Clematis Vines in a container

I just planted a 12' Ramona clematis in a 20 "D X 21" H container on my balcony which gets about 7 hours of sun each day. I'm buying a 6' obelisk trellis for it to grow on, but what do I do when it outgrows the trellis? Does clematis require compost be added to the container's soil? What type/brand of fertilizer works the best and how often should it be fertilized? Thanks.

you can keep a ramona, as well as many other clems in check by pruning it back some before bud formation and after bloom time. as a gardener, you can control the size and shape of any clem with this tactic.