The 'Jackmanii' is a cultivar in its own right, one of the oldest of the LFH's. It is considered a pruning group III. 'Niobe' is usually considered a pruning group II but with a rebloom - so a light initial pruning and again after the first bloom flush. Or you can treat as a group III if you wish with just a slight delay of the bloom season. 'Avant Garde' is a group III - hard prune.

Single entire branches dying back on newly planted clematis are often the result of stem damage rather than wilt. The stems are extremely delicate and break or split easily, resulting in dieback. We see this constantly at the nursery and I spend a good portion of my time carefully removing broken stems and resulting dead foliage. Many clematis authorties do not consider wilt to be a fungal disease but rather the inability of the vascular system of immature plants to support rapid and lush topgrowth. It is extremely common with many LFH's, primarily those in group II but is seldom fatal and most vines will eventually grow out of it. I have yet to see any text or resource that supports spraying as any kind of deterrant.....I wouldn't bother.

When planting new vines, prepare a large, well-amended planting hole and plant the vines deep -- 2-6" deeper than they present in the nursery container. And all clematis regardless of pruning group will benefit from hard pruning for at least the first couple of seasons in the ground. Both of these practices will encourage the development of a large, healthy root system less prone to wilt and the growth of multiple stems from the root crown, resulting in stronger and more vigorous, wilt-resistant plants. And one of the benefits of planting something at the base of clematis is to shield/protect these fragile stems from accidental damage.