.
Dianthus are tough little rebloomers
For a bright whack of reblooming color, it's hard to beat dianthus. Ranging from 5 inches to three feet, the smaller ones make great edging plants. Cheap to purchase, and a piece of cake to grow, these often spicily scented beauties can take a lot of abuse. I grow them along a cement driveway, and deadheading them results in wave after wave of bloom even in 90 degree heat. They grow in zones 3 to 9.
Salvia verticillata and Digitalis 'Husker's Red' for a great combination
I find this to be a fabulous combination.
Digitalis 'Husker's Red' was the Perennial Plant of the Year in 1996. Hardy from zones 3 to 8, with a height of 2 to 3 feet and a spread of 1 to 2 feet. This plant has a bloom period of May to June in my zone, but its beautiful burgundy foliage lasts well into the fall. It requires virtually no maintenance, although it is best to cut it down to the foliage after bloom. The elegant flowers are white with sometimes a tinge of pink. And it tolerated heavy clay soil at my former home.
I paired it with Salvia verticillata and found it a great drought-tolerant combination. This particular salvia is commonly found in the color purple but is also available in white, particularly from seed. Growing in zones 5 to 8, with very low maintenance and a showy flower, it blooms for at least three months, and has handsome foliage when not in bloom. It prefers full sun, and tolerates drought beautifully. I find it almost indestructible. The height is 1 and a half feet and the spread is one and a half to 2 feet.
Both are excellent border plants on their own, but I find the combination magical.
There are many excellent edging plants in commerce, in addition to those that you can grow. I hope that you will explore the topic and come up with your favorites.