Might be too wet and it might be the begining of crown rot.....or too much fertilizer.....did you use Miracle grow?.....I find that miracle grow has its draw backs about burning the roots (too salty and hard on some plants.....

This post was edited by ians_gardener on Thu, Jun 5, 14 at 7:17

Thanks for your advice.
I googled crown rot and agree that it does seem like the culprit. I used Osmocote granules. Maybe I disturbed the roots?
I will cut back the affected stems and check the ground moisture before watering in the future.

See FAQ link below about browning leaves

Here is a link that might be useful: Why have the leaves on my Clematis turned brown?

Browing leaves from the bottom up is typical Clematis behavior. Crown rot is not.

Clematis do not bloom all summer, some bloom for about a month only the entire year, some bloom longer but none bloom all summer like annual vines do.

Neither Osmocote nor Miracle Gro are good fertilizers for perennial plants grown in the ground. They are both made for container annuals and add nothing to your soil like organics like manure or blood meal to name a couple would. They can cause burning too though Osmocote not so much.

Thanks everyone. The FAQ is very helpful.

Your Clematis is not Nelly Moser. It is probably "The President".

I agree, it is not Nellie Moser. Nellie Moser is light lavender with a darker stripe through the center. Hope you figure out what is wrong with it.