Answer: It's hard to successfully overwinter perennials in containers outdoors. Often even the hardiest of plants don't receive enough protection from cold to survive. This could be one reason behind the poor performance of the clematis.
There are countless annual vines (hyacinth bean, morning glories, canary creeper, asarina, cobaea, etc.) that would fare better for you, although they won't provide coverage of the wall during the winter.
If you can construct a large planter for the area, you can keep hardy perennial vines protected through the winter. Other fast-growing perennial vines that enjoy full sun are honeysuckles (Lonicera sempervirens, L.x heckrottii) and trumpet vine (Campsis radicans). Clematis requires sun at the top of the plant to ensure bloom, but prefers a shady spot for its roots. A rich, organic soil that drains well, yet retains moisture, is necessary for good performance. If you enrich the soil and keep it mulched with a loose layer of bark or leaves, and grow an annual flower around the base to shade the soil, you'll probably have better luck. Take care not to nick the roots or trunk when amending soil, since clematis is succeptible to several soil-borne diseases. Hope this info is helpful!
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