Everyone does summer containers but it's not so common to see small trees and shrubs used in containers. I'm not sure why, since you can put a tree or shrub in your containers for the summer, then get them into the ground before it gets too cold and then you...
A) can use them for your containers again the next year
B) You have more trees and shrubs in your yard
As I don't like to repeat myself much,as I don't like to repeat myself much, I shoot for things that can be reused in my yard someplace. Last year I used a super sexy Black Diamond Crepe Myrtle in my pot right next to my front door. I loved the proportions of using a shrub in back of all the annuals, PLUS the dark foliage of this particular plant really lit things UPPPPP!
Scale is important when using shrubs in containers. I like the way almost all shrubs look in a pot with other plants clustered around, but you might not want to use a plant that's more than twice as tall that the other plants in the pot. You want it to look like a happy combination, not 2 separate thoughts in a pot, like some kind of nightmare hi/lo bridesmaid dress. It can be hard to line up all the plants at the garden center to see how they'll look together, since the shrub is most likely in a pot much larger than it's new playmates.
I always admire how they use trees and shrubs in the containers when I visit the Morton Arboretum. No kidding, right?? It's kinda their job but I really think they do it STELLARLY. (Turns out, that's not a word. Should be)