You could put ivy in the pot and then add a couple of smaller clematis to climb up through the ivy and provide some colour in the summer months. I've done this with similar planters on my patio and it works quite nicely.
My RHS book says C.Armandii grows to a height of 3 to 5 m and spread is 2 to 3 metres (10-15ft x 6-10 ft). Mine is 10 years old, in the ground, south facing in SW England and is now at least 3 metres high and about 6-7 metres wide. It is a monster clematis and when happy goes wild. I have seen it right up the side of a 3 storey house. Definitely not a clematis for any planter IMO. The smaller winter flowering evergreen clematis covers a trellis much better and is easier to keep under control.
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One of the variegated ivies would probably be better.
(10-15ft x 6-10 ft). Mine is 10 years old, in the ground, south facing in SW England and is now at least 3 metres high and about 6-7 metres wide. It is a monster clematis and when happy goes wild. I have seen it right up the side of a 3 storey house. Definitely not a clematis for any planter IMO.
The smaller winter flowering evergreen clematis covers a trellis much better and is easier to keep under control.
http://clematisontheweb.org/new-clemdetail.cfm?dbkey=112