Gary, you are likely to get a million and one suggestions for side dressing your clematis. I tend not to be tied to any one fertilizer and use what I have at hand. That has included 10-10-10, rose fertilizer, Espoma organic fertilizer, miracle grow, alfalfa pellets, llama poop, --- you name it and I have probably used it. The only thing I wouldn't use is a high nitrogen rapid release fertilizer since it will stimulate rapid vegetative growth and often this causes a reduction in flower production.