Here is my favorite Jackmanii Clematis in the garden in June/July.
It's not so lovely in March but if you want it to live up to its gorgeous potential you have to make some drastic cuts now. There are 3 different groups of clematis that are defined by when they bloom and each type needs a different approach when it comes to pruning. The Jackmanii is in Group 3(also called C) which means it blooms on new wood. Therefore it needs to be pruned to about 12-14 inches in the spring. I know it sounds drastic and the first time I pruned it this low after about 2 years I wanted to cry. I thought I had killed it. It made a spectacular return that year and grew about 8 feet. So get out your pruners and enforce some tough love on your clematis if it's the type that grows on new wood. You'll be thrilled with the results come July even if it's a bit hard to look at now.