palmtree

Cannas are excelent especially ones of varying heights (there are dwarfs that flower at under 2 feet tall but tall ones that can get to 12 feet before flowering). Dark especially black colored elephant ears can really complement the green color of banana plants. A hardy hibiscus such as texas star or rose of sharon will survive the winter and look great planted right next to cannas. Purple heart plants or wandering jew plants (both are in the same family, will make fast tropical ground covers. You can easily root cuttings to save for next year. I think the wandering jew plants show the most color in partial shade and the purple heart in more sun. They both can handle mostly shade or full sun.

Also crape myrtles might make nice complements and are probably semi-hardy in your area (some cold tolerant varieties probably survive a zone 6).
Hawaiian ti plants, philodendrons, and begonias are all great tropicals you can plant in the ground and dig up when fall comes into pots.

Good luck! Think lots of color and different textures.

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Location: New York City, zone 7b