Walkers low catmint!
Its one of my favorite no maintenance, beautiful, airy ,fabulous plants. Mine has bloomed since april. I never cut it back tho i will next year , because it never quit blooming. I bet it will look great with that clematis.

Here are some in my mailbox bed:
Gallardia 'Burgundy'
Flax, blue
Aquilegia
Mum
Portulaca, I like this plant for the street edge as the soil is not as deep there and very dry.
Thyme, creeping
Sedum, dragonsblood, others
Zinnias, Profusion varieties
Melapodium
Knautia
Oriental Poppies
Dianthus/Sweet William
Dark Purple/Red Castor Bean

Some of the smaller grasses will work fine there too, to give a bit of texture. Also, don't forget spring blooming bulbs. Now is the time to be thinking about those too. My mom has a pretty good sized mailbox garden and some of the things that have done well for her are coreopsis, artemesia, salvia, blue fescue, heuchera, sedums and iceplant. These are good for sunny spots with dry, sandy soil, which my mom has. The heuchera may be a bit iffy, but it's done well there. Though her box gets some light shade during the morning, in the afternoon it's full sun, and light shade again in the evening. I wish I could do a mailbox garden, but ours is across the road on the neighbors property, and there's gravel all around it and the mailman has a gorge driven out in front of them. Oh well, I have lots more space in my yard to grow things.

Karen

First suggestion: amend the soil. That'll increase your choices considerably.

My "mailbox bed" is a circular one constructed around an ornamental pole lantern at the foot of the driveway. Perennials growing there are a no-name Salvia species which is tough as nails, blooms much of the growing season in waves and stays in one spot, the hardy white-flowered Hibiscus laevis (not a dinner-plate type but a smaller species sort I used to grow in Texas, and which has a much longer flowering period than the usual H. moscheutos hybrids; mine is pure white without the central pink blotch seen in the photo) and Mirabilis longiflora (sweet four o'clock). For the rest, I fill in with annuals (this year there's a variety of cannas and Rudbeckia "Indian Summer". It's a dry bed in full sun, but the plants flourish partly because I amend the soil with water retention crystals when planting every spring.

How close is your mailbox flowerbed by the street/roadway? If close, I would think that you will need to consider many conditions. Probably will need drought tolerent plants. Would it get full sun all day? Another consideration. If you are in an area where you have bad winters and a lot of snow, keep in mind road salts being put down by the county, city or state for melting snow/ice. It can kill plants easily. I had the niobe clematis at my previous home and like you said, it is pretty but not a fast grower. Jeanne, the clematis expert usually post on the clematis forum and she can recommend some clematis that might fit your needs. Since you wouldn't probably have access to water, you'd need to depend on Mother Nature..thus my reason for suggesting drought tolerent plants. What about yellow sedum or ice plant or ajuga, creeping phlox would be pretty but it's blooming time is limited to the spring. Portulaca would be great. I even have portulaca that has bloomed all summer long planted in an old bird bath with no drainage holes and it hasn't stopped blooming all summer. Just water it once a week if there is no rain. Yes, BES spread quickly but they are showy and drought tolerent and I see so many people planting Walker's low catmint for that reason. If it gets too scragly looking, they say if you cut it back, it will rejuvenate over again in a few weeks. I plan to plant some myself this coming year. Carpet of snow alyssum would be another one that will bloom all summer. Think about plants that you would plant in a rock garden..drought tolerent.

The two plants that have done best next to my baking in the sun driveway are:

Whirling butterflies Gaura--bloom since may and still going strong. (Pink and white varieties)

Snow Fairy Caryopteris--not well known. Vibrant Variegated Foliage that stays fresh all summer. Blue blooms in Sept. Will lighten up shady spots, but in the sun it vibrates.

Don't forget the human element. Pity the poor letter carrier who has to fight off the bees to deliver your mail!

Hmmm, you are right so I won't be putting any russian sage there. The bees go CRAZY for that. I tried Gaura once and it did not return. While I really liked it, I'm not sure it liked my soil.

Gallardia might be a good option if I can find one that I like. The clematis is mauve so an orange gallardia would really clash. I know they come in yellow too.

What about Echinacea? Do you think that would work? It is pretty dry over there next to the street. Morning sun until about 2:00 or so.

Not so exciting choices but these are drought, salt and alkaline tolerant.

Karl Foerster Grass, Caradonna Salvia, Magic Carpet Spirea, Bevan's Geranium, most daylily cultivars.

I tried several Echinaceas by the mailbox this year. By far, the best performers were Echinacea 'Flame Thrower' and Echinacea 'Pica Bella'. Keep in mind that Echinacea 'Flame Thrower' most likely won't flower the first year. If you plant some now, they should bloom next year. That is one tough plant! It came back after our flood we had this Spring and it performed well in our drought this summer. 'Pica Bella' looks very unique in that it's a compact plant but has tons of the most perfectly formed blooms with pointed tips to the petals. It's also one of the most drought tolerant of all the Echinaceas. 'Pica Bella' will flower the same year you plant it from a plug. It is a blooming machine!

Pica Bella looks like a cute one. Where did you get yours?

The ech I had in mind was Coconut Lime. What does everyone think of that choice? I like catmint but I think it might get too large for the bed.

Heliopsis Summer Sun is my favorite perennial because it is virtually no care. Cheerful yellow daisylike blooms that begin in June and go to October (if you're lucky and if you deadhead). Can't beat it.

Looks like a nice plant but a little taller than what I wanted. Thanks.

I got Echinacea 'Pica Bella' from High Country Gardens.