Do you want to create a garden with a long blooming season? Candytuft is early spring and Hellebores one of the first to bloom very early. Lavender, Dianthus, Thrift, and Bergenia would also be good candidates.

It's been my experience that 'evergreen' and 'perennials' don't necessarily go together. In my Z6 garden, perennials mostly go dormant in cold weather, and as regards shrubs (other than holly) wouldn't necessarily have vibrant foliage in winter.

I'll add my vote for Hellebore/Lenten rose as an early-season bloomer with interesting foliage but it isn't a low/no-maintenance mainstay in my perennial beds once the season warms up since the previous season's foliage looks a bit ratty early on.

You say it's a 'sunny public spot' that will be watered but since we've no clue what the soil conditions are, I'm guessing some suggestions may not take that into account.

I'd hate to recommend something that might suit the site but not survive the soil conditions.

In all honesty, 15 years is the longest I have had a garden in any one place, so I can't be totally sure that any plant will be OK for 20 years. I'll assume that you will choose plants appropriate for soil and temperature conditions since I am not familiar with what grows in NC.

Hellebore was my first thought as well. Can you use groundcovers that might need cutting back, but would not need dividing per se? Can you use short woody plants? Are any of the evergreen daylilies slow enough to spread to make them work here?

If you can use groundcovers, some of the thymes might work well to weave in and out of other plants.

If you can use woodies, some of the small conifers derived from witch's brooms might well work or some of the really small rhododendrons. I've had a Rhododendron 'Checkmate' for somewhere close to 10 years and it is still under 3 feet and is as easy as all the other PJM family plants. Would any of the easy care roses work for you here?

For deciduous plants, peonies are the classic plant that never needs dividing. Baptisia is another one that is happiest left in one spot. There are some fairly short clematis with long bloom periods that might suit the situation as well.

Yours is an interesting question! I'm curious about the size and topography of this garden and also about the plants you're already considering for your zone. Posting a photo or two of the plot could help us visualize your workspace.

You asked for perennials that won't need dividing for about 20 years. In my years of gardening (two homes), I've found that when planted in optimal conditions of great soil, sun and watering, many perennials will go nuts! For example, I have several Pine Knot hellebores. After 7 years these have formed huge clumps that do not die back in the winter. And yes, the leaves do get ratty as Gardenweed said. The clumps are now 3' by 2'6", and I've had to remove many things near them.

I wonder if this garden will have some of the "woodies" that nhbabs suggested? If not, I'd suggest plants with interesting foliage -- like variegated Liriope, Sedum ('Vera Jameson', etc.), Artemisia, Agastache or Sage. Except for the Agastache, which I've only had for one year, I've found most of these to be well behaved.