Some clematis brown on the lower stems without affecting the upper leaves and stems and flowering. Nothing to be done except to hide it with plantings of annuals or perennials in front of the plant. Keeping soil moist might help delay or stop the browning from occurring.

If the stems are black and leaves above are wilting it may be clematis wilt. Trim off the old dead stems as the top growth is going to die anyway. Leave unaffected stems alone. New stems should grow from the crown.

There are different theories about clematis wilt, some think it it caused by a fungus and others think that rampant growth overwhelms the plant and the plant will outgrow the wilt tendency as it matures. Large flowered varieties seem to be the most affected.

Here is a link that might be useful: Clematis wilt

This post was edited by mnwsgal on Tue, May 20, 14 at 19:25

Did you prune the plant this spring? Young plants benefit from hard pruning for 2-3 years which encourages more stems to grow from the crown.

Photos help us understand the problem. Can you post a photo?

This post was edited by mnwsgal on Tue, May 20, 14 at 19:32

Some do that because they weren't pruned properly and some because they aren't getting enough water and some have fungus.

A photo will help.

Fungus and lack of water occur much later in the year for me but your climate may vary.