Or should I say "Howdy neighbor"? I live in Columbia and your zone is 7. Here's a zip code zone finder.
If your planter is like many I've seen made from tires or is a half whiskey barrel, you should be able to plant perennials. There are two plants that I can think of that will bloom most of or all of the summer that I planted in my daughter's garden. Daylilies, botanically known as Hemerocallis, are tough plants, though most bloom for about 3 weeks once mature. There are some that are everblooming and some that are reblooming. You could do a search at www.google.com and put in the search box
Hemerocallis + everbloom
Hemerocallis + rebloom
and find lots of choices. Most large nurseries carry several, though the most popular is Hemerocallis 'Stella D'Oro' and is a short plant that blooms dark gold. My favorite is slightly fragrant, a nice yellow and blooms all summer. Hemerocallis 'Big Time Happy'.
Here's several of this variety with similar habit.
Here's what a good sized clump of daylilies looks like, this one is the 'Stella D'Oro'.
Another plant that will give you a long season of bloom and cascade over the edges of the planter would be a hardy geranium - not the annuals you see in red, white or pink in your neighbors' pots. Those aren't really geraniums, but pelargoniums. The hardy geraniums will come back every year. Geranium 'Rozanne' blooms all summer, is quite hardy and the light purple color will look lovely with the yellow daylily. Here's a picture of the flower, leaves and plant habit.
With this combo, and the size of your planter, one of each should do you fine. In two or three years it will be full and lush.
You might also want to consider putting in bulbs with your perennials if you are going to plant this fall. You don't need a whole lot of them, but some early blooming crocus, daffodils and tulips that will return every year would look lovely until the perennials start to bloom in June. Some tulips don't return every year, so here's some that will and instructions on how to plant them with your perennials on top or mixed in.
One more thing about fertilizing if you are going to plant this fall. Use either a bulb food or just rock phospate to mix into your potting soil which you can get that at any garden center, Home Depot, Lowe's or Myers Seed in the city. If you use a fertilizer that is high in nitrogen now, the plants will put out lots of green growth that will be too tender when the cold weather comes. The rock phosphate will help them grow strong roots.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.