|I plan to buy an obelisk trellis(?)for a 19|
|Unfortunately I am not entirely clear on the volume in your planter, however the larger the better if you want to try to overwinter any plant outdoors. A half whiskey barrel planter might be large enough to insulate the root ball sufficiently. You could then experiment with something like the fall blooming clematis or a trumpet vine (Campsis radicans). These are both very large flowering vines but I have seen them survive outdoors in containers. You would need to make sure the soil is kept moist any time it is not frozen, mulch the soil surface, and provide some wind protection. If possible, in late fall move it to the north side of the building so it is shaded and will stay at a more even temperature, in spring move it to a sunny location. Both of these vines are cut back very hard in spring so you would not need to worry so much about protecting the top growth. I think though they would overwhelm the typical three or four foot tall obelisk.
Chilean jasmine is the common name for a number of plants botanically known as Mandevilla. This is a tropical plant and will not survive outside in the winter no matter what you do to try to protect it. It should not be kept much colder than about 50 degrees or the tuberous root will rot and the plant will die. It is generally available with lovely pink flowers and does well as a container plant if kept well watered and well fertilized. It loves heat, humidity, and sunshine. You could overwinter it indoors to keep it from year to year although you may find that it loses vigor over time due to the short growing season here. Many people like it enough that they are willing to replace it every spring.
Your local professional nursery staff may also have some suggestions.