Answer: With clematis, routine pruning is mostly done to try to encourage maximum flowering and it varies depending on what specific kind of clematis it is. Some types bloom only on old wood produced the previous year so you would not want to prune them much at all, some bloom only on new wood grown the current season so you might prune them back to encourage new growth, and some will bloom early on last year's wood and again later in the summer on this year's wood so they would be pruned if needed immediately after the spring bloom flush.
However, a newly planted clematis will probably not need to be pruned at all for several years because they grow very slowly in the beginning and need to build strength and become established at first. In late spring, simply trim away any bits tha tmay have died back and then allow the plant to grow. Once it begins to bloom nicely for you then you can begin pruning it. By then you will also be able to tell which type of pruning regime will work best for it.
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